detailed crustal thickness variations beneath the illinois basin area implications for crustal evolution of the midcontinent we present high resolution imaging results of crustal and upper mantle velocity discontinuities across the illinois basin area using both common conversion point stacking and plane wave migration methods applied to p wave receiver functions from the earthscope oiink ozark illinois indiana and kentucky experiment the images reveal unusually thick crust up to 62 km throughout the central and southeastern illinois basin area a significant moho gradient underlies the nw trending ste genevieve fault zone which delineates the boundary between the illinois basin and ozark dome relatively thinner crust 45 km underlies most of the precambrian highlands surrounding the illinois basin and beneath the rift related structures of the reelfoot rift and the rough creek graben we consider four hypotheses to explain the presence of thick crust under the central and southeastern illinois basin crustal thickening may have been produced 1 prior to its accretion to north america around 1 55 ga and is an inherited characteristic of this crustal province 2 by underthrusting or shortening during proterozoic convergent margin tectonics around 1 55 1 35 ga 3 by late precambrian magmatic underplating at the base of older crust associated with the creation of the eastern granite rhyolite province around 1 3 ga and 4 through crustal relamination during an episode of proterozoic flat slab subduction beneath the illinois basin possibly associated with the grenville orogeny

imaging p to s conversions with multichannel receiver functions we present a new methodology in the direct imaging of p to s converted phases recorded on broadband seismic arrays our approach is based on conventional three component array processing and receiver function techniques with the key addition of a weighted stack based on an aerial smoothing function this creates synthetic arrays with a specified aperture whose image points vary continuously across the array with this approach it is possible to interpolate data from an array of broadband stations onto an arbitrarily fine grid we have applied this technique to a single deep event recorded by the lodore broadband array located in northern colorado and southern wyoming the resulting images show distinct differences in crustal structure across the array and also image major upper mantle discontinuities

three dimensional prestack plane wave migration of teleseismic p to s converted phases 1 theory 1 we present the theoretical foundations for a prestack migration technique to image teleseismic p to s converted phases the method builds on teleseismic p wave deconvolution pseudostation stacking neal and pavlis 1999 and on the idea of using a plane wave decomposition for imaging as introduced by treitel et al 1982 deconvolution operators are constructed by pseudostation stacking of the array aligned to the incident p wave arrival times to produce a space variable deconvolution operator the resulting data are then muted to remove the deconvolved direct p wave pulse and pseudostation stacked over a grid of feasible slowness vectors the pseudostation stack interpolates the wave field onto a regular grid along earth s surface producing a series one per slowness vector of uniformly sampled three dimensional data cubes two space variables and time the plane wave components can be propagated downward using a form of approximate ray tracing with a three dimensional earth model this yields a series of distorted cubes topologically equivalent to the original uniformly sampled data cubes these data volumes are summed as a weighted stack with the weights derived from an integration formula for inverse scattering based on the generalized radon transform this allows an image of the subsurface to be constructed on an event by event basis beneath the array we apply this technique to data from the lodore array that was deployed in northwestern colorado the results suggest the presence of a major lithospheric scale discontinuity defined by a south dipping boundary

multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

the scots bay formation nova scotia canada a jurassic carbonate lake with silica rich hydrothermal springs during early jurassic time the scots bay formation accumulated in an aerobic lake on the floor of the subtropical fundy rift valley a diverse biota flourished in the lake as evidenced by algal stromatolites oncolites charophytes ostracods gastropods conchostracans fish bones calcispheres and logs carbonate wackestone and packstone were deposited in the littoral zone and lime mudstone accumulated offshore on a proximal slope silica rich hydrothermal springs and seeps around and on the floor of the lake precipitated siliceous tufa and silicified adjacent carbonate strata the tufa is characterized by the progressive sequence 1 open framework porous substrate of goethite spheres 2 encrusting chalcedony spherulites where goethite inclusions decrease in size and number outwards towards the surfaces of the spherulites 3 a fringe of quartz on the chalcedony spherulites and 4 coarsening inwards mosaics of quartz that filled the remaining voids carbonate diagenesis included conversion of low mg calcite ooze to micrite and microspar pseudospar and local dissolution and reprecipitation of calcite cement at times the lake shrank in size concentrating silica in increasingly alkaline lake water which then silicified some of the carbonate strata and logs the latest cements were calcite zeolites and celadonite

