geomorphological impacts of a flood event on ephemeral channels in se spain a flood event occurred on 30 september 1997 in three catchments torrealvilla salada and nogalte in se spain which were being monitored for morphological change peak discharges were high in the torrealvilla catchment moderate in the salada and low in the nogalte detailed topographic surveys were made before and after the flood and from these dems of difference were constructed the two sites with highest flow both underwent significant incision of the channel at other sites both scour and aggradation of the channel took place with scour holes of up to 1 m depth eroded elsewhere within the channels major erosion took place downstream of tracks and structures such as check dams that impeded sediment movement the pattern and amounts of change show little relationship to independent factors other than a scale effect of peak discharge behaviour of channels is closely related to local sediment supply sediment flux calculated from changes in volumes at the site is comparable with the other figures produced for ephemeral channels and much higher than for perennially flowing streams

the upper mantle degree two pattern constraints from geoscope fundamental spheroidal mode eigenfrequency and attenuation measurements we have analyzed fundamental spheroidal mode spectra obtained from very long period geoscope records in terms of global distribution of eigenfrequencies and attenuation with a new data set and a different data processing approach involving time variable filtering and the removal of higher order effects in the spectra we confirm the strong upper mantle degree two signature in the eigenfrequency data the higher frequency content of the geoscope spectra allows us to put better constraints on the depth range at which this pattern originates which appears to be somewhat shallower than was proposed before while some effect may persist down to the bottom of the transition zone the frequency signature of the strongest degree two pattern requires an s velocity heterogeneity concentrated in the depth range 300 500 km and this signature is also apparent in the distribution of q which is indicative of a probable thermal origin the data also resolve a shallow degree 2 pattern well explained by heterogeneity spanning from the surface to depths of about 300 km and associated with the distribution of tectonic provinces it is shifted by about 45 with respect to the deep degree 2 pattern and its expression in the q data is not clear

s to p scattering at the 650 km discontinuity summary a search of seismograms recorded at the warramunga seismic array wra from events occurring below the izu bonin islands shows an arrival on some but not all of the records with an onset at 25 30 s after p which is not predicted by the standard travel time tables the slowness and azimuth of the phase show that it is generated almost in line with p and the variation of arrival time with the hypocentral depth of the earthquake indicates that its origin lies on the receiver side of the source it appears in fact to be an s to p conversion at a depth of 650 700 km which is seen only when the receiver is close to a node of the p radiation pattern and an antinode for s so that its amplitude compared with that of p is at a maximum finally the duration of the phase indicates that it is not simply a refracted wave but that it has a coda of scattered arrivals from lateral heterogeneity in the neighbourhood of 650 km below the izu bonin islands

s p conversion from the transition zone beneath tonga and the nature of the 670 km discontinuity summary using more than 300 recordings of deep tonga earthquakes from the warramunga seismic array central australia we have searched for short period s p conversions from the 670 km discontinuity arriving in the p wave codas the array seismograms were stacked and plotted in sections to optimize observation of s670p yielding the following results for the northern part of tonga 17 3 18 4 s s670p phases are observed with apparent conversion depths between about 660 and 680 km for the central part 20 2 22 3 s s670p is not clearly observed for the southern part 23 1 25 7 s s670p is observed for conversion depths between about 660 and 700 km intermittent occurrence of s p conversion may result from different characteristic source radiation patterns among the three sections overall uncertainties in conversion depths resulting from unmodelled lateral velocity variations and hypocentral location errors are perhaps a few tens of kilometres the s p rays must travel either within or very close to the subducted slab because of the source receiver slab dip geometry so we conclude that large 50 km deformations of the 670 km discontinuity do not occur beneath tonga our observations appear to be inconsistent with a chemical discontinuity at 670 km but are easily explained if the 670 km discontinuity is a strongly pressure dependent and perhaps moderately endothermic phase change in both the slab and surrounding mantle

the problems of radiocarbon dating in their perspective the boon and bane of radiocarbon dating 21 jan p 362 1 t p guilderson et al raise some important issues in radiocarbon 14c dating they discuss the problems of calibrating 14c dates that fall within age plateaus of the 14c calibration curve and conclude

