the shear wave velocity gradient at the base of the mantle the relative amplitudes and travel have been directed toward obtaining global times of scs and s phases are utilized to place averages and the degree of lateral variation in constraints on the shear wave velocity gradient d properties remains an open question above the core mantle boundary a previously a conflicting result was found by mitchell and reported long period scsh sh amplitude ratio helmberger 1973 who utilized the relative minimum in the distance range 65 o to 70 o is shown amplitudes and timing of long period scs and s to be a localized feature apparently produced by phases to constrain the s wave velocity gradient an amplitude anomaly in the direct s phase and in d they found a minimum in the scsh sh therefore need not reflect the velocity gradient amplitude ratio near 68 o which was attributed to at the base of the mantle the amplitude ratios low amplitudes of the scs arrivals unable to that are free of this anomaly are consistent with explain this feature by models with negative or calculations for the jb model or models with mild near zero shear velocity gradients in d they positive or negative velocity gradients in the proposed models with positive s wave velocity lowermost 200 km of the mantle scsv arrivals gradients above the cmb these positive are particularly sensitive to the shear velocity gradients extended over 40 to 70 km above the structure just above the core mantle boundary core reaching velocities at the cmb as high as the apparent arrival time of the peak of scsv is 7 6 to 7 8 km s these models can explain the as much as 4 sreater than that of scsh in the observed amplitude ratio behavior as well as an distance range 75 v to 80 o for sea of okhotsk apparent difference observed in the arrival times events recorded in north america this can be of transversely and radially polarized scs explained by interference effects produced by a mitchell and helmberger also proposed a low q localized high velocity layer or strong positive zone in d or finite outer core rigidity to s wave velocity gradient in the lowermost 20 km explain the baseline of the scsh sh amplitude of the mantle a velocity increase of about 5 ratios while the majority of their data was for is required to explain the observed shift between deep south american events recorded in north scsv and scsh this thin high velocity layer america they did analyze one deep sea of okhotsk varies laterally as it is not observed in event for which the radial and transverse scs similar data from argentine events refined arrival times were not different which suggested estimates of the outermost core p velocity lateral variations in the d velocity structure structure are obtained by modeling sks signals in in this paper we extend the analysis of scs the distance range 75 o to 85 o s

carboniferous ostracods from central honshu japan silicified beyrichiocopid and podocopid ostracods from limestone nodules derived from the middle part of the ichinotani formation within the hida gaien terrane of central honshu island japan are associated with fusulinid foraminifera that indicate strata of the middle moscovian pennsylvanian carboniferous this is a rare record of ostracods from the palaeozoic of japan and the first systematic description of ostracods from the carboniferous of the hida gaien terrane the fauna comprises six ostracod species two new assigned to the genera amphissites kirkbya bairdia aechmina and healdia and additional material of possible cavellinids the numerical dominance of ornamented beyrichiocopids such as kirkbya and amphissites along with smaller numbers of smooth podocopids such as bairdia indicates an eifelian mega assemblage ecotype sensu g becker that is typical of mid palaeozoic shallow marine high energy environments in a fore reef ecosystem

simulation and analysis of 1991 u s geological survey refraction data from the new madrid seismic zone two orthogonal wide aperture seismic refraction lines were acquired in the new madrid seismic zone by the u s geological survey in 1991 these n s and e w oriented lines cross the blytheville arch and adjacent portions of the reelfoot rift ten shots were fired along the e w line eight shots were fired along the n s line iterative numerical modeling including scattering and viscoelasticity has been performed for all shots on both lines using a new eighth order staggered grid two dimensional finite difference solution based on superposition of relaxation mechanisms the main features in the data can be explained by the existence of a strong 15 km wide low velocity cap lying beneath the center of the blytheville arch between 1 5 and 2 5 km depth the data do not allow estimation of its extent along the axis of the arch but it is at least 30 km long and appears to become less prominant toward the southwest the arch itself has a low velocity background that extends to at least 5 km depth an additional low velocity layer the elvins shale lies generally between 3 and 4 km depth is upwarped at the edges of the arch and thins toward the edges of the reelfoot rift there is no evidence for anomalously low intrinsic q but the data may be interpreted to indicate strong local scattering that occurs west of the low velocity cap of the blytheville arch at depths that are similar to and slightly shallower than the cap additional strong scattering occurs at the western edge of the reelfoot rift an alternative interpretation attributes the apparent scattering at the blytheville arch to instrument noise

alternatives to core mantle boundary topography abstract nuclear explosions at the french test sites in the south pacific have been recorded at the regional seismic networks in the western united states a total of approximately 600 good quality short period seismograms were selected from an initial set of 900 the dense receiver spacing particularly in california allows the division of the networks into numerous subarrays and the data from each subarray can be analysed using conventional array techniques such as beam forming and frequency wave number analysis the seismic rays provide dense sampling of a small area of the core mantle boundary cmb beneath the eastern pacific from 5 to 13 n and 133 to 126 w pcp has a shallow incidence angle on the cmb at an epicentral distance of 65 and d is sampled over a large swathe from 2 s to 24 n and from 134 to 123 w beam traces have been formed for the p and pcp phases for each of the subarrays using slowness and azimuth values derived from array methods good quality beams have been selected for further analysis by applying a matched filter to the beamed traces an accurate set of pcp p differential travel times has been determined and compared with iasp91 the residuals show a smooth trend over the area sampled for the given epicentral distances p and pcp have similar paths through the crust and upper mantle so we make the assumption that near source and near receiver structures have negligible effect on the differential times we also assume that the midmantle is relatively homogenous many previous studies have interpreted pcp p residuals in terms of cmb topography by mapping the residuals directly to topography a range in elevation of 40 km over a lateral extent of about 700 km results alternatively we attempted to interpret the residuals in terms of velocity heterogeneity in d a range of d models have been tested including lateral velocity variation in a d layer with constant thickness the pcp p residuals may be explained by allowing the p wave velocity to vary laterally by a factor of 2 over the region sampled with most of the region being marginally fast this suggests that d heterogeneity has large spectral power at relatively short wavelengths in this scenario previous tomographic inversions for cmb topography will have large errors since in these inversions corrections for d structure are based on low order spherical harmonic models

dynamic faulting studied by a finite difference method we have developed a finite difference method that is especially adapted to the study of dynamic shear cracks we studied a number of simple earthquake source models in two and three dimensions with special emphasis on the modeling of the stress field we compared our numerical results for semi infinite and self similar shear cracks with the few exact solutions that are available in the literature we then studied spontaneous rupture propagation with the help of a maximum stress criterion from dimensional arguments and a few simple examples we showed that the maximum stress criterion depended on the physical dimensions of the fault for a given maximum stress intensity the finer the numerical mesh the higher the maximum stress that had to be adopted a study of in plane cracks showed that at high rupture velocities the numerical results did not resolve the stress concentration due to the rupture front from the stress peak associated with the shear wave propagating in front of the crack we suggest that this is the reason why transonic rupture velocities are found in the numerical solutions of in plane faulting when the rupture resistance is rather low finally we studied the spontaneous propagation of an initially circular rupture two distinct modes of nucleation of the rupture were studied in the first a plane circular shear crack was formed instantaneously in a uniformly prestressed medium after a while once stress concentrations had developed around the crack edge the rupture started to grow in the second type of nucleation a preexisting circular crack became unstable at time t 0 and started to grow the latter model appeared to us as a more realistic simulation of earthquake triggering in this case the initial stress was nonuniform and was the static field of the preexisting fault

inhomogeneities near the core mantle boundary inferred from short period scattered pkp waves recorded at the global digital seismograph network we analyze short period pkp precursor wave trains recorded at global digital seismograph network stations in the distance range 120 140 to infer the nature of inhomogeneities near the core mantle boundary travel times and particle motions are consistent with predictions of single scattered waves near the cmb the dominant frequencies are around 1 hz the regions best sampled are beneath indonesia north america central africa and south america based on first order scattering theory the characteristics of the wave number spectrum of the structural inhomogeneities are obtained for two hypotheses volume inhomogeneities and topographic irregularities for the range of wavelengths sampled by these data between 10 and 70 km the spectrum of inhomogeneities is best represented by a power law of index 5 3 for volume inhomogeneities and 6 8 for topographic irregularities if the scattering is due only to volumetric inhomogeneities we estimate the strength of the structural variations to be about 0 5 in p wave velocity for a 200 km thick layer if the scattering is due only to topographic irregularities the topography has an rms height of 280 m at present we can not distinguish between these two types of inhomogeneities

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

seismic energy partitioning and scattering in laterally heterogeneous ocean crust we present finite difference forward models of elastic wave propagation through laterally heterogeneous upper oceanic crust the finite difference formulation is a 2 d solution to the elastic wave equation for heterogeneous media and implicitly calculates p and sv propagation compressional to shear conversion interference effects and interface phenomena random velocity perturbations with gaussian and self similar autocorrelation functions and different correlation lengths a are presented which show different characteristics of secondary scattering heterogeneities scatter primary energy into secondary body waves and secondary stoneley waves along the water solid interface the presence of a water solid interface in the model allows for the existence of secondary stoneley waves which account for much of the seafloor noise seen in the synthetic seismograms for the laterally heterogeneous models

lateral variations near the core mantle boundary differential travel times between s and sks ts sks and skks and sks tskks sks along with amplitude behavior have been used to study lateral variations at the core mantle boundary the data were recorded by the world wide seismographic station network wwssn and the canadian seismograph network csn from source regions in south america and the fiji tonga area a core mantle boundary region traversed by sks and s beneath the mid pacific from the fiji tonga area to north american stations shows the most anomalous behavior in ts sks sks and s are anomalously separated by up to 8 seconds in addition to ts sks anomalies sks and skks are anomalously separated by 2 to 3 seconds a model fitting ts sks for this anomalous region requires one or more of the following 1 up to 5 increase in the top 300 km of outer core velocity 2 a large scale high vs region 2 increase in the lower mantle ne of tonga 3 lateral variations in d shear wave velocities of up to 3 however 1 above predicts a decrease in tskks sks contradicting the observations this suggests that 2 and 3 are more likely explanations in modeling this data set