a thinned lithospheric image of the tanlu fault zone eastern china constructed from wave equation based receiver function migration 1 we apply the newly proposed wave equation based receiver function poststack migration method to the northern china interior structure project broadband data to image the lithospheric structure of the tanlu fault zone area in eastern china our migration result reveals a 60 to 80 km thick present day lithosphere beneath the study region significantly thinned from the paleozoic lithosphere of 180 km the lithosphere asthenosphere boundary lab is coherently imaged along the 300 km east west profile displaying an arc like shape with its apex roughly coincident with the transverse location of the tanlu fault zone on the surface an obvious uplift from 36 km to 32 km of the moho is also clearly detected right below this fault zone the coincidence of the imaged moho uplift and the lab apex with the surface location of the tanlu fault zone provides seismological evidence for the steep geometry and deep penetration of the fault system and indicates that the tanlu fault zone might have acted as a major channel for anthenosphere upwelling during the mesozoic cenozoic continental extension and lithospheric thinning in eastern china frequency analysis and synthetic modeling suggest that both the moho and the lab are sharp and strong the latter in particular is constrained to have a 3 7 drop in s wave velocity over a depth range of 10 km or less such a rapid velocity change at the base of the lithosphere in the study region cannot be solely explained by thermal variation but likely reflects the presence of volatiles or melt in the asthenosphere or is partially attributed to the compositional contrast between the preserved depleted and dehydrated cratonic lithospheric veneer and the uplifted hydrated and fertile asthenospheric materials

experiments in migration processing of ss precursor data to image upper mantle discontinuity structure long period ss precursors result from underside reflections off upper mantle discontinuities by grouping and stacking global seismic data by ss bounce point location it is possible to map lateral variations in depths to the 410 and 660 km discontinuities a process analogous to common midpoint cmp stacking in reflection seismology because this method assumes horizontal reflectors energy arriving from dipping or intermittent reflectors may not be correctly imaged to address this possibility we experiment with techniques based on migration processing of shallow seismic reflection data the problem is complicated by the uneven distribution of sources and receivers for the ss precursor observations but the data are sufficiently dense beneath the northwest pacific ocean that reasonably good coverage can be obtained for this region we parameterize the model as a grid of point scatterers in latitude longitude and depth from the surface to 1000 km depth and compute travel times from each grid point to the source and receiver locations these times are used to construct a matrix equation that yields predicted ss precursor waveforms from the assumed scatterers to recover the model we experiment with both simple back projection and full inversions using a conjugate gradient method tests on noise free synthetic data generated using the same source receiver distribution as the actual data suggest that detailed resolution of discontinuity structure is possible at horizontal scales much smaller than the fresnel zone however the real data do not produce coherent results unless some degree of horizontal smoothing is imposed at least partially defeating the purpose of this approach results for the northwest pacific find structure on the 410 and 660 km discontinuities and hints of intermittent reflectors at other depths random resampling tests however suggest that most of these features are not reliably resolved with the exception of a depression on the 660 km discontinuity seen in the northwest pacific our experiments show that it is unlikely that small scale structure on the 660 km discontinuity near subducting slabs causes significant bias in maps of the large scale 660 km topography derived from long period ss precursor observations

multiparameter two dimensional inversion of scattered teleseismic body waves 2 numerical examples in this paper we investigate the formal inversion of synthetic teleseismic p coda waves for subsurface elastic properties using a ray theoretic approach which assumes single scattering bostock et al this issue we consider a model comprising an idealized lithospheric suture zone whose geometrical configuration is drawn from previous deep crustal seismic studies two dimensional pseudospectral synthetic seismograms representing plane waves propagating through this model are preprocessed to extract an estimate of the scattered wave field generated by short wavelength structure these data are employed in a series of numerical simulations which examine the dependence of multiparameter inversion results on a range of input parameters in particular we demonstrate 1 the contrasting sensitivity which forward and backscattered waves display to structural recovery 2 the diminution of the problem null space accompanied by increased source coverage 3 improvements in model reconstruction achieved through simultaneous treatment of multiple scattering modes and 4 the robustness of the method for data sets with noise levels and receiver geometries that approach those of field experiments

seismic imaging of the crust and upper mantle using regularized joint receiver functions frequency wave number filtering and multimode kirchhoff migration 1 receiver functions provide an indispensable tool for producing discontinuity images of the crust and upper mantle from teleseismic earthquake arrivals however image quality can be significantly compromised by instabilities inherent in the deconvolution process and by irregularities in data coverage driven by source and or receiver geometries we present receiver function estimation and prestack migration techniques which both reduce receiver function deconvolution instability and produce regularized multimode receiver function images the receiver function estimation technique exploits frequency wave number filtering methods using frequency pseudowave number domain filtering of gathers to enhance arrivals with consistent moveout characteristics synthetic tests confirm robust recovery of features in the presence of high noise levels to process regularized receiver functions into accurate images of the subsurface seismic scattering potential we employ a regularized kirchhoff migration that migrates both direct and reverberated p to s converted modes to their correct subsurface locations the subsequent images can be stacked to mute mismigrated modes synthetic studies and model resolution tests demonstrate that this methodology is well suited to irregularly spaced stations and uneven data coverage and does not require the recording of a single earthquake at multiple stations

