seismic reflections from the upper mantle discontinuities beneath the mid atlantic ridge observed by a seismic array in hokkaido region japan following a recent big deep event mb 6 8 in the fiji islands region a seismic array in hokkaido region japan recorded clear impulsive precursors to the p p phases p is another symbol for pkp and p p is the surface reflection of p in the epicentral distance range of 70 72 4 reflection points of the phases are located 1 5 3 5 west of the mid atlantic ridge 13 s 15 w the precursors were also recorded at two wwssn stations in japan travel times and slowness measured by the array are best explained as the precursors being underside reflections from a depth of 660 km the amplitude ratios of p 660p p p are about 0 1 a value similar to previous observations the observation suggests that there exists a sharp discontinuity at a depth of 660 km beneath the area close to the mid atlantic ridge

systematic analysis of deep seismicity 200 centroid moment tensor solutions for earthquakes between 1977 and 1980 summary a systematic study of deep seismicity is performed using waveform data from the gdsn between 1977 and 1980 and the centroid moment tensor method of dziewonski et al two hundred new cmt solutions are found for events deeper than 100km their moments obtained using the same algorithm range between 7 x and 1 4 x lo2 dyne cm a factor of 20 000 a statistical analysis of the cmt parameters provides important conclusions among these the shift in origin time for deep events is found to be almost independent of the seismic moment providing an indication of the complexity of deep earthquakes the deviation from the double couple model appears to be correlated to the state of stress of the subducting slabs mechanisms showing down dip compression are generally dominated by the eigenvalue corresponding to the compression axis and down dip tension correlates with the predominance of the tensional eigenvalue comparison with moment tensors obtained by other authors using different waveform data shows good agreement between different methods comparison with published focal plane mechanisms confirms the precision and reliability of the cmt method examples of cmt solutions used to identify patterns of seismicity associated with deep fault planes are shown

on the importance of surface forcing in conceptual models of the deep ocean in the major ocean basins diapycnal mixing upwells dense antarctic bottom water which returns southward and closes the deepest cell of the meridional overturning circulation moc this cell ventilates the deep ocean and regulates the partitioning of co2 between the atmosphere and the ocean the oceanographic community s conceptual understanding of the deep stratification and moc has evolved from classic abyssal recipes arguments to a more recent appreciation of along isopycnal upwelling in the southern ocean consistent with a weakly mixed ocean interior both the deep stratification and the deep moc are shown here to be sensitive to the form of the surface buoyancy forcing in a two dimensional model that includes a circumpolar channel and northern basin for a fixed surface buoyancy condition the deep stratification is essentially prescribed whereas for a fixed surface buoyancy flux the deep stratification varies by orders of magnitude over the range of diapycnal diffusivity observed in the ocean these cases also produce different scalings for the deep moc with in both weak and strong regimes in addition these scalings are shown to be sensitive not only to the type of surface boundary condition but also to the latitudinal structure of the surface fluxes this latter point is crucial as buoyancy budgets and dynamical features of the circulation are poorly constrained along the antarctic margins this study emphasizes the need for caution in the interpretation of simple conceptual models that while useful may not include all mechanisms that contribute to the moc s strength and structure

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

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

ellesmere greenland fold belt structural evidence for left lateral shearing abstract regardless of whether they were passive or active markers of the eurekan deformation fold axial traces in the ellesmere greenland fold belt define a regional pattern typical of left lateral ductile shear zones it is therefore postulated that a left lateral mega shear zone spanned the entire east west width of ellesmere island plus adjacent northern greenland and that the shear direction was subparallel to nares strait a left lateral motion along nares strait is required by the plate tectonic models of j t wilson and e c bullard to explain the opening of the labrador sea and baffin bay by sea floor spreading these models involve a left lateral displacement between greenland and north america of 200 400 km because of the lack of lithological and structural offsets across nares strait the models have been controversial the mega shear zone proposed herein reconciles all the conflicting evidence accordingly the ductile behaviour of the sedimentary strata on ellesmere island permitted a large left lateral displacement of greenland relative to north america without a large offset of geological features across nares strait

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

investigation of mantle discontinuities from a single deep earthquake converted phases have been identified in the p wave coda of teleseismic seismograms from a single large mw 6 8 deep 360 km earthquake south of honshu japan standard array techniques slantstacking applied to broadband data from the global broadband network the german regional seismic network grsn and the grafenberg grf array and synthetic seismogram modeling reveal that these unusual phases are s to p conversions from under the seismic source timing of the converted phase indicates that subduction has had little if any effect on the depth of the 660 km discontinuity this study shows that upper mantle seismic structure can be constrained from single events using the presently available broadband seismic data and applying array techniques to network data