investigation of laterally heterogeneous shear velocity structure in d beneath eurasia seismic waves are used to explore large scale radial structure and lateral heterogeneity of shear velocity in the d region the chemical and thermal boundary layer at the base of the mantle we analyze long period horizontally polarized sh shear waveforms from earthquakes in the northwest pacific recorded in europe and the middle east for evidence of proposed laterally varying shear velocity stratification in d beneath eurasia specifically our data sample the lower mantle extending from beneath the arctic ocean near novaya zemlya ussr to under northern india using a reflectivity technique to model the sh waveforms we determine a locally stratified shear velocity model sgle that provides a good average travel time and waveform fit to over 80 of the data sensitive to d structure sgle is characterized by a rapid 2 75 increase in shear velocity approximately 290 km above the core mantle boundary similar to previously proposed models for this region unlike for other regions analyzed using similar waveform modeling techniques the percentage of data inconsistent with this average radial model is fairly substantial indicating that laterally heterogeneous d velocity structure is important in this region some waveforms lack the reflected arrival expected for a stratified model while others show evidence of two arrivals both possibly originating within d while our data are insufficient for direct inversion of the three dimensional structure we utilize several simple parameterizations of the waveforms to gain some understanding of the region both the relative amplitude and differential travel time variations of the waveforms indicate that d beneath eurasia is characterized by 1 broad regions in which the structure has large scale stratification giving coherent long period reflections with superimposed velocity variations that scatter shorter wavelength waves and 2 smaller regions which are sufficiently heterogeneous to scatter even the long period energy the scale length of the lateral heterogeneity in d that produces these variations is inferred to be somewhat less than 500 km which contrasts with weaker lateral heterogeneity that is observed in the central region of the lower mantle where heterogeneity scale lengths of approximately 500 1000 km appear to dominate these observations are consistent with a model of the lower mantle in which distributed heterogeneity is concentrated in d generating a locally stratified layer in some regions the effects of a diffuse or topographically variable d discontinuity must be further explored using two and three dimensional synthetic calculations before inversions for the detailed structure can be undertaken

the structure of the core mantle boundary from diffracted waves diffracted p and s waves pd sd traveling around the core mantle boundary cmb of the earth give us information about the velocity structure and therefore the thermochemistry of d the base of the earth s mantle by examining pd and sdarrivals we determined the apparent ray parameter for different regions at the base of the mantle by comparing the data slownesses to those found from reflectivity synthetic seismograms we were able to quantify d average velocities using these averaged velocities with a thermochemical modeling of lower mantle minerals using a birch mumaghan equation of state we have been able to make chemical and physical inferences as to the causes of lateral variations at the cmb examinations found significant lateral heterogeneity at the base of the mantle amounting to 4 for both p and s velocities these velocities did not always vary in parallel and the poisson ratio varied regionally by almost 6 the most unusual region of the cmb found was under indonesia where velocities 3 slower than the preliminary reference earth models were found adjacent to a region of faster than average velocities these regions currently correspond to areas of core up welling and down welling respectively found by voorhies 1986 which if mostly held in place by core mantle coupling might cause a flux of heat and iron into the mantle making the anomaly both thermally and chemically derived at the cmb under the northern pacific rim the fastest shear velocities were found but the same region yielded slower than average p velocities while the presence of fast shear velocities here would support the idea that we are seeing the cold dregs of mantle convection perhaps continuing down from the north pacific subduction zones the presence of slow p velocities suggests additional complications our thermochemical modeling suggests that the d poisson ratio is very sensitive to variations in the silicate oxide ratio and that a decrease in the amount of perovskite relative to magnesiowustite may play an important role in this region

s wave scattering by 3 d heterogeneities at the base of the mantle the hypothesis of s wave scattering by three dimensional heterogeneity in the lowermost mantle is shown to provide a plausible alternative to the spherically stratified models recently proposed by lay and helmberger to explain certain observed complexities in long period s wave data a result of central importance is that contrary to the conclusions of lay and helmberger and most recently young and lay sharp increases of s velocity with depth are not required to explain their data

finite difference seismograms for sh waves the accuracy of the finite difference method for generating synthetic seismograms of sh wave propagation in cylindrically symmetric media is discussed the finite difference method has the advantage that arbitrary density and velocity fields in the medium may be specified a point source is generated by a simple transformation of a line source the accuracy of the finite difference seismograms in flat and dipping layered media is confirmed by comparison with the generalized ray method a source radiation pattern is inserted by introducing a near field which has permanent displacement near the source strong motion synthetics are constructed with this new method for the 1968 borrego mountain earthquake as recorded at el centro good fits to the data are achieved using the laterally varying model determined by a detailed refraction survey and the source parameters determined by teleseismic waveform modeling shallow faulting is no longer necessary to explain the long period surface wave development

influence of near source volcano topography on the acoustic wavefield and implication for source modeling in near source conditions 5 km the atmosphere is often assumed to be fully transparent to acoustic waves and to have a small effect on the acoustic wavefield as a consequence the propagation of the acoustic waves in the atmosphere near the source is often neglected and waveform is compensated only in amplitude by the inverse of the distance from the source however the amplitude distribution at five infrasonic stations deployed around the active craters of the stromboli volcano reveals anomalous decay up to 11 db larger than expected which cannot be explained in terms of energy loss for geometrical spreading the distribution of amplitude decay changes for different source positions suggesting a strong relationship with the source to receiver path geometry by using 2d fdtd modeling we show that the anomalous amplitude distribution of the acoustic waves at stromboli is not necessarily due to the source radiation pattern but it is strongly influenced by the topography of the ground atmosphere interface diffraction and reflection of topography contaminate the acoustic wavefield and have a strong effect in reducing the amplitude and altering the waveform our work demonstrates that line of sight conditions are quite difficult to be achieved in acoustic experiments and that a detailed knowledge of the topography along the crater receiver path is necessary whenever infrasound waveforms are used to invert for the source process

evidence against a discontinuity at the top of d summary we investigate a recent suggestion of lay helmberger that there is a world wide discontinuity in 5 wave velocity about 250 300 km above the core mantle boundary with an increase of 2 5 3 per cent from above to below their model predicts pronounced pulse form changes of long period sh body waves in the epicentral distance range 95 120 a range which has not been studied by them similar pulse form changes are expected in p wave seismograms under the reasonable assumption of a similar increase in p velocity nine long period seismogram sections of sh and p waves from deepfocus earthquakes below fiji west brazil the sea of japan the sea of okhotsk and the banda sea are presented with practically no indication of the complications expected also the decay constants of pdiff as predicted by the new model differ strongly from observed values thus there is clear evidence against this model the pulse form peculiarities between mantle s and scs in transverse component seismograms at distances from 70 to 80 which lay helmberger interpreted as reflections from the discontinuity of their model must have a different cause

holocene cemented beach deposits in belize abstract two types of cemented beach deposits occur on reef islands off the coast of belize these are 1 intertidal beachrock that is dominantly cemented by marine aragonite and high magnesium calcite cements and 2 supratidal cayrock that is cemented mainly by vadose low magnesium calcite cements besides differences in position relative to present sea level and resulting early diagenesic features beachrock and cayrock can be distinguished on the basis of differences in composition texture geographical position and age whereas the composition of beachrock is similar to that of the adjacent marginal reef sediments cayrock is enriched in benthic foraminifera intertidal beachrock is moderately to well sorted and well cemented while supratidal cayrock is very well sorted poorly cemented and friable beachrock occurs preferentially on windward beaches of sand shingle gays on the middle and southern barrier reefs and on the isolated platforms glovers and lighthouse reefs cayrock only occurs on larger mangrove sand gays of the isolated platforms turneffe islands lighthouse reef and the northern barrier reef 14c dating of ten whole rock and mollusk shell samples produced calibrated dates between ad 345 and ad 1435 for beachrock and between bc 1085 and ad 1190 for cayrock the large scale distribution of beachrock in belize supports the contention that physical processes such as water agitation rather than biological processes control beachrock formation and distribution only on windward sides of cays that are close to the reef crest where large amounts of seawater flush the beaches considerable amounts of cements can be precipitated to produce beachrock cayrock forms due to cementation in the vadose zone and is only preserved on larger stable mangrove sand cays

surface motion over a sedimentary valley for incident plane p and sv waves abstract finite difference techniques are applied to study the response of a sedimentary basin in an isotropic elastic half space to vertically incident compressional and shear sources the sedimentary rock of the basin is modeled as a transversely isotropic material improved finite difference approximations to the boundary conditions are developed of primary interest is the formation of rayleigh waves over the flank of the basin displacement time series at points on the free surface and rayleigh orbits are plotted three dimensional spectral amplitude graphs of the entire time series up to a period of temporary quiescence are presented the rayleigh wave is the dominant scattered phase for the basin geometry considered having 20 to 30 per cent of the source displacement amplitude at the free surface in a basin with 26 flanks 0 7 km depth 6 km width over a half space whose acoustic impedance contrast with the basin was two the effect of varying the degree of compressional anisotropy in the basin on the amplitudes of refracted and scattered phases of engineering interest was minimal

amplitude of the core mantle boundary topography estimated by stochastic analysis of core phases abstract the core mantle boundary cmb topography is an important parameter for constraining the models of mantle dynamics and core mantle coupling however the various large wavelength seismological models of the cmb topography which have been obtained up to now are poorly correlated moreover their maximum amplitudes vary considerably from one model to another with values ranging from 4 to 12 km these large discrepancies may be due to the difficulty to separate in the travel time anomalies the contribution of the cmb topography from that of the highly heterogeneous d region at the base of the mantle in order to better constrain the amplitude of the cmb topography we perform a stochastic analysis of the core phases which sample the cmb as transmitted waves and or as reflected waves in particular we analyse underside reflected pkkp phases which help to discriminate between cmb topography and d structure and have in addition a great sensitivity to cmb topography the other phases used are the upperside reflected waves pcp and the transmitted waves pkp the analysis is performed on the travel time residual file obtained by engdahl et al engdahl e r van der hilst r buland r p 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seismol soc am 88 722 743 after earthquake relocation and phase re identification after careful travel time data selection the stochastic analysis allows us to separate coherent signal from random signal in the data at different length scales ranging from 300 to 1500 km then the cmb topography variance is obtained at the different length scales from a joint analysis of the coherent signal in the different core phases in taking into account their different sensitivities to cmb topography the estimated cmb topography variance has a significant signal in the wavelength range that we have investigated showing that 95 of cmb topography amplitude is in the range 4 0 km for wavelengths larger than 300 km this value decreases to 1 5 km for wavelengths larger than 1200 km indicating that the long wavelength cmb topography is significantly lower than previously proposed however it has not been possible to invert the data for deriving a map of the cmb topography a checkerboard pattern analysis reveals that despite the introduction of the pkkp phase the relative contributions of d heterogenieties and cmb topography cannot be separated with the presently available data

inhomogeneities near the core mantle boundary evidenced from scattered waves a review statistical properties of small scale inhomogeneities wavelengths between 20 and 70 km near the core mantle boundary are inferred from scattered core waves observations of scattered core waves at large seismic arrays and worldwide networks indicate that the inhomogeneities have a global nature with similar characteristics however there may exist a few regions having markedly stronger or weaker strengths scattering by volumetric inhomogeneities of about 1 inp wave velocity in the lower mantle or by about 300 m of topographic relief of the core mantle boundary can explain the observations at present it is not possible to rule out either of these two alternatives or a combination of both