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

seismic migration processing of p sv converted phases for mantle discontinuity structure beneath the snake river plain western united states we experiment with backprojection migration processing of teleseismic receiver functions from the snake river plain srp broadband seismic experiment previous analyses of data from this experiment have used a common midpoint cmp stacking approach a method widely applied for analysis of p sv converted phases receiver functions to obtain high resolution imaging of upper mantle discontinuities the cmp technique assumes that all p sv conversions are produced by flat lying structures and may not properly image dipping curved or laterally discontinuous interfaces in this paper we adopt a backprojection migration scheme to solve for an array of point scatterers that best produces the large suite of observed receiver functions we first perform synthetic experiments that illustrate the potential improvement of migration processing over cmp stacks application of the migration processing to the srp data set shows most of the major features as in the original cmp work but with a weaker 410 km discontinuity and a more intermittent discontinuity at 250 km apparent depth random resampling tests are also performed to assess the robustness of subtle features in our discontinuity images these tests show that a 20 km elevation of the 660 km discontinuity directly beneath the snake river plain is robust but that the variations in 410 km discontinuity topography that we observe are not stable upon resampling bright spots near 250 km apparent depth are robust upon resampling but interpretation of these features is complicated by possible sidelobe artifacts from topside moho reverberations

teleseismic waveform modelling with a one way wave equation summary it is now widely accepted that elastic properties of the continental lithosphere and the underlying sublithospheric mantle are both anisotropic and laterally heterogeneous at a range of scales to fully exploit modern three component broad band array data sets requires the use of comprehensive modelling tools in this work we investigate the use of a wide angle one way wave equation to model variations in teleseismic 3 d waveforms due to 2 d elastic heterogeneity and anisotropy the one way operators are derived based on a high frequency approximation of the square root operator and include the effects of wave propagation as well as multiple scattering computational cost is reduced through a number of physically motivated approximations we present synthetic results from simple 1 d layer over a half space and 2 d subduction zone models that are compared with reference solutions the algorithm is then used to model data from an array of broad band seismograph stations deployed in northwestern canada as part of the iris passcal lithoprobe canoe experiment in this region radial component receiver functions show a clear continental moho and the presence of crustal material dipping into the mantle at the suture of two palaeo proterozoic terranes the geometry of the suture is better defined on the transverse component where subduction is associated with a 10 km thick layer exhibiting strong elastic anisotropy the modelling reproduces the main features of the receiver functions including the effects of anisotropy heterogeneity and finite frequency scattering

teleseismic shot profile migration the shot profile migration approach of wave equation migration generates subsurface images using the interferometric principle of crosscorrelating two passive wavefields these wavefields are typically a source wavefield containing energy from an excited source and a receiver wavefield comprised of scattered source wavefield energy by the discontinuous earth structure shot profile migration can be recast as a novel way of imaging the earth s lithosphere using teleseismic wavefield data where the source wavefield is the directly arriving wavefront and the receiver wavefield is the following wavefield coda we demonstrate that the shot profile technique can be tailored to suit teleseismic acquisition geometry and wavefields assuming an acoustic framework and 2 5d experimental geometry we develop procedures that enable kinematic and structural imaging migration using both transmission and free surface reflected passive wavefields experiments with synthetic data demonstrate the method s applicability a

unraveling the geometry of the farallon plate synthesis of three dimensional imaging results from usarray abstract we compare 12 recent three dimensional 3d seismic imaging results that made extensive use of data from the earthscope transportable array ta our goal is to sort out what can be said about the geometry of the farallon plate our main approach is 3d visualization using a kinematic plate motion model as a framework comparison of results from all 12 image volumes indicates that the results are most consistent with a single coherent farallon slab overridden by north american the farallon can be tracked from the trench in the pacific northwest to its remnants in the lower mantle under eastern north america from the trench the lithosphere has a low dip to the volcanic arc immediately east of the arc the slab steepens sharply before undergoing a decrease in dip above the 410 km discontinuity the gently dipping section varies along strike under washington the deflection is minor but to the south the slab flattens to become nearly horizontal beneath southern idaho there is a strong agreement that the high velocity anomaly associated with the slab vanishes under eastern oregon scattered wave imaging results however suggest the top of the anomaly is continuous these can be reconciled if one assumes the wavespeed anomaly has been neutralized by processes linked to the yellowstone system we find that all results are consistent with a 4d kinematic model of the mendocino slab window under nevada and utah in the eastern us the larger scale models all show a lower mantle anomaly related to the older history of farallon subduction the link between the lower mantle and new results in the u s cordillera lies under the high plains where the required usarray coverage is not yet complete image volumes in a unified format are supplied in an electronic supplement

