closure of the tornquist sea constraints from mona lisa deep seismic reflection data deep seismic reflection profiles west of denmark across the suture between baltica and eastern avalonia reveal weak southward dipping reflectors within the crystalline basement these reflectors are interpreted as thrusts resulting from emplacement of ea

paraxial ray tracing in 3d inhomogeneous anisotropic media1 many investigations of the propagation elastic waves within the earth require a technique for producing synthetic seismograms which is capable of modelling 3d propagation effects ray methods are an excellent option for these problems because they can be made fully 3d and allow a relatively quick and flexible computation of synthetic seismograms however the two point problem of finding the ray which connects exactly a specific source and receiver may still be difficult and time consuming therefore application of the paraxial method which allows extrapolation of the information on a given ray to nearby receiver locations is very valuable with this approach great savings in computation time and significant simplification of computer codes are possible we investigate the application of the paraxial ray method to two problems in which the effects of 3d seismic wave propagation are important the first is a model of a reef structure in this case we consider synthetic seismograms for a vsp experimental configuration when the sv source and well are located along the axis of the reef only 2d propagation effects are observed if the source receiver plane is located to the side of the reef however the 3d shape of the reef causes significant amplitudes to be predicted for shear wave arrivals on the transverse component of the synthetic seismograms the second example is a id layered earth model but it includes two layers which are azimuthally anisotropic due to the presence of aligned vertical fractures this anisotropy leads to 3d raypaths synthetic seismographs are presented for a cross hole geometry both for an equivalent isotropic model and for the direction parallel to the fractures and at an angle of 45 to the fractures in the anisotropic case these synthetics show that the differences between the isotropic case and the case for source and receivers aligned with the plane of the fractures are small and subtle on the other hand the predictions for the direction at 45 to the cracks show shear wave splitting and significant transverse component signal these results have important implications for both modelling and for applications such as tomography it is clear that for some of the cases considered a 2d algorithm will lead to errors in interpretation of data in addition ray based tomographic techniques will have great difficulty in obtaining a well defined 2d planar image when the signals are propagating in regions outside the image plane

radiation from point sources in general anisotropic media summary a procedure to evaluate the geometrical properties of the slowness surface for arbitrary anisotropic media is presented this allows us to use results for computing the displacement field from point sources in general anisotropic media utilizing the gaussian curvature of the slowness surface of the considered waves since the formulae used are based on high frequency asymptotics the procedure gives reliable results as long as no parabolic points of the slowness surface are encountered radiation patterns of point sources for models representing rocks in the crust and upper mantle are presented and discussed the radiation from point sources in anisotropic models is compared with the radiation from point sources in average isotropic models to emphasize the differences anisotropy is either caused by aligned cracks crustal rocks or by aligned olivine crystals upper mantle rocks considering models of aligned cracks differing only in the physical parameters of the crack material water or gas significantly different radiation patterns are obtained models with wet cracks exhibit focusing of quasi s wave energy for some directions whereas this effect is not observed for the corresponding model with dry cracks the quasi s wave radiation pattern from explosions are multilobed and the wet crack models exhibit more lobes than the dry crack models the radiation of single forces in olivine shows strong energy focusing wave tuning of quasi s waves however for realistic compositions of upper mantle rocks with some 10 per cent of preferentially orientated olivine this wave tuning is less pronounced

shear waves from explosions in granite revisited lessons learned from the new england damage experiment abstract we studied seismic wave generation from five small 60 122 kg fully contained explosions detonated in barre granite as a part of the new england damage experiment nede the explosions were conducted using three types of explosives with different velocities of detonation vod black powder ammonium nitrate fuel oil anfo emulsion and composition b comp b empirical evidence suggests that the low vod explosives produce more shear wave energy than high vod explosives the proposed mechanisms to explain this effect include a inhibition of gas driven fracture propagation by thicker pulverized zone for high vod explosions and b fracture toughness increase at higher loading rate the main objective of the experiment was to study differences in shear wave generation between different types of explosives and to determine the likely mechanism responsible for these differences seismic amplitude analysis revealed that comp b releases more energy and larger amplitude p waves for the same weight of explosives while producing smaller amplitude s waves furthermore large radial cracks were observed on the surface after the anfo and black powder shots while there was no surface fracturing after the comp b shots thus longer fractures correlate with higher s wave amplitudes however drilling into the source region indicates that high vod explosions may actually produce a smaller pulverized zone which means that fracture inhibition is a less plausible explanation therefore we hypothesize that increase in loading rate in combination with shorter impulse duration for high vod explosives inhibits fracture processes and subsequently reduced s wave radiation