finite difference seismograms for laterally varying marine models summary the method of finite differences is applied to the elastic wave equation to generate synthetic seismograms for laterally varying seafloor structures the results are compared with borehole seismic data from the gulf of california deep sea drilling project site 485 in which lines were shot over flat and rough topography the significant new phenomenon observed in both the synthetic seismograms and the field data is the generation of a double head wave due to the interaction of the incident wavefront with the side of a hill and the flat seafoor adjacent to the hill in these models the hills are on the order of a seismic wavelength in height and steep velocity gradients occur over distances comparable to wavelengths ray theoretical methods would not be suitable for studying such structures true amplitude record sections are obtained by the finite difference method which show for these models that the head wave generated at the flat seafloor adjacent to the hill is lower in amplitude than if the hill were not present and is lower in amplitude than the head wave generated at the hill a second feature which is important for borehole receivers is the existence of the direct wave root in the upper basement this energy occurs below the sharp interface when the direct wave impinges on the interface from above there is no corresponding snell s law ray path for this energy and the energy is evanescent with depth in the lower medium the properties of both the double head wave and the direct wave root are clearly demonstrated in the finite difference snapshot displays

aragonite relic preservation in jurassic calcite replaced bivalves shells of the aragonite bivalve neomiodon great estuarine group jurassic scotland replaced by coarse neomorphic calcite contain oriented relics of the original aragonite ultrastructure the presence of these relics in such old altered shells as well as the high sr content of the replacement calcite indicate that the process of calcite replacement of aragonite is not a cumulative slow process involving repeated alteration events but rather a rapid one step process aragonite relics once encased in neomorphic spar will survive as unequivocal evidence of original aragonite mineralogy barring total remobilization of the enclosing stable calcite a generally unlikely event the retention of this residual aragonite and high sr calcite supports recent isotopic studies which suggest that the multiple phases of alteration recrystallization invoked in earlier literature are unlikely events in the diagenesis of most undolomitized limestones retention of aragonite relics appears to be independent of whether alteration occurs in shallow meteoric or as in the case of our neomiodon material deeper burial environments pseudopleochroism of the replaced neomiodon shells appears to be due to organic largely graphitic relics not to the aragonite relics

seismic wavefield calculation for laterally heterogeneous whole earth models using the pseudospectral method summary a method for simulating seismic wave propagation in a laterally heterogeneous whole earth model is presented by solving the elastodynamic equations in 2 d cylindrical coordinates using the pseudospectral method psm the psm is an attractive timedomain technique that uses the fast fourier transform for an accurate diierentiation of eld variables in the equations since no dispersion error arises in fourier diierentiation even when using a large grid spacing computer memoryand time are reduced by several orders of magnitude compared to traditional nite diierence methods in order to examine body wave phases with current computing resources a slice through the sphere is approximated with a 2 d cylindrical model an irregular grid spacing is used in the vertical coordinate to improve the treatment of the various structural boundaries appearing in the earth model by matching the heterogeneity in the model synthetic seismograms obtained by the psm calculation are compared with those calculated from an exact simulation method for a spherically homogeneous 1 d earth model and achieve good agreement the psm method is illustrated by constructing the seismic p sv wave eld for strongly heterogeneous earth models including a shield structure near the free surface and velocity perturbations just above the core mantle boundary the visualization of the evolution of seismic p and s waves in time and space is tracked using a sequence of snapshots and synthetic seismograms these displays allow direct insight into the nature of the complex seismic wave behaviour in the earth s interior

evidence of a laterally variable lower mantle structure from p and s waves summary examples of grafenberg array data showing anomalous p waves which typically arrive 3 5s after the direct p wave and which have a slowness 0 7 0 8 sdeg smaller than direct p are presented this additional phase is most frequently observed for events located in the ne portion of the southern kurile island subduction zone 73 80 from grafenberg but systematically disappears for events in the sw portion of this zone because of the magnitude of the slowness difference these observations cannot be attributed to a complex source rupture process nor to multipathing through the descending slab likewise they may not be accounted for by near receiver structure because these phases are not seen for all kurile events if present they appear at all stations of the array but they follow direct p too closely to be a multiple from the moho therefore we conclude they are very likely caused by lower mantle velocity structure the most likely explanation is the presence of a p velocity jump of about 3 per cent approximately 290 km above the core mantle boundary since such a reflector in the lowermost mantle not only gives a good fit of traveltimes and slowness but is also able to model the waveform and the amplitudes of this additional p phase the distribution of bounce points on this reflector for the kurile events indicates a lateral extension of this velocity anomaly under northern siberia of about 150 km by at least 200 km the best fitting s wave model has a reflector in the same depth but the velocity contrast seems to be only about 2 per cent suggesting a different behaviour of the p and s velocity in d few events from other regions in this distance range are suitable for a definitive analysis of this kind from among this group some observations indicate a lower mantle anomaly under the lomonosow ridge and under northern greenland but since the lower mantle under western siberia northern novaya zemlya the azores islands region and the ussr afghanistan border region does not produce an additional phase in the grafenberg recordings it is very unlikely that such a velocity anomaly in the lowermost mantle is a global feature

elastic reverse time migration elastic prestack reverse time finite difference migration of two component seismic surface data requires data extrapolation and application of an imaging condition data extrapolation involves synchronous driving of the vertical component and horizontal component finite difference meshes with the time reverse of the recorded vertical and horizontal traces respectively extrapolation uses the coupled elastic wave equation for variable velocity solved with a second order explicit finite difference scheme the imaging condition at any point in the grid is the one way traveltime from the source to that point elastic migrations of both synthetic test data and real two component common source gathers produce simpler images than acoustic migrations because of the coalescing of double reflections compressional waves and shear waves into single loci

seismology of the lower mantle and core mantle boundary the thermal and chemical processes operating near the core mantle boundary cmb play a major role in the planet s evolution influencing the magnetic field behavior chemical cycling in the mantle irregularities in the rotation and gravitation of the planet and the configuration of mantle convection the past decade has witnessed extensive investigation of the core mantle transition zone using seismological geodynamical geomagnetic and mineral physics experimental techniques the many multidisciplinary contributions on this region have been surveyed in several recent review articles e g loper and lay 1994 jeanloz and lay 1993 bloxham and jackson 1991 which provide a comprehensive survey of the recent literature i confine my attention here to recent seismological evidence for laterally heterogeneous structure in the d region the lowermost 200 km of the mantle while seismic models for d are still low resolution we have attained general constraints on the extent of heterogeneity which raise provocative questions as to the origins of the complex structures the idea that chemical and thermal heterogeneity in d is a manifestation of boundary layer processes is a long standing one bullen 1949 but we are far short of quantifying this notion in particular it is not known whether heterogeneities in d are relics of subducted slabs in situ accumulations of core mantle chemical reaction products primordial residuum from the core formation process or thermal heterogeneities within a hot unstable thermal boundary layer

magnesian calcite cements and their diagenesis dissolution and dolomitization mururoa atoll the most ubiquitous syn sedimentary cements affecting mururoa atoll are composed of magnesian calcite three main types are distinguished fibrous bladed and sparitic on the basis of petrography morphology and mgco3 concentration of the constituting crystals while peloid infills a particular form of hmc chemical precipitation also exist petrographic evidence and isotopic signatures are compatible with marine precipitation mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of mg calcite cements these alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology the first stage of the partial dissolution of mg calcite induces the development of chalky white friable zones within the initially crystalline hard cement layers at ultrascale this is due to the creation of micro voids along the elongate cement fibres advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and or between superposed cement layers mg calcite dissolution is related to meteoric diagenesis during periods of quaternary exposure the creation of voids within mg calcite layers is due to the mechanical removal of previously altered calcium carbonate a process suggesting marine or non marine water flow probably in the vadose environment selective dolomitization of mururoa cements involves alternations of calcite and dolomite which form successive cement like rinds within primary cavities at mururoa these alternations are the result of selective dolomitization of the pre existing mg calcite cements rather than successive precipitation of calcite and dolomite selective dolomitization of mg calcite cements at mururoa indicates that a given cement succession is not necessarily a simple chronological sequence oxygen isotope values of dolomites are enriched in 186 by about 3 pdb within calcite dolomite pseudo alternations the dolomitizing fluid at mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization

diagenesis of quaternary bahamian beachrock petrographic and isotopic evidence abstract carbonate beachrock is a common feature on many beaches of san salvador island bahamas these well indurated slabs of cemented beach sand are jointed normal and parallel to the shoreline and slope gently seaward constituent sediment grains are identical to those of adjacent unlithified beach sands beachrock exposed along the northern western and southern coastline of the island is commonly characterized by aragonite needle cement micritized grains and micritic envelopes which are features of early marine phreatic diagenesis these beachrocks have whole rock 13c values of 4 75 pdb and 18o values of 0 75 pdb which are identical to carbon and oxygen isotopic compositions of isolated aragonite needle cement and unlithified beach sand inclusion of recent artifacts in the rock indicates that these deposits are holocene beachrock from one locality on the western side of the island is covered with a calcrete crust indicating prolonged subaerial exposure and vadose weathering associated with the formation of organic rich soils petrographic features include low mg calcite cement evidence of neomorphism of aragonitic cement and micritic rims to low mg calcite random needle fiber calcite calcified filaments and micritic networks whole rock 13c values of these beachrocks range from 3 66 to 6 85 and 18o from 2 16 to 3 68 the wide range of 13c values in these samples is probably a function of the relative proportions of meteoric vadose to meteoric and or marine phreatic cements in individual samples isolated meteoric phreatic cements are only slightly enriched in the light isotopes relative to marine cements whereas meteoric vadose cements are significantly lighter the diagenetic features suggest the beachrock passed through marine phreatic to meteoric phreatic and meteoric vadose diagenetic environments this diagenetic sequence commonly forms in response to sea level lowering evidence of deposition in a shoreline environment and prolonged residence in meteoric phreatic and meteoric vadose environments suggests that these beachrocks may be late pleistocene