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

a detailed map of the 660 kilometer discontinuity beneath the izu bonin subduction zone dynamical processes in the earth s mantle such as cold downwelling at subduction zones cause deformations of the solid state phase change that produces a seismic discontinuity near a depth of 660 kilometers observations of short period shear to compressional wave conversions produced at the discontinuity yield a detailed map of deformation beneath the izu bonin subduction zone the discontinuity is depressed by about 60 kilometers beneath the coldest part of the subducted slab with a deformation profile consistent with the expected thermal signature of the slab the experimentally determined clapeyron slope of the phase transition and the regional tectonic history

determination and analysis of long wavelength transition zone structure using ss precursors summary global mapping of 410 and 660 km discontinuity topography and transition zone thickness has proven to be a powerful tool for constraining mantle chemistry dynamics and mineralogy numerous seismic and mineral physics studies suggest that the 410 km discontinuity results from the phase change of olivine to wadsleyite and the 660 km discontinuity results from the phase change of ringwoodite to perovskite and magnesiowustite underside reflections of the 410 and 660 km discontinuities arrive as precursors to ss with the recent development of a semi automated method of determining ss arrivals we have more than tripled the flanagan and shearer 1998a data set of handpicked ss waveforms we are able to increase resolution by stacking waveforms in 5 rather than 10 radius bins as well as increasing data coverage significantly in the southern hemisphere the resulting ss s410s and ss s660s times are heavily influenced by upper mantle velocity structure we perform a joint inversion for discontinuity topography and velocity heterogeneity as well as performing a simple velocity correction to the precursor differential times and find little difference between the two methods the 660 km discontinuity topography and transition zone thickness are correlated with velocities in the transition zone whereas the 410 km discontinuity topography is not in addition the 410 km discontinuity topography is not correlated with the 660 km discontinuity topography rather anticorrelated as expected due to the opposite signs of the clapeyron slopes of their respective phase changes these results suggest that whereas the topography of 660 km discontinuity could be dominated by thermal effects the topography of the 410 km discontinuity is likely dominated by compositional effects in addition unlike previous studies which find less topography on the 410 km discontinuity than on the 660 km discontinuity our 410 and 660 km topography have similar amplitudes

deep seismicity and modes of deformation in tonga subduction zone the geometry of deep 400 km earthquakes in the subducting western edge of the pacific plate when related to the local distribution of seismic activity reveals a complex pattern of planes of shear failure enabling the modes of deformation by which the descending plate shortens and thickens to be identified the deepest part of the slab is displaced laterally suggesting that it fails to penetrate the discontinuity at 670 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

the effect of the descending lithosphere beneath the tonga island arc on p wave travel time residuals at the warramunga seismic array abstract p wave travel time residuals at the warramunga seismic array wra in the northern territory australia have been studied from 49 earthquakes with epicenters south of 19 s in the fiji tonga region focal depths are between 42 and 679 km as determined from pp p using the jeffreys bullen and the herrin travel time tables the epicentral parameters have been redetermined by considering only normal seismic stations in the location procedure these are those stations where p wave travel times are probably not affected by lateral heterogeneities caused by the lithosphere descending beneath the tonga trench epicenters of deep earthquakes below 300 km have been relocated by using stations at 25 only epicenters from shallower depth earthquakes have been recalculated without using stations between 35

transition zone velocity gradients and the 520 km discontinuity stacks of long period global digital seismograph network gdsn seismograms at 110 to 180 epicentral distance reveal precursors to ss that result from underside reflections off upper mantle seismic discontinuities the 410 and 660 km discontinuities are obvious in these stacks but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410 and 660 km reflections these sidelobes result from the limited bandwidth of the gdsn instrument responses and the effect of crustal reverberations on the ss reference phase the crustal effects can be minimized by restricting the records to oceanic bounce points where the 6 km thick crust has little effect on the long period waveforms over 2000 long period transverse component seismograms with oceanic ss bounce points recorded by the gdsn from 1976 to 1991 are manually edited aligned on ss and then stacked using a new procedure that weights the records by data quality the resulting image shows a clear reflection from a 520 km discontinuity that cannot be explained as a sidelobe artifact confirming earlier results of shearer 1990 1991 and revenaugh and jordan 1991 by stacking along the expected travel time curves for discontinuity phases the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time the features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces using geometrical ray theory and assuming a constant velocity versus density scaling relationship i fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion the inverse problem is addressed by performing a deconvolution of the profile with the ss reference phase after a correction for attenuation followed by a direct mapping of reflectivity versus time into velocity versus depth velocity depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models except that the ss precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660 km discontinuity estimated discontinuity shear impedance changes are 6 7 1 1 at 420 km 2 9 0 7 at 519 km and 9 9 1 5 at 663 km the impedance change near 520 km is consistent with current mineral physics results for the olivine to phase change and places constraints on the fraction of olivine in the transition zone