moho depth and crustal composition in southern africa abstract we present new results on structure thickness and composition of the crust in southern africa based on 6300 seismic receiver functions at 85 stations application of hk stacking to the entire sase dataset and use of multi frequency bands improve resolution substantially we observe a highly heterogeneous crustal structure with short wavelength variations in thickness h vp vs ratio composition and moho sharpness which defines 20 blocks that do not everywhere coincide with surface tectonic features in the zimbabwe craton the tokwe block has h 35 38 km and vp vs 1 74 1 79 whereas the thicker crust in the tati block h 47 51 km may be related to deformation of the archean crust along the cratonic margin two distinct crustal blocks with similar crustal thickness 42 46 km but significantly different vp vs ratios are recognized in the limpopo belt extreme values of 1 90 1 94 at the dyke swarms in eastern limpopo and 1 84 at the olifants river dyke swarm and easternmost bushveld intrusion complex bic indicate voluminous magmatic intrusions in the whole crust we find no evidence for magmatic intrusions in the central inferred part of bic where the crust is thick 45 50 km and vp vs is low 1 68 1 70 this thick crustal root may have deflected rising magmas to form the two bic lobes most of central kaapvaal has thin 35 40 km crust and vp vs 1 74 these characteristics are similar to the tokwe block in zimbabwe craton and may indicate delamination of pre existing lower crust which is further supported by a very sharp moho transition the exposed cross section in the vredefort impact crater is non representative of cratonic crust due to shallow moho 34 km and high vp vs 1 80 attributed to shock metamorphism high vp vs 1 76 is typical of the witwatersrand basin and anomalously low vp vs 1 66 1 67 marks the kaapvaal kheis namaqua transition highly heterogeneous crust both in thickness and vp vs ratio is typical of the namaqua natal and cape fold belts

origin of hibonite pyroxene spherules found in carbonaceous chondrites we have studied both of the known glass free hibonite pyroxene spherules mysm3 from murray cm2 and y17 6 from yamato 791717 co3 they consist of hibonite plates 2 wt tiotot2 enclosed in al rich pyroxene that has such high amounts of cats caal2sio6 component up to 80 mol that it must have crystallized metastably within the pyroxene abundances of mgo and sio2 are strongly correlated with each other and are anticorrelated with those of al2o3 reflecting an anticorrelation between the diopside and cats components of the pyroxene in contrast with previous results for type b fassaite however we do not observe an anticorrelation between mgo and tiotot2 possibly reflecting different relative distribution coefficients for ti3 and ti4 in the aluminous pyroxene of the spherules from those found for fassaite in type b inclusions previously described hibonite silicate spherules have 26mg deficits but the present samples do not furthermore the pyroxene in y17 6 has excess 26mg while the hibonite it encloses does not indicating that the two phases either had different initial 26al 27al ratios or different initial 26mg 24mg ratios the ti isotopic compositions of the present samples are highly unusual 50ti 103 4 5 2 o in mysm3 and 61 4 4 1 0 in y17 6 which are among the largest 50ti anomalies reported for any refractory inclusion the textures suggest that hibonite crystallized first but based on the calculated bulk compositions of both spherules it is not the liquidus phase in either sample which suggests that the hibonite in both samples is relict the presence of ragged hibonite grains in mysm3 and rounded hibonite grains in y17 6 and a lack of isotopic equilibrium between pyroxene and hibonite support this conclusion the spherules crystallized from liquid droplets that probably formed as a result of the melting of solid precursor grains that included hibonite the heating events were too short and or not hot enough to melt all the hibonite the droplets cooled quickly enough that cats rich pyroxene crystallized instead of anorthite based on the observed differences in isotopic composition it is unlikely that the precursors of the present samples formed in the same reservoir as each other or as the previously described hibonite silicate spherules providing further evidence of the isotopic heterogeneity of the early solar nebula

beyond receiver functions passive source reverse time migration and inverse scattering of converted waves 1 we present a wave equation prestack depth migration to image crust and mantle structures using multi component earthquake data recorded at dense seismograph arrays transmittedp and swaves recorded on the surface are back propagated using an elastic wave equation solver the wave modes are separated after the reverse time continuation of the wavefield from the surface and subjected to a cross correlation type imaging condition forming an inverse scattering transform reverse time migration rtm does not make assumptions about the presence or properties of interfaces notably it does not assume that interfaces are locally horizontal with synthetic experiments and different background models we show that passive source rtm can reconstruct dipping and vertically offset interfaces even in the presence of complex wave phenomena such as caustics and point diffraction and that its performance is superior to traditional receiver function analysis e g common conversion point ccp stacking in complex geological environments