new insight into a subduction related orogen a reappraisal of the geotectonic framework and evolution of the japanese islands abstract the geotectonic framework and the evolutionary history of the japanese islands need revision in accordance with the various geophysical geological evidence gathered by new methodologies in the recent years including seismic tomography vibroseis ground breaking seismic experiments and detrital zircon chronology these investigations have addressed various themes such as 1 seismic profile of the crust and mantle beneath the japanese islands 2 high precision ages of the protoliths of high p t metamorphic rocks and 3 provenance of terrigenous clastics the results have led to a number of important findings including 1 detection of a large mass of slab around the mantle boundary layer suggesting the long term oceanic subduction beneath japan 2 confirmation of the subhorizontal piled nappe structure for the entire crust of japan 3 finding a new high p t metamorphosed accretionary complex unit that represents the youngest blueschist in japan 4 finding of the oldest early cambrian arc batholith and cover sediments and 5 the identification of plural arc batholiths which have already been erased from the surface based on a synthesis of these new data this article presents a re evaluation of the conventional geotectonic subdivision of the subduction related orogen in japan re definition of the elements and their mutual boundaries and re consideration of the geotectonic evolution of the japanese islands in particular the historical change in provenance suggests that proto japan has experienced large scale tectonic erosion in multiple stages and the corresponding large amounts of continental crust materials were subducted for understanding the orogenic growth of japan during the last ca 500 million years the significance of tectonic erosion coupled with continental contraction as well as the oceanward accretionary growth requires further attention

finite difference synthetic acoustic logs the finite difference method is a powerful technique for studying the propagation of elastic waves in boreholes even for the simple case of an open borehole with vertical homogeneity the snapshot format of the method displays clearly the interaction between the borehole and the rock and the origin and evolution of phases we present an outline of the finite difference method applied to the acoustic logging problem including a boundary condition formulation for liquid solid cylindrical interfaces which is correct to second order in the space increments absorbing boundaries based on the formulations of reynolds 1978 and clayton and engquist 1977 were used to reduce reflections from the grid boundaries results for a vertically homogeneous sharp interface model are compared with the discrete wavenumber method and excellent agreement is obtained the technique is also demonstrated by considering sharp and continuous transitions damaged zones at the borehole wall and by considering the effects of wa

south kitakami region an allochthonous terrane in japan the pre cretaceous geology of the japanese islands may be interpreted as an accretion complex in which many terranes of several origins are juxtaposed and a number of allochthonous blocks are involved the south kitakami and abukuma regions of northeast japan and probably also silurian and devonian blocks in the kurosegawa fault zone and the border fault zone of the hida metamorphic terrane are the allochthonous blocks the stratigraphic sequence and paleobiofacies of the regions are similar to each other and their lithologic characters are of continental affinity but however are quite different from those of the other terranes having oceanic characters the south kitakami and abukuma regions have been fragments of a lost continent less remotely situated to gondwana during the middle paleozoic and may have drifted and incorporated with the other part of the japanese islands before late mesozoic to early tertiary igneous activity

a sharp and flat section of the core mantle boundary the transition zone between the earth s core and mantle plays an important role as a boundary layer for mantle and core convection1 this zone conducts a large amount of heat from the core to the mantle and contains at least one thermal boundary layer2 3 the proximity of reactive silicates and molten iron leads to the possibility of zones of intermediate composition4 here we investigate one region of the core mantle boundary using seismic waves that are converted from shear to compressional waves by reflection at the boundary the use of this phase known as scp the large number of receiving stations and the large aperture of our array all provide higher resolution than has previously been possible5 7 for the 350 km long section of the core mantle boundary under the northeast pacific sampled by the reflections the local boundary topography has an amplitude of less than 500 m no sharp radial gradients exist in the 400 km above the boundary and the mantle to core transition occurs over less than 1 km the simplicity of the structure near and above the core mantle boundary argues against chemical heterogeneity at the base of the mantle in this location

aminostratigraphy and ages of pleistocene limestones of bermuda the pleistocene stratigraphy of bermuda offers important information for sea level history corals are rare in bermuda pleistocene carbonates and so many deposits are not appropriate for u series dating we have extended the geochronological base in bermuda by determining the extent of isoleucine epimerization a i ratios in 1 marine shells glycymeris and others from deposits of sea level high stands 2 land snails poecilozonites from protosols associated with the eolianites that comprise the bulk of bermuda and 3 whole rock samples of bioclastic calcarenites the three distinct types of samples provide independent aminostratigraphies aminozones c e f g h j and k that are mutually consistent and in agreement with the mapped lithostratigraphy correlation of the amino acid ratios with ages based on previously published u series coral dates verifies the utility of the apparent parabolic kinetics model for estimation of amino acid racemization ages extrapolation of ages based on the model to deposits beyond the range of u series dating reveals that a significant middle pleistocene section is present in bermuda

cementation patterns and genetic implications of holocene beachrocks from northeastern brazil abstract this paper describes the complex patterns of diagenesis in holocene beachrock ledges located along both the eastern and northern coasts of the state of rio grande do norte northeastern brazil these beachrocks show highly variable diagenetic processes fabrics and textures carbonate cements are exclusively composed by high mg calcite and occur in seven main morphologies 1 cryptocrystalline coatings 2 isopachous prismatic rims 3 equant spar 4 cryptocrystalline or micritic pore filling 5 pseudo peloidal aggregates 6 radial aggregates 7 isolated and randomly oriented clumps of scalenohedral crystals other diagenetic features include a carbonate siliciclastic mixed sediment b infiltrated micrite c authigenic smectitic clay minerals d organic films e chalcedony pore filling cement f vadose silty internal sediment the texture and composition of the cements are indicative of precipitation in the active marine phreatic zone the presence of vadose features and planktonic foraminifers within the infiltrated materials suggest that they were introduced in the backshore sands by muddy waters dislocated from the shelf during storms a general succession of diagenetic phases can be recognized comprising the precipitation of authigenic smectite micritic coatings radial aggregates isopachous rims of prismatic crystals or equant spar cryptocrystalline and pseudo peloidal pore filling cements and vadose infiltration of micritic carbonate siliciclastic mixed sediments or silty materials the absence of organic structures such as microbial bacteria or fungi filaments and bodies within the cements suggests that the mechanism behind beachrock cementation is essentially inorganic most probably the evaporation of entrapped seawater in response to the dry climatic conditions

p and s wave reflections from anomalies in the lowermost mantle summary seismic p and s waves recorded at the grf array in germany are used to study the inhomogeneous structure of the boundary layer d at the base of the mantle the use of the seismic array allows the detection of small scale anomalies in the lowermost mantle the lateral resolution attainable is about 100 to 400 km i e more than 10 times better than with tomographic methods the analysis of 13 years of grf broad band array data yields 255 events with high signal to noise ratio which are used to map the lowermost mantle 74 of these events show anomalous p waves pdp which arrive 3 6 s after the direct p wave and have a slowness 0 7 0 8 s smaller than the slowness of direct p other events close to the events with pdp do not have such an anomalous phase using slowness traveltime amplitude and waveform information it is demonstrated that pdp is caused by an anomalous lower mantle velocity structure below the turning point of the p wave if the pp phase i e a p wave first reflected at the free surface near the source is used together with the p phase distinct and well sepaarted p velocity anomalies can be determined under the nansen basin the kara sea and northern siberia the areas of the bounce points of pdp in the lower mantle have a lateral extension of about 100 by 200 km but this is not the size of the anomaly since the resolution of p waves at 1 hz in this depth is 130 km by 260 km fresnel zone the accuracy to which the depth of the reflector 2612 km under the nansen basin and 2605 km under northern siberia respectively can be determined for 1 d models is 10 to 20 km the velocity contrast at the lower mantle discontinuity is about 3 per cent 1 to 1 5 per cent areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas 2 d models of the anomalies reveal the range of adequate models and possible trade offs if the lateral extension of the anomaly is about 7 the reflector has to be 40 km deeper than in the 1 d model if the dip of the reflector in the lowermost mantle is only about 1 5 it is difficult to resolve if the reflector is tilted towards the source or towards the receiver for the anomaly under the nansen basin deviations from the great circle path are observed for pdp indicating 3 d effects below the three anomalies the core mantle boundary cmb can be located by pcp with a depth in agreement with standard earth models under northern siberia has a region where p and s velocity anomalies coincide but also a region where only an s velocity anomaly but no p velocity anomaly is observed this results in changes of the poisson ratio from 5 9 per cent to 4 8 per cent 1 5 per cent across the discontinuity in the lowermost mantle for the regions studied the analysis of the s waves s sds and scs reveals two s velocity anomalies in the lowermost mantle under northern siberia the first anomaly coincides with the p velocity anomaly under northern siberia and can be explained by a 1 d model with a reflector depth of 2610 km 15 km and a velocity contrast of 2 3 per cent the second s velocity anomaly is in an area where no p velocity anomaly can be detected the corresponding 1 d s velocity model has a reflector depth of 2575 km 15 km and an s velocity contrast of 2 6 per cent the smallest structures that can be resolved with the s waves in this depth are about twice as large as for the p waves i e 230 km by 460 km fresnel zone the joint analysis of p and s waves therefore shows a region with a p velocity anomaly together with weak indications for an s velocity anomaly nansen basin and a second region with a p but no s anomaly kara sea the lowermost mantle

quasi cylindrical 2 5d wave modeling for large scale seismic surveys 1 the most common form of seismic exploration remains a nearly linear survey with data acquisition lines including the source and receivers the interpretation of amplitude and waveform information for such linear acquisition requires consideration of 3d seismic wavefields in many scenarios the structure is approximately 2d but still modeling is needed for point sources in recent years 2 5d modeling methods have been developed for the simulation of 3d seismic wavefields in media varying in two dimensions although the computer memory requirements are only slightly larger than those for 2d computations the computation times are too long for ready application to actual surveys to overcome this problem we propose a new approach for modeling 2 5d seismic wavefields using a quasi cylindrical representation we show an implementation of this approach using the finite difference method and demonstrate the validity and efficiency of the technique with numerical examples including a profile across a realistic model of subduction zone structure

geomagnetism earth rotation and the electrical conductivity of the lower mantle abstract a new approximation is derived for the electromagnetic torque acting on an electrically conducting mantle as a result of fluid flow in the core the torque considered is that associated with the toroidal field induced in the mantle by advection of poloidal field which is assumed to be frozen into an infinitely conducting liquid core by the flow of liquid at the top of the core the expression relies on the assumption that the mantle is an insulator apart from a thin with respect to the core radius layer of finite conductance adjacent to the core mantle boundary this allows the toroidal field scalar in the mantle to be expressed as a first order taylor approximation the time dependent torque calculated at a sequence of epochs this century is compared with the torque which has previously been inferred from astronomical observations of the length of day although the initial results appear unpromising a significant correlation exists when poorly determined components of the velocity field which contribute substantially to variations in the calculated torque are ignored by regression analysis the conductance of the assumed thin layer is determined as 6 7 108 s and the lag of the electromagnetic torque behind the astronomical torque as 6 years which is interpreted as the delay time for electromagnetic signals through the mantle finally the implications of these results for the conductivity of the lower mantle are discussed they imply that the bottom few hundred kilometres of the mantle probably have a conductivity of a few hundred to a few thousand siemens per metre previously published lower mantle conductivity models are examined in light of this conclusion the close correlation of the variations in the calculated electromagnetic torque which depend primarily on fluctuations in the core velocity field with the length of day torque provides independent evidence that core flow is not steady on the decade time scale