direct hydrocarbon detection using comparative p wave and s wave seismic sections two field experiments in the sacramento basin and one in the green river basin demonstrate that comparative p wave and s wave cdp seismic sections can be used to detect gas directly in sandstone reservoirs the lines in the sacramento basin were shot over producing gas fields known to correlate with amplitude anomalies on p wave sections reflections of comparable strength are present on the p wave and s wave sections at lithologic boundaries but gas saturated zones correlating with p wave bright spots show no equivalent s wave amplitude anomalies the responses are consistent with laboratory observations that p wave velocity is more sensitive to the introduction of gas into liquid saturated pore space than s wave velocity the line in the green river basin was shot over a relatively deep gas field producing from an overpressured reservoir not associated with a conventional p wave bright spot the p wave reflection strength of the reservoir is about 50 percent greater than the s wave reflection strength

an electrical resistivity crustal section from the alps to the baltic sea central segment of the egt abstract electrical resistivity models derived from magnetotelluric and magnetovariational measurements in the past 10 years have been compiled along the central segment of the european geotraverse along the profile low resistivity structures exist particularly in the middle and upper crust and probably mask deeper low resistivity zones these structures can be related to different tectonic units along the profile the prominent features are the two dimensional low resistivity structures at the transition from the moldanubian to the saxothuringian and from the saxothuringian to the rhenohercynian unit of the variscan belt the depth correlation of the low resistivity horizon in the rhenish massif with a seismic low reflectivity band the very high conductance of an upper to middle crustal layer in the north german lowlands and its cessation at the river elbe towards the baltic sea the reduction in the resistivity of these zones is very likely due to graphite at mid crustal depths it is obvious that there exists a general increase of electrical conductance in the upper half of the crust from south to north of the central segment which may be due to increasing accumulation of graphite and successive tectonically controlled ordering processes this phenomenon may by relating it to the growing crustal age from south to north be the indicator of a maturing continental crust

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

velocity anisotropy of shales and depth estimation in the north sea basin it is known that in the north sea basin the depths to major reflectors as determined from surface seismic data are often larger than the well log depths from a study of data sets which tie 21 wells i found a strong correlation between the occurrence of the depth error and the presence of shales in the subsurface assuming that the error is caused by elliptical velocity anisotropy in shales i measured the anisotropy from a comparison of the well log sonic data and the interval velocity profile obtained from the surface seismic data and also from a comparison of the seismic depth and the well log depth it was found that the two methods of measurements agree with each other and also agree qualitatively with the previous laboratory measurements of anisotropy in shale samples the results strongly suggest that the depth anomaly in the north sea basin is caused by the velocity anisotropy of shales a simple method to correct the seismic depth is given

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

effects of rock damage on seismic waves generated by explosions in studying the physical processes involved in the generation of seismic waves by explosions it is important to understand what happens in the region of high stresses immediately surrounding the explosion this paper examines one of the processes that takes place in this region the growth of pre existing cracks which is described quantitatively as an increase in rock damage an equivalent elastic method is used to approximate the stress field surrounding the explosion and a micro mechanical model of damage is used to calculate the increase in damage simulations for a 1 kt explosion reveal that the region of increased damage can be quite large up to ten times the cavity radius the damage is initiated on a damage front that propagates outward behind the explosive stress wave with a velocity intermediate between that of p and s waves calculations suggest that the amount of increased damage is controlled primarily by the initial damage and the extent of the region of increased damage is controlled primarily by the initial crack radius the motions that occur on individual cracks when damage increases are converted to seismic moment tensors which are then used to calculate secondary elastic waves which radiate into the far field it is found that while the contribution from an individual crack is small the combined effect of many cracks in a large region of increased damage can generate secondary waves that are comparable in amplitude to the primary waves generated by the explosion provided that there is asymmetry in the damage pattern this process is quite effective in generating s waves thus providing a quantitative explanation of how s waves can be generated by an explosion two types of asymmetry are investigated a shear pre stress and a preferred orientation of cracks and it is found that both produce similar effects