a wave equation migration method for receiver function imaging 1 theory 1 a wave equation based poststack depth migration method is proposed to image the earth s internal structure using teleseismic receiver functions by utilizing a frequency wave number domain one way phase screen propagator for wave field extrapolation in the migration scheme common conversion point ccp stacked receiver functions are backward propagated to construct a subsurface structural image the phase screen propagator migration method takes into account the effects of diffraction scattering and travel time alternation caused by lateral heterogeneities and therefore it is particularly useful for imaging complex structures and deep discontinuities overlain by strong shallow anomalies synthetic experiments demonstrate the validity of the migration method for a variety of laterally heterogeneous models the migrated images show considerable improvement over the ccp images in recovering the model features influences of several factors on the image quality of the poststack migration are further investigated including interstation spacing noise level of the data velocity model used in migration and earthquake distribution incident direction of source fields on the basis of the sampling theorem and previous statistic results we discuss the relation of spatial resolution and signal to noise ratio of the migrated image with the frequency of the data surface station spacing and number of receiver functions used in stacking we show that both ccp stacking and poststack migration of receiver functions need to be designed in a target oriented way for reliable and efficient imaging our results also suggest that careful consideration of earthquake source distribution is necessary in designing seismic experiments aimed at imaging steeply dipping structures

imaging of structure at and near the core mantle boundary using a generalized radon transform 1 construction of image gathers 1 we introduce a method for imaging heterogeneity at and near interfaces in earth s lowermost mantle with broadband three component seismograms from global seismograph networks our approach is based on inverse scattering and allows the extraction of pertinent signal from large data sets and requires few a priori assumptions about the heterogeneity under study which makes it complementary to the forward modeling of selected waveforms here we construct a generalized radon transform for heterogeneous anisotropic elastic media to map broadband seismogram windows comprising main arrivals with their coda and precursors into multiple images of a target structure the common image point gathers thus produced reveal multiscale variations in elastic properties near deep interfaces the grt can be applied to narrow and wide angle data and the automated extraction of signal from data over a wide range of epicentral distances enables exploration of core mantle boundary cmb regions that cannot with present day data coverage be imaged with the triplicated waveforms used in forward modeling studies tests with synthetic data produced both with idealized and actual source receiver distributions illustrate pertinent aspects of the theory and show that multiple weak interfaces can be detected and located correctly with a radial resolution of 15 km for the frequencies used even in the presence of random noise that would prohibit visual inspection and modeling of the subtle signals we transformed the transverse component scs waveforms into image gathers of a cmb patch beneath central america juxtaposition of stacks of these gathers involving 35 000 seismograms produces a two dimensional 2 d image profile revealing contrasts in elastic properties near the target depth of the cmb and 280 340 km above it the latter may mark the top of the so called d region the images also reveal a richness of structures in between these depths combined with a statistical analysis of these singularities described in paper 2 the approach to imaging presented here paves the way to large scale seismic exploration of the lowermost mantle

inverting interpolated receiver functions with surface wave dispersion and gravity application to the western u s and adjacent canada and mexico we use p wave receiver functions from the western u s and adjacent regions to construct a receiver function wavefield interpolation scheme that helps to equalize the lateral sampling of the receiver functions and the surface wave dispersion and to greatly simplify the receiver functions spatial interpolation and smoothing suppress poorly sampled and difficult to interpret back azimuthal variations and allow the extraction of the first order features in the receiver function wavefield including observations from several ray parameter ranges we combine the interpolated receiver functions with rayleigh wave dispersion estimates and surface gravity observations to estimate the 3 d shear wave speed beneath the region speed variations in the 3 d model correlate strongly with expected geologic variations and illuminate broad scale features of the western u s crust and upper mantle the model is smooth self consistent and demonstrates the compatibility of the interpolated receiver functions and dispersion observations

three dimensional prestack plane wave migration of teleseismic p to s converted phases 2 stacking multiple events 1 in part 1 we developed the theoretical foundations of a prestack migration procedure to image forward scattered p to s pds converted waves in the coda of teleseismic p waves this paper addresses the issue of how to optimally stack data from multiple events migrated by this procedure we apply matrix perturbation theory to develop an objective way to quantify noise in deconvolved pds data application of the theory demonstrates that an optimal stack requires weighting the migrated data from each event by a signal to noise ratio criterion we also find that the migrated pds images have to be binned by back azimuth and balanced prior to the final stack this is necessary to mitigate coherent noise that results from aliased microseism noise that is enhanced by our processing method we processed 23 events recorded by the lodore array in northwestern colorado with our procedure the results indicate the presence of a major lithospheric scale discontinuity defined by a south dipping boundary within the crust that we interpret as the subsurface expression of the cheyenne belt the suture is also marked by a transition in crustal thickness from 35 km on the archean side to over 40 km on the colorado plateau side we also observe a strong difference in the lithospheric mantle pds conversion signature on opposite sides of the suture that suggests delamination and northward convergence of the colorado lithosphere beneath the wyoming province