geoacoustic scattering from seafloor features in the rose area a strong refraction branch diffraction has been observed on ocean bottom hydrophone data from the rivera ocean seismic experiment rose modeling of the interaction between seismic acoustic energy and the ocean bottom has shown that this arrival could be caused by scattering from common seafloor features seismic acoustic wave propagation was computed for models of marine seismic refraction lines using a 2d heterogeneous formulation of the elastic wave equation these models demonstrate that a significant amount of energy is diffracted from seafloor structures such as hills and valleys when both the direct water wave and the refracted compressional wave traveling through the upper crust are incident upon a structure since much of the diffracted energy travels through the crust the diffracted arrivals appear on the refraction branch of the seismograms at large ranges energy partitioning at the seafloor of the two incident wave types produce both compressional p and shear s diffracted phases in

a review of finite difference methods for seismo acoustics problems at the seafloor understanding seismic wave propagation in realistic seafloor environments is essential for many problems in marine seismology finite difference methods are becoming increasingly popular in solving propagation problems as the limitations of other techniques which apply only at high frequency or for flat lying media become fully appreciated the seafloor problem a high contrast in poisson s ratio at a rough sharp interface is particularly challenging and many published formulations fail to solve it accurately the purpose of this paper is to summarize the published finite difference formulations for the elastic wave equation and to outline their applicability to seafloor problems

body wave traveltimes and a spherically symmetric p and s wave velocity model summary the derivation of a spherically symmetric reference earth model is a subject which has recently regained interest after a decade of studies on lateral heterogeneity we now have the tools to approach the problem an iaspei working group on traveltime tables was formed in 1988 with the goal of deriving a reliable and up to date radial model of seismic velocities following the guidelines developed by that working group we have investigated the body wave traveltimes for several branches frequently reported in the isc bulletins and propose here a model for p and s wave velocities consistent with these data approximately 16000 best recorded shallow focus earthquakes are selected from 24 years of isc bulletins 1964 1987 and relocated using among others corrections for lateral heterogeneity from this dataset we derive summary traveltime curves specified for each degree of epicentral distance for p pcp p pkpab pkpbc pkikp and s to constrain velocity at the top of the outer core we include data for sks and skks sks from hales roberts 1970 1971 our discrete time distance curves are obtained by evaluating the shape of the residual density distribution independently for each 1 interval to limit contamination due to the interference of other branches precursors or late readings in the upper mantle the resulting model sp6 differs from iasp91 kennett engdahl 1991 only by slightly higher p and s velocities between 410 and 660 km in the lower mantle new velocity profiles have lower velocity gradients this is also true in comparison with prem dziewonski anderson 1981 at the top of the outer core vp is smaller than in prem and the velocity jump at the inner core boundary is reduced to the value of 0 62 km s 1 the new inner core radius is 1215 km our model predicts traveltimes that differ from the iasp91 tables considerably although generally less than 1 s except for sks and skks for which the differences are greater

constraints on the core mantle boundary topography from p4kp pcp differential travel times 1 p4kp pcp differential travel times are examined to infer the core mantle boundary cmb topography a total of 362 p4kp pcp times are measured with a measurement error of 0 5 s the travel times are corrected for the earth s hydrostatic ellipticity and mantle heterogeneity using a p wave tomographic model spherical harmonic expansion up to degree 4 is adopted for model parameterization the p wave velocity heterogeneity is then estimated in the lowermost 150 km of the mantle to overcome the problem of underestimation of the velocity perturbation at the base of the mantle in the global 3 d p wave mantle model subsequently the cmb topography is inferred using the residues of the above processes since the odd degree components of the cmb topography are insensitive to the p4kp pcp times only the components of degrees 2 and 4 are solved for the resultant features indicate that the maximum amplitude of the cmb topography does not exceed 2 km with an uncertainty of less than 0 5 km a numerical test confirms that the pattern of degree 4 is more reliable with less amplitude recovery the obtained degree 4 pattern shows an amplitude of less than 1 km and indicates the presence of depressions under the circum pacific the central pacific and south africa

history of stage 5 sea level in bermuda review with new evidence of a brief rise to present sea level during substage 5a abstract oxygen isotope substage 5e is represented by the devonshire formation in bermuda the occurrence of these coastal marine deposits to about 5 m indicates pleistocene sea level data in bermuda do not need tectonic corrections a few published u series coral dates suggest that a conglomerate spencer s point formation may have formed during substage 5c geologic mapping however indicates that the eolianite beneath the conglomerate is pre rather than post devonshire formation in age as was previously thought and the aminostratigraphy of marine sheels also confirms the correlation of the spencer s point and devonshire formations substages 5c and 5a are represented by the southampton formation large eolianites comprise the bulk of this formation they are onlapped by a few small younger marine 1 m and eolian deposits aminostratigraphy confirms that these late southampton age deposits are significantly younger than the devonshire formation and u series dates indicate a substage 5a age the setting of some of these deposits along a protected shoreline precludes discounting their position as anomalous due to large storms this evidence that sea level in bermuda was at about present datum late in substage 5a agrees with interpretations of neotyrrhenian deposits of mediterranean shorelines but not with inferences from the deep sea oxygen isotope record or with data from uplifted coral reefs

estimation of fracture height using microseismicity associated with hydraulic fracturing the ratio of horizontal to vertical h v particle velocity in background microseismic radiation associated with hydraulic fracturing is substantially higher in the dilatant low velocity fractured zone than it is outside this provides a useful diagnostic for determining the height of the fractured zone numerical synthesis of guided wave phenomena within the low velocity fractured zone accounts for much of the observed behavior but measured h v patterns are not totally consistent with either pure tensile or pure shear sources a composite model containing both tensile compressional sources and asperity shear failures appears to satisfy the main observations better than either source type does alone this composite is consistent with current models of earthquake aftershock sequences which also have different mechanisms at the edges and in the interior of a fracture zone tensile and shear respectively the h v phenomenon is consistent with a predominance of energy with shear wave polarization traveli

aspherical structure of the core mantle boundary from pkp travel times maps of aspherical structure in the vicinity of the core mantle boundary have been constructed from a tomographic analysis of p df and p ab travel times in the distance range 150 180 these maps reveal long wavelength features not previously detected by mantle tomography the peak to peak amplitude of these features is on the order of 1 s in vertical travel time too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core mantle boundary we hypothesize the existence of one or more chemical boundary layers whose large scale accumulations on the surface of the core may in some ways be analogous to continents at the earth s surface

anomalous difference traveltimes and amplitude ratios of sks and skks from tonga fiji events difference traveltimes skks sks for 12 tonga fiji events observed from 96 117 at north and south american wwssn and csn stations exceed the difference times of standard earth model prem by 2 3 s also the amplitude ratios sks skks at american stations from 96 107 are lower than the prem ratios no such anomalies occur in directions from tonga fiji towards eurasia and africa and for a sea of japan event and paths to north america and africa the regional variations in the data point to pronounced lateral variations in earth structure the anomalous difference times imply either an advance of sks or a delay of skks relative to prem the interpretation of the anomalies is not unique our preferred explanation involves s velocity anomalies in the lower mantle on the source side east of tonga fiji

on the structure of the lowermost mantle beneath the southwest pacific southeast asia and australasia abstract the region of the lowermost mantle beneath the southwest pacific australasia and southeast asia 50 s 50 n and 80 190 e has been studied using a wide variety of seismic techniques we complement these studies with results obtained from long period global digital seismograph network gdsn data using a recently developed phase stripping technique that permits the isolation of d reflections from the stronger neighbouring s and scs signals we identify patches with d reflections and areas where we cannot confidently determine the presence or absence of a d reflection a synthesis of our results with other studies suggests that d varies dramatically through the region generally thickening from 100 150 km in a central zone to about 300 km at the eastern and western margins however there are irregularities in this overall pattern including areas where there seems to be little evidence for a d discontinuity inspection of waveform amplitudes shows considerable scatter in not only the d reflected phases but also core mantle boundary reflected phases experiments with synthetic seismograms for a variety of d models and the observed lateral variability through the region suggest that this is to be expected furthermore ray theory calculations for large scale 3d earth models predict significant variations 30 in s wave amplitudes of lowermost mantle turning rays finally we investigate possible correlations between lower mantle velocity flow and d thickness we find some correlation between d thickness and lower mantle velocities obtained from tomographic inversions with a thin d layer in high velocity regions and a thickening of the layer toward slower regions the relationship between predicted lower mantle flow and d thickness is less clear these results are qualitatively consistent with thermal boundary layer predictions for d but do not preclude the possibility of compositionally distinct material in the layer

observations of pkikp pcp amplitude ratios and implications for earth structure at the boundaries of the liquid core 1 mining of the database produced by seismic array stations of the international monitoring system has resulted in hundreds of new observations of precritical reflections from earth s inner core here we present short period amplitude ratios between these pkikp phases and corresponding reflections pcp from the core mantle boundary cmb pkikp pcp amplitude ratios are one of the few seismic observations that directly constrain the change in s velocity and density across the inner core outer core boundary we measure the amplitude ratios from optimally tuned array beams and use a bootstrap technique to estimate observational uncertainties we also use a model resampling technique to estimate uncertainties associated with the background reference model the optimal models have s velocities at the top of the inner core less than about 2 5 km s and density jumps across the inner core outer core boundary of less than about 0 45 g cm3 these values can be reconciled with the higher estimates from normal mode constraints if strong radial gradients are present in the top of the inner core or if the outermost inner core has material properties distinct from the bulk of the inner core we also find evidence for an ultralow velocity zone in the lowermost mantle beneath the eastern coast of australia a unique best fitting model cannot be determined but an example that is consistent with the data has a reduction in p velocity of 10 a reduction in s velocity of 35 and an increase in density of 20 this type of structure could be explained by the presence of partial melt and increased iron content just above the cmb