ductile normal faulting along the west bohemian shear zone moldanubian tepla barrandian boundary evidence for late variscan extensional collapse in the variscan internides structural and kinematic investigations of the west bohemian shear zone wbs clearly indicate late variscan orogen parallel wsw ene extension within the variscan internides along the wbs the western part of the tepla barrandian tb was downthrown to the east against the adjacent moldanubian according to seismic data the steeply east dipping wbs flattens with depth forming a prominent detachment zone the western part of the tb was tilted along this zone producing the patterns of metamorphic isograds the age of which is probably cadomian cross cutting relationships of wbs mylonites and carboniferous granites as well as the overall cooling ages of hornblende and mica suggest that ductile normal faulting along the wbs was active from about 330 to 310 ma

measured anisotropy in pierre shale the five elastic coefficients which characterize a transversely isotropic medium have been measured for the pierre shale the first arrival times deduced from two closely spaced vertical seismic profiles provide values of magnitude and direction of the phase velocity at a given depth for different locations of the sources from these velocities we can estimate the five elastic coefficients by assuming that the p and sv wave velocities have only a small dependence on the angle of propagation on the other hand it is shown that the accuracy in the difference between the directions of propagation and particle motion is not sufficient to determine the anisotropy this work was industrially sponsored through the integrated geophysics project

palaeozoic terranes and their lithospheric boundaries within the trans european suture zone tesz a review the trans european suture zone tesz is a broad and complex zone of terrane accretion separating ancient lithosphere of the baltic shield and east european craton eec from the younger lithosphere of western and southern europe there is debate about the number of terranes involved and their origins the most significant terrane boundaries originally oceanic sutures are poorly exposed and are defined using faunal provinciality prominent geophysical lineaments may represent sub terrane boundaries within composite terranes the avalonia composite terrane was amalgamated to baltica in latest ician time the provenance of the malopolska and moravo silesian terranes whose neoproterozoic basement is suspect with respect to baltica but with which they nonetheless share faunal affinities is still debated the extent of the variscide rhenohercynian zone defines the southern limit of the early palaeozoic accreted terranes terranes within the bohemian massif exhibit hp metamorphism recording subduction driven collision prior to incorporation in the tesz collage itself associated with a hp event in romania the saxothuringian zone and rheic suture are directly juxtaposed with the eec and the crustal structure of the tesz has been much modified by post palaeozoic tectonism deep seismic reflection data where available show that the oceanic sutures are frequently associated with inclined reflectivity zones irz in the lithospheric mantle here inferred to mark relict subduction zones interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the irz resulting from early orogenic lithospheric delamination at the moho interface and or post orogenic reordering processes the mantle irz appear to indicate a n directed polarity for early palaeozoic subduction apparently incompatible with the geological evidence which indicates subduction of the tornquist sea beneath avalonia and a large amount of underthrusting of avalonia by baltica possible explanations for these apparently incompatible observations are evaluated

causes of compressional wave anisotropy in carbonate bearing deep sea sediments forty indurated sediment samples from dsdp site 516 were studied with the principle objective of determining which of several proposed mechanisms is the cause of acoustic anisotropy in carbonate bearing deep sea sediments recovered from sub bottom depths between 388 and 1222 m the samples have properties exhibiting the following ranges wet bulk density 1 90 to 2 49g cm3 fractional porosity 0 46 to 0 14 carbonate content 34 to 88 percent compressional wave velocity at 0 1 kbar 1 87 to 4 87 km sec anisotropy 1 to 13 percent velocities were measured in three mutually perpendicular directions through the same specimen in 29 of the 40 samples studied calcite fabric has been estimated by x ray pole figure goniometry the major findings of this study are 1 carbonate bearing deep sea sediments may be regarded as transversely isotropic media with symmetry axes normal to bedding 2 calcite c axes are weakly concentrated in a direction perpendicular to bedding but the preferred orientation of c

anisotropy and fracture zones about a geothermal well from p wave velocity profiles the feasibility of locating fracture zones and estimating their crack parameters was examined using an areal well shoot method centered on utah state geothermal well 9 1 beaver county utah high resolution traveltime measurements were made between a borehole sensor and an array of shotpoints distributed radially and azimuthally about the well the directional dependence of velocity in the vicinity of the well was investigated by comparing traveltimes from different azimuths velocity anisotropy was detected this condition is consistent with but not required by the existence of fractures in the basement rock having orientations subparallel to the latest episode of local faulting the interpretation is complicated by a variable thickness of overburden around the well traveltime delays also suggest the presence of a low angle fracture zone intersecting the well at a depth of 1 500 ft