gaussian beam migration just as synthetic seismic data can be created by expressing the wave field radiating from a seismic source as a set of gaussian beams recorded data can be downward continued by expressing the recorded wave field as a set of gaussian beams emerging at the earth s surface in both cases the gaussian beam description of the seismic wave propagation can be advantageous when there are lateral variations in the seismic velocities gaussian beam downward continuation enables wave equation calculation of seismic propagation while it retains the interpretive raypath description of this propagation this paper describes a zero offset depth migration method that employs gaussian beam downward continuation of the recorded wave field the gaussian beam migration method has advantages for imaging complex structures like finite difference migration it is especially compatible with lateral variations in velocity but gaussian beam migration can image steeply dipping reflectors and will not produce unwanted reflectio

roughness of the mantle transition zone discontinuities revealed by high resolution wavefield imaging we processed two independent data sets 2 458 973 three component seismograms from all earthscope transportable array usarray stations and 141 080 pairs of high quality receiver functions from the earthscope automated receiver survey using the generalized iterative deconvolution method and shaped the output with a 0 1 hz ricker wavelet we used these data as input to our 3 d plane wave migration method to produce an image volume of p to s scattering surfaces under all of the lower 48 states model simulations show that compared to the widely used common conversion point stacking method our method is capable of resolving not only dipping discontinuities but also subtler details of amplitude and topography variations at scales near the diffraction limit maps of attributes derived from these discontinuities reveal several previously unobserved features of the 410 and 660 discontinuities d410 and d660 topography with many tens of kilometers is resolved at a range of scales additionally we observed large variations of amplitude on the radial component and in the radial to transverse amplitude ratio that correlate with inferred variations in discontinuity topography we argue that this combination of observations can be explained by roughness at a range of scales the structural fabric we imaged is preferentially aligned orthogonal to the farallon north american relative motion two regions we infer to have exceptional roughness show spatial correlation with locations where tomography models indicate that the subducted slab is passing through the transition zone we suggest that these rough regions are indicators of vertical mantle flows through the transition zone

improved green s functions for passive source structural studies over the past two decades teleseismic receiver functions have proved to be a useful tool to investigate crustal structure because they represent a first order approximation to the s wave component of the teleseismic p green s function receiver functions provide valuable information on physical properties related to shear modulus however the implicit use of the p component seismogram as a proxy for the source precludes the recovery of information on discontinuous structure involving contrasts in compressional modulus by deconvolving improved estimates of complex source time functions generated by earthquakes one may move beyond the conventional receiver function paradigm to a more accurate approximation of the earth s green s function using a new deconvolution method we present estimates of the p component of the teleseismic p green s functions at several stations of the canadian national seismic network cnsn that clearly show the receiver side pure p wave crustal multiple the identificat

strength and rheology of the western u s cordillera effective elastic thickness te depends primarily on temperature composition and state of stress of the lithosphere in this paper we examine high resolution spectral estimates of te and their relationships to regional heat flow age of the lithosphere seismic properties stress orientations and earthquake focal depths of the western u s cordillera the relationship of te to heat flow indicates that ductile flow accommodates long term 106 to 108 years isostatic response at different levels of the crust and upper mantle depending principally on age and by implication bulk composition of the lithosphere isostatic response is primarily controlled by the upper mantle in archean lithosphere of the middle rocky mountains whereas te depends on lower crustal flow in early proterozoic lithosphere of the colorado plateau the yellowstone snake river plain volcanic field and significantly extended regions in the basin range and northern rocky mountains are associated with latest proterozoic aged lithosphere and indicate middle to upper crustal control of long term te we also show that azimuthal variations of te reflect deviatoric stress in the lithosphere te is found empirically to approximate the 95th percentile focal depth of background seismicity the latter relationship is inconsistent with brittle ductile control of focal depth indicating that another rheological transition e g from stick slip to stable sliding frictional behavior is responsible tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere moreover we find a spatial correlation of the intermountain seismic belt to a marked transition in te implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current day plate boundary interactions

a wave equation migration method for receiver function imaging 2 application to the japan subduction zone 1 the newly developed wave equation poststack depth migration method for receiver function imaging is applied to study the subsurface structures of the japan subduction zone using the fundamental research on earthquakes and earth s interior anomalies freesia broadband data three profiles are chosen in the subsurface imaging two in northeast ne japan to study the subducting pacific plate and one in southwest sw japan to study the philippine sea plate the descending pacific plate in ne japan is well imaged within a depth range of 50 150 km the slab image exhibits a little more steeply dipping angle 32 in the south than in the north 27 although the general characteristics between the two profiles in ne japan are similar the imaged philippine sea plate in eastern sw japan in contrast exhibits a much shallower subduction angle 19 and is only identifiable at the uppermost depths of no more than 60 km synthetic tests indicate that the top 150 km of the migrated images of the pacific plate is well resolved by our seismic data but the resolution of deep part of the slab images becomes poor due to the limited data coverage synthetic tests also suggest that the breakdown of the philippine sea plate at shallow depths reflects the real structural features of the subduction zone rather than caused by insufficient coverage of data comparative studies on both synthetics and real data images show the possibility of retrieval of fine scale structures from high frequency contributions if high frequency noise can be effectively suppressed and a small bin size can be used in future studies the derived slab geometry and image feature also appear to have relatively weak dependence on overlying velocity structure the observed seismicity in the region confirms the geometries inferred from the migrated images for both subducting plates moreover the deep extent of the pacific plate image and the shallow breakdown of the philippine sea plate image are observed to correlate well with the depth extent of the seismicity beneath ne and sw japan such a correlation supports the inference that the specific appearance of slabs and intermediate depth earthquakes are a consequence of temperature dependent dehydration induced metamorphism occurring in the hydrated descending oceanic crust