3 d viscoelastic anisotropic modeling of data from a multicomponent multiazimuth seismic experiment in northeast texas three component seismic data were acquired along six azimuths at a site in northeast texas sources were inclined impacts combined to produce equivalent vertical and crossline horizontal components the data were analyzed by iterative modeling using a new 3 d staggered grid finite differencing scheme the 3 d model is defined in terms of spatially variable anisotropic viscoelastic parameters p velocity s velocity density qp and qs and converted to elastic tensor components and relaxation times for computation simultaneous fitting of amplitudes of near surface reverberations surface waves converted waves and deeper reflections gives an anisotropic viscoelastic model that provides constraints on earth properties the assumption of transverse isotropy in previous experiments is supported by the results of this experiment the final model consists of a stack of flat layers each of which has isotropic p velocity qp and qs the wills point and kincaid shales are strongly transversely isotropic

small scale changes of core mantle boundary reflectivity studied using core reflected pcp abstract short period pcp phases from earthquakes in the western aleutians recorded at the canadian yellowknife array yka sample the core mantle boundary cmb beneath the alaskan kenai peninsula and the cook inlet the pcp to p amplitude ratios for these events show large variability with some amplitude ratios more than an order of magnitude larger than predicted by radial earth models such as iasp91 or prem the amplitude ratios vary laterally with pcp core mantle boundary reflection points showing a 1 region with exceptionally large amplitude ratios it is likely that the variability in relative pcp amplitudes is due in large part to cmb topography other mechanisms such as radiation pattern mantle attenuation focusing and defocusing and core mantle boundary velocity and density perturbations could cause variations on scale lengths of less than a degree as found in this study but are unable to produce variations as large as those we observe

inference of core mantle boundary topography from isc pcp and pkp traveltimes we invert isc pcp and pkp absolute and differential traveltimes in an attempt to infer the long wavelength topography of the core mantle boundary cmb the data selection and processing methods are described and evaluated these travel time data are very noisy and the geographic distribution of the data is highly non uniform inhibiting reliable inference of cmb topography spatial averaging enhances the coherent component of the residual variance related to heterogeneity however the random component of the variance is much larger than the coherent component we show that for pcp data the coherent signal due to mantle heterogeneity overshadows that arising from the cmb and that the effects of mantle heterogeneity are mapped into our inferred cmb solutions the pcp data are not correlated across the spatial averaging bins and seem to have a strong bias due to small scale structure and or noise the non uniform geographic sampling of the data plays a role in the mapping of mantle heterogeneity onto the cmb spatial patterns of cmb models inferred from different phases do not agree amplitudes of seismically inferred cmb undulations vary greatly the sensitivity of inferred cmb models to the processing spatial averaging procedure and inversion techniques are investigated topographic amplitudes increase strongly with increasing input residual variance the power spectrum of inferred topography indicates that there are unmodelled heterogeneities that must be described with spherical harmonics of degree 6 and higher based on this work we conclude that reliable inference of long wavelength cmb topography is not likely with the current isc data set or with a spherical harmonic expansion truncated to degree and order 6

a lower mantle s wave triplication and the shear velocity structure of d summary a lower mantle s wave triplication detected with short and longperiod wwssn and csn recordings indicates a substantial shear velocity discontinuity near 280 km above the core mantle boundary the triplication can be observed in rotated sh seismograms from intermediate and deep focus events throughout the distance range from 70 to 95 three distinct source region receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity modelling of the observations with synthetic seismograms produced with the cagniard de hoop and reflectivity methods constrains the shear velocity increase to be 2 75 k 0 25 per cent comparable to upper mantle discontinuities short period observations indicate that the velocity increase may be a sharp first order discontinuity or may extend over a transition zone no more than 50 km thick the shear velocity gradient below the discontinuity within the d layer is not well constrained by the sh data but slightly positive or near zero velocity gradients are consistent with the long period amplitude ratios of scsh sh

beachrock occurrence characteristics formation mechanisms and impacts beachrocks are hard coastal sedimentary formations consisting of various beach sediments lithified through the precipitation of carbonate cements the objectives of this contribution are to a collate and review information on the reported occurrences characteristics and formation mechanisms of beachrocks and b consider their impacts on the coastal zone the analysis of the available information has shown that a beachrock formation is a global and diachronic phenomenon and b the great majority of beachrocks are found in tropical subtropical and low temperate latitude microtidal coasts the cementing agents of beachrocks are composed predominantly of the metastable carbonate phases high magnesian calcite hmc and aragonite ar appearing in a diverse crystalline morphology it has been suggested that cement precipitation in the coastal environment is controlled by i the physicochemical conditions ii the presence of organic compounds and microbes iii the magnitude and distribution of the wave energy along the coast and iv the textural characteristics of the constituent sediments various theories have been proposed to explain beachrock formation itself linking the phenomenon to either physicochemical or biological processes these theories however do not seem to be of universal validity and acceptance as each is able to explain only some of the reported occurrences the presence of beachrocks appears to affect beach morphodynamics by i locking the beach profile ii modifying the nearshore hydrodynamics iii changing the porous character of the beach and thus its response to wave forcing and iv differential bed erosion at the margins of the beachrock outcrops that can alter significantly the long and particularly the cross shore sediment transport therefore although relict submerged beachrock outcrops may provide some coastal protection by reducing the wave energy impinging onto the coastline modern beachrocks may promote offshore loss of unconsolidated beach sediments and buried beachrock outcropping finally the presence of beachrocks may have also significant ecological impacts as the indigenous mobile substrate fauna and flora of the beach is replaced by hard substrate benthic assemblages which are commonly arranged in hydrodynamically controlled zones

evidence for a p wave velocity anomaly in d a number of strong p coda phases have been observed on european wwssn station short period recordings of intermediate magnitude kurile arc earthquakes which occurred at several different depths of focus these phases have been observed for several different events and cannot be explained by known phases such as pcp or source multiplicity a lower mantle model containing a first order discontinuity with a 2 75 p wave velocity contrast and located about 344 km above the core mantle boundary produces synthetic seismograms which approximately reproduce the observed amplitudes and arrival times of the coda phases however the fact that these phases are not always observed and that their amplitudes vary significantly for different events and stations suggests that the discontinuity at the top of d may be a transition zone and may be laterally variable in thickness and contrast

simulation of long period 3 d elastic responses for whole earth models summary improvements in gdsn seismic data quality and advances in numerical algorithms for solution of the wave equation now make it useful and feasible to simulate long period elastic responses for 3 d whole earth models the algorithm used is eighth order finite differencing on a staggered grid this allows stable computation at boundaries with large contrasts in poisson s ratio such as the core mantle boundary as the solution automatically generates all wave types it may lead to prediction of previously unobserved or misinterpreted phases and allow more detailed analysis of all phases simulations of large e g nuclear explosions and double couple earthquake sources reveal the effects on relative amplitudes of their respective radiation patterns in both wavefield snapshots and seismogram profiles a large number of phases are affected even by a single smooth bump on the core mantle boundary a 3 d whole earth model implicitly contains the travaltimes for all source receiver positions and for any source configuration the corresponding amplitudes of all phases thus 3 d numerical modelling of whole earth responses is a potentially valuable diagnostic and interpretive tool and a basis for globalization in contrast to regionalization of traveltime tables

the p wave velocity structure near the base of the mantle travel times from 16 events recorded at two temporary networks of short period seismometers deployed across australia have been used to study the p wave velocity structure of the mantle below depths of 1700 km the slowness distance curve derived from these times shows small deviations from the slownesses predicted from the preliminary reference earth model of dziewonski and anderson 1981 that may be significant the most important feature is a rapid decrease of more than 0 5 s degree between distances of 85 and 88 velocity filtering of seismograms from 5 nuclear explosions in kazakh recorded at these australian networks indicates the presence of two arrival branches at distances between 83 and 91 whose slownesses differ by 0 40 to 0 55 s degree and which intersect at a distance of 87 processed p wave seismograms from the yellowknife array in canada also indicate the presence of a similar phenomenon in the same distance range in two separate regions the results can be explained by a widespread rapid or sharp increase in velocity of 2 5 to 3 0 about 200 km above the core mantle boundary

reflections from a randomly grooved interface ultrasonic modelling and finite diff erence calculation1 this study utilizes ultrasonic water tank modelling to examine the complexities which can result when seismic energy is diffracted by even a simple 3d interface structure ultrasonic laboratory modelling is carried out in our in house water tank using an aluminum block to simulate very simple ocean bottom topography the topography consists of parallel depressions which simulate valleys running parallel to a mid ocean spreading ridge this study shows that the phase and amplitude of primary ocean bottom reflections can vary strongly with the azimuth of propagation the trailing energy coda also varies dramatically as a function of propagation direction showing the importance of developing a seismic model which includes the full 3d azimuthal distribution of interface structure

cathodoluminescence in quaternary carbonate deposits abstract the cathodoluminescent oscillatory and sectoral growth zones common in crystals formed in ancient limestone successions in a variety of putative environments appear to be rare or absent from recent and pleistocene marine carbonate sequences the factors controlling cathodoluminescence and reasons for this disparity are examined the cathodoluminescent zones in the cements of ancient rocks have been interpreted as responses to variations in the redox potential of formative pore waters during crystal growth although similar cathodoluminescent behaviour is recorded from some deposits including travertines and quaternary speleothems formed in what are thought to have been strongly oxidizing environments the apparent absence of cathodoluminescence in the most recent and pleistocene marine deposits that presumably reflect deposition and diagenesis in environments that are also characteristically oxidized therefore seems anomalous the controlling influences on cathodoluminescence are reviewed together with evidence relating to observations of pleistocene marine deposits and likely conditions of formation but where it is present the mechanism s for its development remain elusive

azimuthal anisotropy in the lithosphere from observations of long period s waves summary the paper gives a detailed description of the technique for measuring azimuthal anisotropy which was suggested earlier by vinnik kosarev makeyeva 1984 and used by kind et ai 1985 the technique is based on the observations of long period converted phases like sks additionally we describe a generalization of this approach which makes use of long period s waves of arbitrary polarization both modifications were applied to records of the grf array in southern germany the results of this analysis if combined with the data on p velocities bamford 1977 suggest that the direction of the fast velocity in the lithosphere of the region varies with depth

models of the core mantle boundary and the travel times of internally reflected core phases topographic maps of the core mantle boundary cmb obtained by inverting different subsets of international seismological centre arrival time data involving either waves refracted through the boundary pkp or waves reflected from the topside pcp do not agree we report on the implications for models of cmb structure when data from bottomside reflections notably pkkp are included in the inversion travel time residuals with respect to a reference earth model prem of all core phases are positively biased but the mean residual is different for different phases and must be accounted for prior to inversion the new data illustrate that models of cmb topography can explain only a small fraction of the data variance the new cmb model involves both topography and a laterally varying boundary layer the cmb itself is smooth compared to previous results