ultrasonic velocities in cretaceous shales from the williston basin compressional and shear wave velocities were measured in the laboratory from 1 bar to 4 kbar confining pressure for wet undrained samples of cretaceous shales from depths of 3200 and 5000 ft in the williston basin north dakota these shales behave as transversely isotropic elastic media the plane of circular symmetry coinciding with the bedding plane for compressional waves the velocity is higher for propagation in the bedding plane than at right angles to it and the anisotropy is greater for the 5000 ft shale for shear waves the sh wave perpendicular to bedding and the sv wave parallel to bedding propagate with the same speed which is about 25 percent lower than that for the sh wave parallel to bedding in general compressional and shear velocities are higher for the indurated 5000 ft shale than for the friable 3200 ft shale all velocities increase with in increasing confining pressure to 4 kbar the 3200 ft shale exhibits velocity hysteresis as a function of pressure whereas this effect is a

dispersion of rg waves in southeastern maine evidence for lateral anisotropy in the shallow crust abstract group velocities of short period rayleigh waves rg are sensitive to variations in the velocity structure of the upper few kilometers of the crust in this study we analyzed dispersive properties of rg waves recorded from blasts detonated as part of the maine seismic refraction experiment conducted in 1984 by the u s geological survey the period range of the rg waves in this study is 0 4 to 1 6 sec group velocities in this period range are sensitive to the velocity structure at depths ranging from very near the surface to about 2 km group velocities were found to vary significantly from one path to the next a simple explanation of the observed pattern of group velocities is that higher velocities were observed for paths parallel to the structural grain of the appalachians than for paths transverse to the grain for example 1 sec rg waves have group velocities of about 3 0 to 3 2 km sec for paths parallel to the grain and about 2 6 to 2 7 km sec for paths transverse to the grain thus our results suggest that the shallow crust beneath the appalachians of southeastern maine is laterally anisotropic rg group velocities from this study were inverted to obtain shear wave velocity models of the upper 1 5 to 2 0 km of the crust underlying southeastern maine to compare our results with refraction and laboratory results v p was estimated from v s using a v p v s ratio of 1 78 inverting the average group velocities for all paths in this study resulted in a v p model with velocities ranging from 4 9 km sec in the upper 0 3 km to 6 2 km sec at 2 km depth the range in observed rg group velocities from one path to the next is evidence of a variation in shallow crustal structure depending on geographical location and or orientation of the paths in the upper 0 3 km v p appears to range from about 4 8 to 5 3 km sec and at 2 km depth v p appears to range from about 5 6 to 6 6 km sec

anisotropy effects in p wave and sh wave stacking velocities contain information on lithology depths calculated from s wave stacking velocities and event times almost always exceed actual depths sometimes by as much as 25 percent in contrast depths from corresponding p wave information are often within 10 percent of actual depths discrepancies in depths calculated from p and s wave data are attributed to velocity anisotropy a property of sedimentary rocks that noticeably affects s wave moveout curves but leaves the p wave relatively unaffected two careful studies show that discrepancies in depths and hence in constituent layer thicknesses correlate with lithology discrepancies ranged from an average of 13 percent midland basin to greater than 40 percent paloma field in shales but were within expected errors in massive sandstones or carbonates hence anisotropy effects are indicators of lithology analysis of seismic data involved determining interval velocities from stacking velocities calculating layer thicknesses and then comparing layer thicknesses from s wave data with thickne

borehole flexural modes in anisotropic formations a perturbation method of solution is an efficient way of analyzing elastic wave propagation along a borehole in anisotropic formations the perturbation model allows us to calculate changes in the modal dispersion curves caused by the differences in elastic constants between the anisotropic formation of interest and a reference or unperturbed isotropic formation the equivalent isotropic constants in the reference formation are obtained from the appropriate compressional and shear wave velocities for the selected propagation and polarization directions of the flexural mode this choice of the unperturbed solution means that the required perturbation is minimal resulting in enhanced accuracy of the perturbed solution computational results are presented for the dispersion curves of borehole flexural waves in a transversely isotropic ti formation as a function of borehole deviation from the ti symmetry axis in addition radial distributions of displacement and stress fields associated with the flexura