anisotropic upper mantle stratigraphy and architecture of the slave craton the earth s physical properties show a dominantly radial structure which is the result of compositional differentiation isochemical phase changes1 and rheological layering2 rheological layering is perhaps the most difficult to investigate using conventional seismological techniques because the seismic manifestation of this property elastic anisotropy may closely mimic the effects of isotropic heterogeneity3 nonetheless an improved characterization of earth s rheological structure promises important insights into such processes as plate dynamics and continental evolution here i present a methodology for effectively characterizing sharp transitions in anisotropic upper mantle structure using the coda of teleseismic p waves application to seismograms from the slave craton reveals a well developed stratigraphy at least in part anisotropic with major boundaries occurring at nominal depths of 75 135 and 195 km the geometry and sharpness of these discontinuities suggest a structural origin perhaps involving shallow subduction

lithospheric imaging via teleseismic scattering tomography summary the coda of the teleseismic p phase consists largely of energy scattered by small inhomogeneities in the receiver side lithosphere given large collections of teleseismic data from dense permanent networks previous workers have successfully back propagated coda energy back to scattering source points using various kinematic migration schemes as well as by inverting using an inverse scattering radon transform approach under the born approximation seismic scattering is a linear process therefore it is possible to approach coda scattering as a linear waveform inversion problem mathematically similar to transmission based tomography assuming ray theoretical propagation and rayleigh scattering we pose the inverse scattering problem in tomographic form and recover perturbations in density and p and s velocities from pp and ps scattered data the method is applied to data from the southern california seismic network scsn covering the san jacinto anza region the results show a considerable correlation between seismicity and velocity perturbation structure particularly in the region between the mission creek and banning fault branches features connecting the coyote creek and elsinore faults at right angles are correlated with seismicity lineations and may represent conjugate faulting with no surface expression

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

a seismic image of the upper mantle beneath the north american craton teleseismic p waves recorded on the canadian national seismograph network are used to investigate upper mantle structure beneath the north american craton 1090 broadband seismograms are source normalized and stacked in 100 bins between 30 and 100 epicentral distance the resulting image reveals a number of p to s converted phases which include prominent ps conversions from the 410 and 660 km discontinuities reduction of these signals to single traces via simultaneous deconvolution along theoretical moveout curves permits the analysis of p 410 s and p 660 s over a range of frequencies relative amplitudes vary with p 410 s dominating at higher frequencies this results in a greater pulse width for p 660 s in broadband stacks and supports previous suggestions that the phase transformation responsible for the 410 km discontinuity occurs over a narrower transition than that at 660 km the image and single trace stacks are examined for evidence of discontinuities at other levels notably at 520 km and 220 km depth no obvious expression of p 520 s is evident at high frequencies and evaluation of lower frequency stacks is complicated by sidelobe interference from p 410 s and p 660 s evidence in support of a weak 220 km interface is present in the form of an amplitude maximum at the expected time and slowness of a pp 220 s phase

receiver function arrays a reflection seismic approach summary the receiver function method rfm is a commonly used technique to study the crustal and upper mantle velocity structure early receiver function rf investigations were performed mostly at individual permanent stations they were focused on crustal structures and later on upper mantle velocity discontinuities 410 km and 660 km discontinuities only recently has research been directed towards the study of the lateral 2 and 3 d variability of major velocity boundaries in the crust and upper mantle by receiver function arrays using temporary and permanent three component short period and broad band seismic stations to improve the signal to noise ratio receiver functions are calculated for individual earthquakes and are then binned moveout corrected and stacked we show that this processing sequence is similar to that applied routinely in exploration seismology therefore existing tools from the near vertical data processing can be adopted for receiver functions velocity analysis tools solutions for static and residual static problems coherence enhancement of seismic phases migration etc the high spatial density of seismic stations of recent and future receiver function experiments provides the opportunity and obligation to use the more sophisticated migration methods full wavefield migration commonly and successfully used in exploration seismics parasynthetics calculated by the finite difference method for simple 2 d crustal models are employed here to test our processing approach and to show the potentials and limitations of stacking and migrating rf data we show that binning normal move out nmo corrections stacking and post stack migration of the synthetic data can reconstruct the models reliably with a high spatial resolution