lower mantle inhomogeneities inferred from pcp precursors a search of phases arriving between p and pcp the reflection from the core mantle boundary cmb was performed in the bulletins of the international seismological center bisc the distribution of the turning reflection points of anomalous waves in the lowermost mantle indicates the areas which are possibly anomalous regions the regions exhibiting these anomalies cover only a small fraction of the lowermost mantle sampled the areas which can be associated with anomalous arrivals are the lowermost mantle under the north pole region northern siberia central siberia western mongolia afghanistan northeastern china korea and the north atlantic ridge these are the regions which should be studied in more detail by analyzing the seismograms i e the amplitude and waveform information at the corresponding stations in addition to the travel time information reported to the isc

polarization anisotropy and fine scale structure of the eurasian upper mantle we have observed shear wave splitting with tsh tsv tsv up to 1 5 on long period records of multiply reflected s waves bottoming in the upper mantle beneath the russian and siberian platforms the dispersion of love and rayleigh waves over these paths shows discrepancies of comparable or larger magnitude with respect to smooth isotropic si structures consistent with a model of the uppermost mantle having significant apparent vertical anisotropy although the splitting and dispersion data can be fit by smooth anisotropic sa models we have investigated the apparent anisotropy associated with fine scale rough structure beneath stable eurasia we fit the data with a rough isotropic ri model having an rms shear velocity fluctuation that varies from 14 in the uppermost mantle to zero at 400 km depth these fluctuations are larger than the variation in isotropically averaged parameters expected for even a diverse assemblage of upper mantle ultrabasic rocks which we take to be evidence for some sort of intrinsic local anisotropy

effects of local geologic structure on yucca flats nevada test site explosion waveforms two dimensional linear finite difference simulations abstract two dimensional linear elastic finite difference calculations were performed for a two dimensional geologic model of yucca flats nevada test site nevada the calculations were used to produce synthetic teleseismic p wave seismograms for explosive line sources in yucca flats p wave coda in the first 5 sec is observed to be highly dependent on take off angle for the teleseismic synthetics p wave coda also varies with the position of the source in the valley structure and may produce variations in the individual station teleseismic p wave m b magnitude of up to 0 3 magnitude units however these magnitude variations should be substantially reduced by averaging over stations at multiple azimuths the reverberant coda appears to rise from scattered modal waves that are initially excited in the low velocity near surface structures of the yucca flats deposits of alluvium and tuff scattering of the waves occurs at offsets in the basement structure and at the sides of the valley the combined effects of scattering source function intrinsic attenuation and instrument response serve to obscure the p pp spectral scalloping that is expected from a linear model this loss of spectral resolution is the product of p coda filling in the p pp interference notches and the lengthening of the initial p wave source time function by a convolution of the source time function intrinsic attenuation operator and instrument response therefore short time windows 1 to 2 sec that do not include p coda energy do not have sufficient resolution to reliably detect the p pp interference notches

the marine permian of east and northeast asia an overview of biostratigraphy palaeobiogeography and palaeogeographical implications abstract the permian marine biostratigraphy faunal successions and mutual correlations where possible throughout east and northeast asia are synthesized region by region based on both published literature and the author s field observations in certain parts of the region the correlation of the permian marine successions of ne asia with the permian international timescale and in particular with gondwanan permian marine sequences remains a major challenge due to profound marine provincialism during the permian however by employing biogeographically mixed faunas from east asia se mongolia ne china south primorye of far east russia and the south kitakami terrane of japan as biostratigraphic gateways coupled with some bipolarly and bi temperately shared permian marine taxa and faunas it has been possible to correlate with reasonable confidence some of the high palaeolatitude permian marine rock units and faunas of ne asia with those of the tethyan region and gondwana palaeobiogeographically the permian marine faunas of east and ne asia are assigned to four major provinces verkolyman sino mongolian japanese cathaysian and panthalassan provinces on the basis of their palaeogeographical distribution patterns and characteristics of faunal assemblages of these the sino mongolian japanese province has considerable significance for regional palaeogeographical plate tectonic and palaeoceanographical reconstructions during the middle permian because of its conspicuously mixed cool and warm water marine biota the origin of this biogeographically mixed marine biota is interpreted to have resulted from a combination of some key factors including the increased tectonic convergence between the bureya jiamusi terrane and the sino korean platform during the permian and the intermingling of both warm and cold water ocean currents off the eastern coastal areas of the bureya jiamusi terrane and the sino korean platform during the middle permian

imaging global body wave phases by stacking long period seismograms stacked record sections combining over 5 years of long period global digital seismograph network gdsn seismograms from shallow events show dozens of seismic body wave phases separate images are obtained for the vertical radial and transverse components using an automatic gain control algorithm which normalizes the amplitude of each trace prior to stacking these images provide an indication of the relative signal to noise characteristics of different parts of the gdsn data set core mantle boundary diffracted phases are particularly prominent in these stacks which show pdiff to 150 pkpdiff to 200 ppdiff to 230 sdiff to 160 and ssdiff to 220 high order s surface multiples can be seen extending through the surface wave train up to ranges of 540 two examples of possible upper mantle discontinuity reflected phases are visible one following ppp between about 95 and 125 the other following scs3 between about 210 and 240 an apparent offset between sh and sv arrivals is seen which becomes larger for the higher order s multiples sss ssss etc and appears consistent with models containing transverse isotropy in the upper mantle

evaluation of meteoric calcite cements as a proxy material for mass 47 clumped isotope thermometry abstract meteoric diagenetic cements are ubiquitous throughout geologic history affecting most carbonate exposures worldwide they can often be difficult to interpret as it is usually difficult to separate the influences of water 18o and temperature on isotopic signals contained within the carbonate rock body despite this difficulty in interpretation meteoric phreatic cements can potentially be a useful proxy material as they form slowly in equilibrium at mean annual temperature and are not affected by any biogenic effects that can bias other proxy materials we applied the mass 47 clumped isotope paleothermometer to pleistocene and holocene carbonates from bermuda and barbados in order to investigate the effects of meteoric diagenesis on 47 signals and to determine their suitability as a paleotemperature proxy phreatic calcite cements are found to record the same temperatures as unaltered carbonate sediments while any sample exhibiting vadose characteristics is biased towards unreasonably hot apparent formation temperatures burial heating and re equilibration are not geologically viable explanations for the anomalously hot temperatures recorded in vadose cements as neither bermuda or barbados has any burial history instead it is likely that precipitation in the vadose zone occurs on timescales quicker than isotopic equilibrium can be achieved driven by a combination of co2 degassing and evaporation which have been previously shown to cause problems in speleothems and pedogenic carbonates we conclude by suggesting that meteoric phreatic calcites may be an ideal phase for paleotemperature reconstruction as they accurately record mean annual temperatures and form under equilibrium conditions while also being resistant to further mineral driven diagenesis vadose cements and any sample likely affected by processes similar to vadose diagenesis should be avoided for climate reconstructions using the mass 47 clumped isotope thermometer

modelling seismic reflections from d using the kirchhoff method abstract the observation of seismic reflections from the top of d is restricted to the epicentral distance range near the critical angle where the amplitude is large and a phase distortion occurs this phase advance in the reflected wavelet is examined and implications for commonly used seismic processing techniques such as phase picking stacking and deconvolution are made kirchhoff synthetics are generated to study several models of d reflectors such as steeply inclined interfaces and laterally varying velocity contrasts observations of strong amplitude variations in seismic reflections the bounce points of which lie well within a fresnel zone are successfully explained in a search for reflections from d investigators have exclusively used models of the d layer parallel to the core mantle boundary we speculate about a steep slope as it may be produced by deposited slab material and model the influence of such features on travel time and waveform

theory of equivalent staggered grid schemes application to rotated and standard grids in anisotropic media the previous finite difference numerical schemes designed for direct application to second order elastic wave equations in terms of displacement components are strongly dependent on poisson s ratio this fact makes theses schemes useless for modelling in offshore regions or even in onshore regions where there is a high poisson s ratio material as is well known the use of staggered grid formulations solves this drawback the most common staggered grid algorithms apply central difference operators to the first order velocity stress wave equations they have been one of the most successfully applied numerical algorithms for seismic modelling although these schemes require more computational memory than those mentioned based on second order wave equations the goal of the present paper is to develop a general theory that enables one to formulate equivalent staggered grid schemes for direct application to hyperbolic second order wave equations all the theory necessary to formulate these schemes is presented in detail including issues regarding source application providing a general method to construct staggered grid formulations to a wide range of cases afterwards the equivalent staggered grid theory is applied to anisotropic elastic wave equations in terms of only velocity components or similar displacements for two important cases general anisotropic media and vertical transverse isotropy media using respectively the rotated and the standard staggered grid configurations for sake of simplicity we present the schemes in terms of velocities in the second and fourth order spatial approximations with second order approximation in time for 2d media however the theory developed is general and can be applied to any set of second order equations in terms of only displacement velocity or even stress components using any staggered grid configuration with any spatial approximation order in 2d or 3d cases some of these equivalent staggered grid schemes require less computer memory than the corresponding standard staggered grid formulation although the programming is more evolved as will be shown in theory and practice with numerical examples the equivalent staggered grid schemes produce results equivalent to corresponding standard staggered grid schemes with computational advantages finally it is important to emphasize that the equivalent staggered grid theory is general and can be applied to other modelling contexts e g in electrodynamical and poroelastic wave propagation problems in a systematic and simple way