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

the eclogite to garnetite transition experimental and thermodynamic constraints we have found that the forms of the experimentally determined phase diagrams for pyroxene garnet transformations require the volume changes for these reactions to be much smaller at 130 kbar 400 km than at 1 atm using these volume data we calculated the width of the eclogite garnetite transition in natural quartz tholeiite and alkali olivine basalt compo sitions taking account of mixing in multicom ponent pyroxene and garnet phases we find that the eclogite garnetite transition does not exhi bit any discontinuity in bulk sound velocity in the pressure range 110 to 230 kbar in addition this velocity when plotted as a function of pressure possesses a curvature which is in the opposite sense to that given by the preliminary reference earth model prem this phase transi tion therefore does not appear to be a satis factory model for the 400 km seismic discon tinuity

aspherical earth structure from fundamental spheroidal mode data free oscillation data reveal heterogeneity in the earth s mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid the heterogeneity can be modelled as localized in the transition zone 420 670 km depth and may be related to a large scale component of mantle convection

seismic velocity structure using array data abstract the fine structure of regional travel time curves is investigated about 200 events from the region north of australia recorded at the warramunga array are analysed to give the p wave velocity structure for that region the events were confined to a very narrow azimuth range results show that the 20 discontinuity is more complicated than a single velocity increase near 400 km depth and that there are regions of rapid or discontinuous velocity increase at depths below 650 km

short period s p conversion in the mantle at a depth near 700 km summary data from 16 deep earthquakes originating in the tonga subduction zone and recorded at the warramunga seismic array wra in central australia show the presence of a distinct later arrival in the coda of p this phase which is not predicted by smooth velocity depth models of the earth has been called x in the paper slownesses and arrival azimuths of x are similar to those of the p wave it is unlikely that x was radiated as the result of multiple focal activity because a corresponding phase was not observed neither in the coda of scp nor on seismograms of seismic stations located at close epicentral distances relative to p the variation in arrival times with focal depth of the earthquakes indicates that x is produced by s p conversion below the source at a depth near 700 km most of the observed x phases have approximately the same signal duration as the p waves thus suggesting a transition zone near 700 km which is only a few kilometres thick synthetic seismograms calculated for a model with a first order seismic discontinuity at a depth of 700 km show reasonable agreement with the observations as demonstrated by the synthetic seismogram modelling the amplitude of x depends on the radiation pattern of the earthquake source variations in the radiation pattern offer a plausible explanation that first x is only intermittently detected at wra and secondly that x has most frequently been observed from the southern part of the tonga subduction zone

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity stacked images of long period seismic data reveal many phases some not previously observed which are caused by reflections and conversions at discontinuities in the earth s upper mantle these images support the existence of discontinuities at 410 520 and 660km depth but no evidence is found for a significant interface at 220 km

a simple model of the 1982 83 californian el nino steric height steric height anomaly sea level and sea level anomaly show that the 1940 41 and the 1982 83 californian el nino events were resonant responses of the california current to the expansion and intensification of the aleutian low and decrease in strength of the north pacific high in both cases the subsurface anomalies were dynamically produced by onshore transport of subarctic water and a downward tilt of the inshore thermocline sea surface temperature anomalies however resulted from thermodynamic as well as dynamic processes most previous californian el nino or anti el nino events probably were resonant or anti resonant responses of the seasonal cycle of the california current to anomalous large scale atmospheric forcing

heterogeneities in the mantle transition zone from observations of p to sv converted waves abstract the method of detection of p to sv converted waves from distant earthquakes vinnik 1977 was applied to sets of long period records from a few seismograph stations in europe and the west of north america the results obtained suggest that the converted phases related to the major boundaries in the mantle can be reliably detected and the depths of conversion evaluated with an accuracy of a few kilometres the depth of the olivine spinel transition is close to 400 km and no difference between the estimates for the north of europe and the west of north america is found the depth of the boundary separating the upper and lower mantle is close to 640 km which is 30 km less than in the recent earth reference models fine s velocity stratification of this transition changes laterally from a high gradient layer 50 km thick terminated at the bottom by a sharp discontinuity to a gradient layer 100 km or more thick without the discontinuity a striking anomaly of the mantle transition zone is found in the rio grande rift area where a well pronounced boundary is found at 510 km depth