teleseismic wavefield interpolation and signal extraction using high resolution linear radon transforms summary we present a new application of a method based on the radon transform for data interpolation and signal to noise separation of teleseismic wavefields recorded by regional seismic arrays the method exploits the plane wave nature of direct arrivals and receiver side scattered ar rivals from regional scale structure by decomposing the recorded wavefield into a plane wave basis linear radon transform casting the radon transform as an inversion problem allows the incorporation of time dependent weighting schemes and model variance tuning which are helpful in minimizing artefacts related to the transform process while enhancing lower ampli tude arrivals the procedure begins with an estimate radon transform of the input data panel a mute is applied in the radon domain to isolate portions of the radon panel that represent plane waves with different moveouts 025 s km 1 relative to the direct arrival the inverse transform of the estimated noise is subtracted from the input data to produce an estimate of the desired signal without plane waves following undesired moveouts white ambient noise and arrivals not represented well by plane waves diffractions inverse transformation of estimated signal in the radon domain produces a natural way to interpolate in the data domain due to the implicit assumption that a plane wave basis provides the most compact representation of the teleseismic wavefield this processing scheme presents an important addition to standard teleseismic imaging processing flows that attempt to exploit irregularly sampled data

deconvolution of teleseismic body waves for enhancing structure beneath a seismometer array abstract we discuss three stacking deconvolution techniques for enhancing signal of earth structure contained in the seismograms of teleseismic body waves recorded by a seismometer array the goal of the deconvolution is to remove from the seismograms the complications due to the source for three component data the removal of the source can be efficiently achieved by deconvolving one component from another as in the receiver function analysis in the case of an array if the structure beneath individual stations varies the source wavelet can be estimated and removed from the seismograms by stacking this allows structural analysis in the situations when only single component seismometers are used or when a particular component can provide additional independent information here we compare the efficacy of conventional time domain stacking cepstral averaging and autocorrelation stacking the latter is a new approach developed here for cepstral averaging we present conditions that stabilize the convergence examples are presented using synthetics and real data from a broadband seismic array experiment conducted in the cascadia subduction zone oregon in 1993 to 1994 data from subduction zones are particularly well suited because of the spatially rapidly varying structure we find that all three methods work well for low frequencies which are not sensitive to the time alignment of the seismic traces the latter two methods do not require precisely aligned seismograms and their advantage becomes evident at higher frequencies where the appropriate time alignment is harder to achieve at high frequencies time domain stacking is likely to fail

3 d crust and mantle structure in southern ontario canada via receiver function imaging abstract a teleseismic data set from the polaris project is used to obtain 3 d images of southern ontario using two imaging techniques scattering tomography and common conversion point stacking the resulting images reveal a layered crust the layering being interrupted by discontinuities associated with major crustal scale faulting breaks in crustal continuity and moho deflections associated with the ottawa bonnechere graben indicate that the graben is associated with faulting on a whole crust scale we also detect similar discontinuities across the mississauga domain supporting the previous interpretation that the domain is bounded by crustal scale faults we locate discontinuous sub lithospheric negative polarity arrivals which indicate complex three dimensional structures within the lithosphere and may be associated with subduction remnants or a mid lithosphere discontinuity

lithosphere scale seismic image of the southern urals from explosion source reflection profiling explosive source deep seismic reflection data from the southern ural mountains of central russia provided a lithosphere scale image of the central eurasian plate that reveals deep reflections 35 to 45 seconds in travel time 130 to 170 kilometers deep from the mantle the data display laterally variable reflectivity at the base of the crust that deepens beneath the central part of the profile documenting a crustal thickness of 55 to 60 kilometers beneath the axis of the orogen these data provide an image of the structure of the crust and underlying mantle lithosphere in a preserved collisional orogen perhaps to the base of the lithosphere

correlation migration using gaussian beams of scattered teleseismic body waves correlation migration for structural imaging using gaussian beams is described for the inversion of passively recorded teleseismic waves gaussian beam migration is based on an overcomplete frame based representation of the seismic wave field and uses localized slant stack windows of the data paraxial gaussian beams are then utilized for the backpropagation of the seismic waves into the me dium the method can provide stable imaging of seismic data in smoothly varying background media where caustics and triplicated arrivals exist we develop gaussian beam migration for structural imaging which allows for incident teleseismic waves from beneath the structure using directly scattered or surface reflected phases the method is applied to synthetic data computed for a collisional zone model and the inversions show that the gaussian beam migration can image structure using passive teleseismic data the method is next applied to synthetic data that have been con volved with an observed pulse from the 1993 cascadia experiment the autocorre lation is computed and the autocorrelated data are migrated by using gausian beams for direct p to s scattered waves and surface reflected waves although the migra tions of the autocorrelation data with the source pulse included have more noise than the migrations of the impulsive source data the subduction zone structure can still be clearly identified in the migrated results without removal of the source pulses