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

detrital composition of ordovician sandstones from the rugen boreholes implications for the evolution of the tornquist ocean the lower palaeozoic sequences of the rugen boreholes are composed of pelitic clastic sediments which range in age from the cambro ordovician boundary to the late ordovician provenance studies have been carried out on cambro ordovician sandstones from the loissin borehole and on middle upper ordovician greywackes of the rugen 5 borehole

shear wave velocity anisotropy in sedimentary rocks a laboratory study a systematic laboratory study of shear wave velocity anisotropy in sedimentary rocks has been carried out as a function of both hydrostatic and uniaxial stress the presence of shear wave velocity anisotropy in sedimentary rocks was confirmed by transmitting a polarized shear wave in a rock sample and receiving a signal on an orthogonally polarized receiver the magnitude of the observed velocity anisotropy is dependent upon the magnitude and nature of the applied stress and the lithology of the rock sample shales exhibit significant velocity anisotropy independent of both uniaxial and hydrostatic stress which suggests the presence of preferentially aligned minerals sandstones also exhibit significant velocity anisotropy but the anisotropy is strongly dependent upon the stress hydrostatic stress was found to diminish the velocity anisotropy and uniaxial stress was found to enhance it this implies the presence of preferentially oriented cracks limestones exhibit weak velocity anisotropy these laboratory observations may be helpful in in situ identification of lithology

the matter of size on the moment magnitude of microseismic events abstractwe investigated the method of estimating seismic moment and moment magnitude for microseismic events we determined that the m0t defined by bowers and hudson is the proper scalar moment to be used in microseismic studies for characterizing the size of an event and calculating its moment magnitude for non double couple sources the proportional relationship between body wave amplitude and seismic moment in the brune model breaks down so under such situations the brune model is not an appropriate way to estimate the seismic moment and magnitude moreover the s wave alone is not sufficient for determining the total seismic moment instead the p wave must be analyzed an example barnett shale data set was studied and the results concluded that the magnitudes estimated with the brune model could be off by as much as 1 92 with an absolute average of 0 35 the moment magnitudes based on the scalar moment m0t also gave a significantly different event size distribution and b value estimation finall

radiation patterns of a shear wave vibrator in near surface shale two shallow surface to borehole field tests were conducted to investigate the radiation patterns of a shear wave vibrator in near surface shale in the in line direction for which baseplate motion is transverse to the source geophone raypath the vibrator produces a broadband high amplitude signal of the sh type and virtually no signal of the sv or p types the amplitude of the sh radiation is approximately constant as a function of polar angle but it falls off as the cosine of the azimuthal angle measured with respect to the in line direction in the broadside direction the vibrator produces a strong uncontaminated signal of the sv type the radiation pattern of the sv signature is lobate in the polar plane and peaks at an incident angle of 30 to 35 degrees in the horizontal plane the sv radiation varies as the sine of the azimuthal angle measured with respect to the in line direction the field tests were conducted on a permian shale in oklahoma and an eocene shale in east texas comparison between sh and sv wave surfaces indicates that both shales behave approximately like transversely isotropic solids with a vertical axis of symmetry theoretical wave surfaces are fit to the observed data by using elastic moduli determined from the field measurements and by making the assumption that the near surface shale at each field site is a single homogeneous inherently transversely isotropic material the analyses indicate that horizontal sh group velocity is about 30 percent greater than horizontal sv group velocity in the permian shale and about 25 percent greater in the eocene shale

focal mechanisms in anisotropic media summary focal mechanisms of seismic sources in anisotropic media are more complicated than in isotropic media planar shear faulting produces pure double couple dc mechanism in isotropy but generally non double couple non dc mechanism in anisotropy the non dc mechanism can comprise both the isotropic iso and compensated linear vector dipole clvd components the amount of the iso and clvd components depends on strength and symmetry of anisotropy and on the orientation of faulting shear faulting in anisotropy generates a pure dc mechanism provided that faulting is situated in symmetry planes of orthorhombic or of higher anisotropy symmetries the fault plane solution i e the orientation of the fault normal and the slip direction can be retrieved from the seismic moment tensor and from elastic parameters describing anisotropy at the source numerical modelling shows that shear faulting in anisotropic rocks present in the earth crust the earth mantle or in the subduction zones can produce mostly mechanisms with the clvd up to 30 per cent and with the iso up to 15 per cent the fault plane solutions calculated under the assumption of isotropy typically deviate from the true solutions by an angle of 5 to 10