composition of the upper mantle geophysical tests of two petrological models the elastic properties of candidate mantle phases are used to test the viability of olivine rich pyrolitic and cao al 2o 3 rich eclogitic assemblages for the mantle high temperature adiabats for each phase of interest are constructed and compared to mantle seismic properties both pyrolitic and eclogitic assemblages satisfy the seismic properties between 200 and 400 km between 400 and 670 km depth an eclogitic assemblage yields a superior match to velocities and velocity gradients the 400 km seismic discontinuity may represent a chemical boundary between pyrolite and picritic eclogite piclogite or phase transformations in the olivine orthopyroxene components of a piclogitic assemblage containing about 16 olivine high velocity gradients in the transition zone may be explained by the transformation of ca rich cpx to majorite garnet seismic properties at the top of the lower mantle are consistent with pyrolite piclogite or perovskite implying that the 670 km discontinuity may be a chemical boundary

an experimental investigation of the pyroxene garnet transformation in a pyrolite composition and its bearing on the constitution of the mantle abstract the pyroxene garnet transformation in a model pyrolite composition has been investigated over the pressure interval 4 0 to 17 5 gpa at temperatures of 1200 1400 c the transformation is largely completed in the pressure interval 14 16 gpa and a single phase aluminum deficient garnet majorite becomes stable above 16 gpa the mineral assemblages and the density and seismic velocity distributions for pyrolite are calculated as a function of depth on the basis of this and other recent experimental data density and seismic velocity profiles calculated for pyrolite are consistent with models based on seismic observations to a depth of 550 km if the existence of a high density velocity gradient between 550 km and 650 km is confirmed the presence of further phase transformations in majorite and or some degree of chemical heterogeneity in this depth interval would be indicated

is subducted lithosphere trapped at the 670 km discontinuity recent work1 2 on phase equilibria has shown that subducted oceanic lithosphere which is differentiated into a basaltic crust and a harzburgitic residue layer could be significantly denser than undifferentiated peridotite at depths between 650 and 700 km but would be less dense in the range 700 740 km it might thus be trapped in a gravitational stable layer between upper and lower mantle3 here using numerical experiments of isoviscous convec tion i explore the stability of such a layer assuming that it had previously been formed the results suggest that the survival of the intermediate layer is at best marginally feasible provided that new subducted lithosphere is efficiently trapped

subduction back arc spreading and global mantle flow the shapes and orientations of benioff zones beneath island arcs interpreted as marking the location of subducted lithosphere provide the best presently available constraints on the global convective flow pattern associated with plate motions this global flow influences the dynamics of subduction subduction zone phenomena therefore provide powerful tests for models of mantle flow we compute global flow models which while simple include those features which are best constrained namely the observed plate velocities applied as boundary conditions and the density contrasts given by thermal models of the lithosphere and subducted slabs two viscosity structures are used for one flow is confined to the upper mantle while for the other flow extends throughout the mantle instantaneous flow velocity vectors match observed benioff zone dips and shapes for the model which allows mantle wide flow but not for the upper mantle model which has a highly contorted flow pattern the effect of trench migration on particle trajectories is calculated it is not important if subduction velocities are greater than migration rates two dimensional finite element models show that including a coherent high viscosity slab does not change these conclusions a coherent high viscosity slab extending deep into the upper mantle would significantly slow subduction if flow were confined to the upper mantle the maximum earthquake magnitude m w for island arcs correlates well with the age of the subducted slab and pressure gradient between the trench and back arc region for the whole mantle but not the upper mantle flow model the correlations with orientations of benioff zones and seismic coupling strongly suggest that the global return flow associated with plate motions extends below 700 km for both models regions of back arc spreading have asthenospheric shear pulling the back arc toward the trench regions without back arc spreading have the opposite sense of shear suggesting global flow strongly influences back arc spreading

phase transformations in diopside camgsi2o6 at pressures up to 25 gpa phase transformations in diopside have been studied at pressures up to 25 gpa and at temperatures of 1100 and 1500 c by a multiple anvil apparatus the following phase assemblages are found to be stable at given pressures diopside 15 and 18 gpa mg2sio4 spinel stishovite casio3 perovskite 19 and 20 gpa mgsio3 ilmenite casio3 perovskite 22 gpa and mgsio3 perovskite casio3 perovskite 24 5 gpa electron microprobe analysis reveals that the solubility of the mgsio3 molecule in casio3 cubic perovskite is limited to between 2 5 the solubility of the casio3 molecule in mgsio3 orthorhombic perovskite is more restricted being of the order of 2 or less these results are contradictory to earlier work in which diopside is reported to transform into a cubic perovskite structure of the same composition above 22 23 gpa

measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy 3 inversion lateral heterogeneity in the earth s upper mantle is investigated by inverting dispersion curves of long period surface waves 100 330 s models for seven different tectonic regions are derived by inversion of regionalized great circle phase velocity measurements from our previous studies we also obtain a representation of upper mantle heterogeneities with no a priori regionalization from the inversion of the degree 6 spherical harmonic expansion of phase and group velocities the data are from the observation of about 200 paths for love waves and 250 paths for rayleigh waves for both the regionalized and the spherical harmonic inversions corrections are applied to take into account lateral variations in crustal thickness and other shallow parameters these corrections are found to be important especially at low spherical harmonic order the trench region and fast velocities down to 250 km under shields below 200 km under the oceans both s velocity and s anisotropy support a model of small scale convection in which cold blobs detach from the bottom of the lithosphere when its age is large enough the spherical harmonic models clearly demonstrate a posteriori the relation between surface tectonics and s velocity heterogeneities in the first 250 km all shields are fast most ridges are slow below 300 km a belt of fast mantle follows the pacific subduction zones however at greater depths large scale heterogeneities that seem to bear no relationship to surface tectonics are observed the most prominent feature at 450 km is a fast velocity region under the south atlantic ocean smaller scale heterogeneities that are not related to surface tectonics are also mapped at shallower depths an anomalously slow region centered in the south central pacific is possibly linked to intense hot spot activity a very fast region southeast of south america may be related to subduction of old pacific plate between 200 and 400 km a belt of sv sh anisotropy follows part of the ridge and subduction systems indicating vertical mantle flow in these regions the spherical harmonic results open new horizons for the understanding of convection in the mantle perspectives for the improvement of the models presented are discussed

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

upper mantle converted waves beneath the nars array several registrations of teleseismic events on the nars array show a phase which can be identified as a p to s conversion at the 670 km discontinuity beneath western europe variations in the amplitude and arrival time of the so called ps phase are observed for different azimuths a single station multiple event stacking technique was applied to the data to determine the consistency of the phase the results are indicative of amplitude arrival time and slowness variations of the converted phase on a local scale these observations are most logically explained by a model with depth variations of the 670 km discontinuity of the order of 20 km over a distance of 200 km

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

precursors to p p re examined using broad band data summary observations of underside p wave reflections from discontinuities deep within the upper mantle which precede the p p phase have provided important constraints on the existence and physical properties of these dynamically critical features in this study we extend earlier work by examining for the first time broad band recordings of those earthquakes for which one would expect to observe p p and its associated precursors p dp an exhaustive search of 20 yr of carnegie data and 11 yr of grafenberg data uncovered a number of fine p p observations but in only one or perhaps two instances are there evidence of a precursor from near 670 km depth these null results are consistent with what has been reported in earlier studies and provide us with an opportunity to estimate the variability of p velocity near 670 km or alternatively the scale length of discontinuity topography which acts to defocus the precursor this has been done by using the reflectivity and gaussian beam methods to compute synthetic seismograms for a suite of 1 d and 2 d earth models which vary slightly from prem we find that the null data may be satisfied either by i reducing the prem impedance jump at 670 km from 8 5 to 4 5 per cent ii smoothing this 8 5 per cent jump linearly over a zone 15 30km thick or iii imposing deformations in the 670 km discontinuity as small as 10 km in amplitude and 300 km in wavelength for the one case of an observed precursor the relative size of the underside reflection and main p p phase suggests that in this instance the precursor s amplitude is correctly predicted by prem

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

detection of underside p reflections at mantle discontinuities by stacking broadband data underside p reflections at mantle discontinuities are difficult to observe directly on seismograms mainly because of their low amplitude it is shown that stacking seismograms obtained at several epicentral distances but having their pp bounce points in the same region enables the identification of such phases this method is applied to data from japanese and taiwan events recorded by the nars array and other broadband european stations discontinuities at about 670 and 300 km are revealed beneath northern eurasia