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

holocene tectonic uplift of the st peter and st paul rocks equatorial atlantic consistent with emplacement by extrusion abstract st peter and st paul rocks spspr are emergent portions of a peridotitic mass within the st paul fracture zone of the equatorial mid atlantic ridge where local seismicity is dominated by strike slip mechanisms we used subtidal red algae in living position death assemblages in intertidal conglomerates and marine terraces as sea level markers the highest deposits correspond to a marine terrace with a shoreline angle at 8 1 m asl that formed ca 6400 6200 cal yr bp after correction for the sea level changes derived by glacioisostatic modelling ams and radiometric 14 c age measurements on the holocene sea level markers indicate emergence during the last 6600 a at a minimum average rate of 1 5 mm a we propose that uplift resulted from peridotite emplacement by extrusion rather than volcanic activity or fault slip with the implication that the ridge has been subject to compression during the holocene

u series and amino acid racemization geochronology of bermuda implications for eustatic sea level fluctuation over the past 250 000 years abstract bermuda is a stable mid oceanic carbonate platform for which a particularly complete record of late pleistocene eustatic sea level fluctuation has been reconstructed from a detailed study of geological field relationships combined with an extensive programme of u series and amino acid racemization geochronology only twice in the past 250 000 yr has sea level in bermuda been above its present level once at approximately 200 k y when it stood at about 2 m and most recently at 125 4 k y when it stood at 5 1 m these times of interglacial high sea level are characterized by the development of patch reefs and marine calcarenites at elevations above present sea level episodes of lower sea stand onto the bermuda platform at elevations higher than 20 m are observed within the two interglacial periods and are characterized by the deposition of eolianites by contrast glacial periods are times of residual soil formation and deposition of speleothems in caves at elevations below present sea level excellent correlation is observed between the bermuda glacio eustatic sea level record and other marine and terrestrial paleoclimate records

sea level and diagenesis a case study on pleistocene beaches whalebone bay bermuda pleistocene fluctuations of sea level have left marine and aeolian limestones intercalated with glacial red soils on the bermuda carbonate platform atlantic 64 50 w 32 20 n successive eustatic highstands of similar amplitude drowned the tectonically stable platform and piled up similar sets of sediments up to three pleistocene beaches are stacked in shorelines sections post depositional diagenetic histories of these beaches can be linked to repeated changes in sea level and pore waters

seismological mapping of fine structure near the base of the earth s mantle the earth s core mantle boundary cmb juxtaposes liquid iron and crystalline silicates and is a region of large vertical thermal gradients the d region which extends up to 200 300 km above the cmb often has elevated shear wave velocity and suggestions of lateral variations in structure1 recent improvements in our ability to assemble and analyse records from regional seismic networks have allowed us to examine long profiles of travel times amplitudes and waveforms from more than a thousand short period seismometers2 we observe across canada and the united states p waves that have grazed the cmb from the powerful nuclear test in lop nor china on 21 may 1992 first arrival travel times and large secondary arrivals are consistent with a 1 5 compressional velocity increase with depth 130 km above the cmb about half the thickness of d in this locality3 our observations together with evidence for the absence of such a thin fast layer in neighbouring regions suggest the presence of lateral heterogeneity in composition or phase at the base of the mantle

energy flux model of seismic coda separation of scattering and intrinsic attenuation a new model of seismic coda is presented based on the balance between the energy scattered from the direct wave and the energy in the seismic coda this energy flux model results in a simple formula for the amplitude and time decay of the seismic coda that explicitly differentiates between the scattering and intrinsic anelastic attenuation of the medium this formula is valid for both weak and strong scattering and implicitly includes multiple scattering the model is tested using synthetic seismograms produced in finite difference simulations of wave propagation through media with random spatial variations in seismic velocity some of the simulations also included intrinsic dissipation the energy flux model explains the coda decay and amplitude observed in the synthetics for random media with a wide range of scattering q in contrast the single scattering model commonly used in the analysis of microearthquake coda does not account for the gradual coda decay observed in the simulations for media with moderate or strong scattering attenuation scattering q less than or equal to 150 the simulations demonstrate that large differences in scattering attenuation cause only small changes in the coda decay rate as predicted by the energy flux model the coda decay rate is sensitive however more to the intrinsic q of the medium the ratio of the coda amplitude to the energy in the direct arrival is a measure of the scattering attenuation thus analysis of the decay rate and amplitude of the coda can in principle produce separate estimates for the scattering and intrinsic q values of the crust we analyze the coda from two earthquakes near anza california intrinsic q values determined from these seismograms using the energy flux model are comparable to coda q values found from the single scattering theory the results demonstrate that coda q values are at best measures of the intrinsic attenuation of the lithosphere and are unrelated to the scatteing q less

evidence of a lower mantle shear velocity discontinuity in s and ss phases teleseismic recordings of direct s and ss body wave phases and their core reflected counterparts scs and sscs from intermediate and deep focus earthquakes are used to analyze the lowermost mantle shear velocity structure beneath alaska a model with a 2 75 shear velocity increase 280 km above the core mantle boundary accurately matches waveform complexi ties in both the s and ss wavetrains variations in source depth produce systematic shifts in the timing of the triplication arrivals between the s and ss travel time branches that are readily ob served in long period wwssn tangential component recordings the systematic range and depth dependence of the observed shifts are well predicted by the discontinuity model and preclude explanations of the waveform complexity as resulting from multi ple ruptures at the source receiver reverberations or near source scattering from slab structure accurate models of the seismic velocity structure in the lower most 200 km of the mantle d region are essential for calcula tions of the thermal history and dynamics of the core and mantle several recent investigations of seismic body waves have indi cated the presence of velocity stratification above the core mantle boundary lay and helmberger 1983 wright et al 1985 while models of long wavelength lateral velocity variations in the lower mantle indicate strong heterogeneities in the d region dziewon ski 1984 clayton and comer 1983 current thermal models indicate a large temperature contrast of about 1000 o between the core and mantle requiring a thermal boundary layer at the base of the mantle jeanloz and richter 1979 loper 1984 this is consistent with the general decrease in velocity gradients observed in the lowermost mantle doornbos 1983 doornbos et al 1986 the combined effects of stratification lateral hetero geneity and decreased velocity gradients complicate interpreta tion of seismic data sampling the d region it is particularly important to resolve whether velocity discontinuities are actually present within d since compositional stratification would strongly affect the thermal boundary layer dynamics as well as the interpretation of the lateral velocity variations this paper presents an analysis of a combination of seismic body waves that is particularly sensitive to the presence of a velocity discontinuity in d lay and helmberger 1983 found evidence for a 2 75 shear velocity discontinuity about 280 km above the core in s waves from intermediate and deep focus earthquakes the long period tangential component seismograms they analyzed show wave form complexities in the distance range 75 o to 95 o that were attributed to a triplication produced by an abrupt velocity in crease this interpretation was criticized by schlittenhardt et al 1985 who argue that such a discontinuity is incompatible with

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

few 2 50 km corrugations on the core mantle boundary short period pcp amplitudes have been observed to vary by a factor of about 20 in the epicentral range 40 85 this behavior is due to the reflection coefficient at the core mantle boundary which varies from a maximum of about 0 5 at an angle of incidence of about 65 to zero at an angle of incidence of about 85 corresponding to an epicentral ranges of about 60 90 we present physical model experiments in which we study the effect of corrugations of the core mantle boundary on pcp amplitudes we find that corrugations with 2 50 km scale lengths would radically alter the amplitude range behavior of short period pcp reducing its variation with range the observed pattern of short period pcp amplitudes indicates that the core mantle boundary is smooth on these scales with amplitudes less than a few hundred meters

pkp bc versus pkp df differential travel times and aspherical structure in the earth s inner core pkp bc versus pkp df differential travel times show coherent patterns of residuals indicating aspherical structure within the earth s inner core using a waveform correlation method we measure 318 differential pkp bc versus pkp df travel times from 5 years of global digital seismograph network gdsn short period data at ranges between 145 and 155 in addition we obtain 19 470 probable bc versus df travel times at these ranges by searching through 23 years of the international seismological centre isc catalog plots of residuals versus turning point location and residuals versus ray direction show clear patterns which indicate that the differential travel times are highly correlated along similar ray paths this pattern is robust with respect to range and source depth these anomalies are almost certainly due to aspherical inner core structure since pkp bc and pkp df have nearly coincident ray paths in the mantle and outer core the gdsn and isc residual patterns are in general agreement but the isc residuals underpredict the gdsn residuals by about a factor of 2 the isc data have better spatial coverage particularly for n s ray paths within the inner core these n s ray paths show positive residuals of about 0 5 s indicating faster inner core p velocities in this direction a result consistent with previous studies of absolute pkp df travel times but derived from an independent data set due to the sparse ray coverage of our data either heterogeneity or anisotropy within the inner core with about 1 velocity variations could explain these residual patterns however inversions of the isc data for heterogeneity require many more free parameters to achieve the same variance reduction as a simple two parameter anisotropy model

detailed investigation of a spot on the core mantle boundary using digital pcp data abstract a number of deep earthquakes from the fiji tonga region were used to study the properties of the core mantle boundary cmb the investigation was restricted to an area with a diameter of only a few hundred kilometers at the cmb the use of a cluster of neighboring events yielding redundant sampling and rather good resolution the components of the investigation are i a classical stacking procedure to search for additional reflectors near the cmb ii a deconvolution of seismograms and a stack of the inferred reflector series for the same purpose iii a matched filter method to determine pcp travel times and pcp p amplitude ratios precisely and iv a comparison with synthetic seismograms to check the consistency of derived travel time residuals and pcp amplitude variations in the presence of cmb topography while we do not see any additional reflector for epicentral distances around 35 we detect by stacking seismograms recorded at 70 an additional phase which might be caused by a reflection from the top of d we show that pcp travel time residuals must not be interpreted as being caused exclusively by cmb topography an additional investigation of synthetics provides some constraints on dominant wavelengths and corresponding amplitudes of cmb undulations for wavelengths of 50 400 km the elevation cannot exceed 2 3 km if pcp is observed in this particular area

3d eight order elastic finite difference modeling of refraction and strong motion data from the coyote lake region california abstract 3d elastic modeling of vertical component explosive source refraction data is performed to constrain the shallow part of a velocity model followed by 3d modeling of three component strong motion data from a spatially and temporally extended earthquake source in that model modeling is by eighth order finite differencing on a staggered grid which automatically generates all body and surface waves including multiples and conversions application is to the 1981 usgs refraction survey and the 6 august 1979 coyote lake event in central california 3d modeling was able to reproduce the main observed features in both the refraction and earthquake data for the refraction data the synthetics explain local variations in first arrival travel times reverberations in the near surface sediments and surface waves for the strong motion data the initial arrival times and amplitudes are determined by the position and orientation of the initial rupture the shapes of the trailing edges of the pulses are modified by the actual source distribution changes of source position of 1 to 2 km are easily resolvable through changes in the position of nodal planes discontinuities in material properties e g layering produce a coda of multiples and converted waves lateral variations in velocities especially in the upper few kilometers change the energy distribution among the trace components by 3d refraction and so shift the apparent source directivity and position as well as contributing to the coda remaining differences between the observed and synthetic data are attributed to local seismometer coupling effects very local site structures and phenomena such as anelasticity and anisotropy which are not explicitly modeled

finite difference seismic modeling of axial magma chambers we tested the feasibility of using finite difference methods to model seismic propagation at 10 hz through a two dimensional representation of an axial magma chamber with a thin liquid lid our technique produces time series of displacement or pressure at seafloor receivers to mimic a seismic refraction experiment and snapshots of p and s energy propagation the results indicate that our implementation is stable for models with sharp velocity contrasts and complex geometries we observe a high energy downward traveling shear phase observable only with borehole receivers that would be useful in studying the nature and shape of magma chambers the ability of finite difference methods to model high order wave phenomena makes this method ideal for testing velocity models of spreading axes and for planning near axis drilling of the east pacific rise in order to optimize the benefits from shear wave imaging of sub axis structure