perovskite and baddeleyite from kimberlitic intrusions in the tarim large igneous province signal the onset of an end carboniferous mantle plume several tens of kimberlitic pipes and dykes are exposed in the wajilitag area in the western tarim large igneous province here we report for the first time secondary ion mass spectrometric u pb age data on perovskite and baddeleyite grains in a kimberlitic pipe and a kimberlitic dyke from the tarim craton the perovskite yielded a well defined intercept age of 299 87 4 3 ma which is consistent with its corresponding concordia and 206 pb 238 u ages corrected for the common pb contribution of about 300 ma the baddeleyite separated from two kimberlitic samples from a dyke display identical concordia u pb ages of 300 87 4 7 ma and 300 57 4 4 ma our age data show that the kimberlitic intrusions were emplaced at ca 300 ma rather than in the late permian as previously regarded these new ages are slightly older than the eruption ages of tarim flood basalts 291 273 ma offering a critical regional time marker for the onset of permo carboniferous magmatism in the tarim craton detailed petrographic observations did not reveal any ultrahigh pressure mineral assemblage in the wajilitag kimberlitic intrusions phlogopites from these intrusions show end t values of 3 7 to 4 2 the baddeleyites which are texturally primary and therefore inferred to have crystallized directly from the kimberlitic magma yield a range of ehf t from 4 8 to 8 7 these results combined with previously reported geochemical data suggest that the wajilitag kimberlitic intrusions were most likely derived from a moderately refractory and depleted subcontinental lithosphere mantle metasomatized by subduction components associated with an early middle paleozoic convergent regime the kimberlitic magma was generated by small degree partial melting of the lithospheric mantle in response to the impingement of the tarim mantle plume thus our new geochronological data suggest the arrival of the mantle plume beneath the tarim lithosphere at least 10 million years before the onset of tarim flood basalt volcanism the end carboniferous wajilitag kimberlitic intrusions the oldest known phase associated with carboniferous magmatism in the tarim craton signals the initial magmatic pulse triggered by mantle plume impingement

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

tsunami generation and associated waves in the water column and seabed due to an asymmetric earthquake motion within an anisotropic substratum in this paper closed form integral expressions are derived to describe how surface gravity waves tsunamis are generated when general asymmetric ground displacement due to earthquake rupturing involving both horizontal and vertical components of motion occurs at arbitrary depth within the interior of an anisotropic sub sea solid beneath the ocean in addition we compute the resultant hydrodynamic pressure within the seawater and the elastic wavefield within the seabed at any position the method of potential functions and an integral transform approach accompanied by a special contour integration scheme are adopted to handle the equations of motion and produce the numerical results the formulation accounts for any number of possible acoustic gravity modes and is valid for both shallow and deep water situations as well as for any focal depth of the earthquake source phase and group velocity dispersion curves are developed for surface gravity tsunami mode acoustic gravity rayleigh and scholte waves several asymptotic cases which arise from the general analysis are discussed and compared to existing solutions the role of effective parameters such as hypocenter location and frequency of excitation are examined and illustrated through several figures which show the propagation pattern in the vertical and horizontal directions attention is directed to the unexpected contribution from the horizontal ground motion the results have important application in several fields such as tsunami hazard prediction marine seismology and offshore and coastal engineering in a companion paper we examine the effect of ocean stratification on the appearance and character of internal and surface gravity waves this article is protected by copyright all rights reserved

seismic source theory in stratified anisotropic media we extend seismic source theory to general anisotropic media for the numerical evaluation of spectral amplitudes of point sources in multilayered anisotropic crustal models first we obtain an explicit representation of the spectral elastodynamic green s tensor in general homogeneous anisotropic media as a sum of three integrals over the corresponding three slowness surfaces the multidimensional stationary phase principle is then applied to derive an asymptotic approximation at the far field in the case of azimuthally isotropic media we offer an alternative representation of the green s tensor and its ensuing displacements fields in the form of an exact hankel transform over the horizontal wave number variable the total field is specified here in terms of two potentials an sh potential and a mixed quasi transverse quasi longitudinal potential both of which assume the role of two scalar green s functions the availability of the green s tensor in analytical form enables one to obtain readily numerical solutions for a wide selection of media and sources it is shown that the radiation field of an explosion has the following new features 1 quasi transverse waves are created with four and eight lobe patterns 2 quasi longitudinal waves are generated for the collatitudinal displacement with four lobe patterns 3 the energy ratio sv p may reach the value of 20 for more than 50 of the azimuths in crustal structures such as tuff and shales and 4 radiation patterns for vertical shear waves are created which are indistinguishable from corresponding waves produced by earthquake faults our formalism allows us to establish a haskell type matrix algorithm for a multilayered azimuthally isotropic half space which enables us to calculate body waves and surface waves in real earth crustal models