upper mantle shear structure beneath the northwest atlantic ocean we have modeled the sh motion from earthquakes in the northwest atlantic ocean to derive an oceanic upper mantle shear velocity model the signals were recorded on long period wwssn and canadian network stations on the east coast of north america the travel times and waveforms of seismograms in the distance range 11 16 were used to constrain the lid structure these data indicate a fast 4 75 km s lid of about 100 km thickness in the older western atlantic from 16 to 28 the s waves pass through triplications due to velocity jumps near 400 and 660 km the branches from both discontinuities are visible in the s wave data these arrivals were modeled using synthetic seismograms to obtain accurate travel times the two triplications are also apparent in ss data from 30 to 50 the triplication curves derived from the s waves agree with the ss data given the lid structure the waveforms and travel times from the more distant data put tight constraints on the shear velocities at greater depths we find shear velocities from 200 to 400 km to be slow in relation to a shear model of the canadian shield derived by a similar technique indicating continent ocean heterogeneity to about 400 km

multiple discontinuities near 660 km beneath tonga area 1 we study the discontinuities beneath tonga subduction zone using the deep earthquakes recorded by the pacific northwest seismic network pnsn the multiple discontinuities around 660 km beneath tonga subduction zone are found using converted phase sdp the amplitude ratios of the converted phase with the direct p wave ac ap are also determined the largest number of robust converted phases with large ac ap comes from the depth range of 660 690 km its peak is at 680 km the second one appears in the depth range of 700 745 km its peak is at 740 km the two discontinuities are obviously depressed by the subduction slab according to experimental and numerical studies a possible explanation of the observation is that the first discontinuity is formed by the transition of spinel to perovskite and magnesiowustite and the second is formed by the transition of ilmenite to perovskite

the thickness of upper mantle discontinuities as inferred from short period j array data the thickness of upper mantle discontinuities at depths around 410 km and 660 km provides a critical clue to our understanding of deep earth since the amplitude of reflected or s p converted waves is controlled by the thickness of discontinuities short period studies of such waves may give a fine constraint on the thickness of upper mantle discontinuities in this study we analysed short period and vertical waveform data from 14 deep events in the fiji tonga region the data were obtained from j array a short period and large aperture seismic array in japan with slant stack analyses of these data we detected short period reflections or s to p conversions from 410 km and 660 km discontinuities near sources which implies that both discontinuities are quite sharp to directly measure the frequency content of these waves linear slant stacks were made with various frequency pass bands 0 2 0 5 0 5 1 0 and 1 0 2 0 hz synthetic traces were generated for the models with various thicknesses of the 410 km and the 660 km discontinuities comparison of slant stacks with synthetic traces indicates that the thickness of both discontinuities is at most 5 km

effects of phase transions and possible compositional changes on the seismological structure near 650 km depth if the lower mantle is enriched in fe si relative to the upper mantle the compositional boundary is expected to be an interface with a density jump of 5 2 and with depth varying laterally by 100 300 km but with changes in seismic velocities that are nearly unobservable 0 to 1 0 2 5 discontinuities in compressional vp and shear vs wave velocities in contrast the polymorphic transformation of transition zone minerals to the perovskite bearing assemblage of the lower mantle is estimated to produce a jump of 5 10 percent in density vp and vs near 650 km depth and the depth of the seismological discontinuity due to the phase transition is relatively independent 50 150 km variation of temperature or horizontal position if present a compositional boundary is not in general expected to occur at the identical depth 650 km as the phase transition to the perovskitebearing assemblage although the depths of the two may be linked

topography of the transition zone seismic discontinuities phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities nominally at 410 520 and 660 km depth thermodynamic principles govern phase transformations making them sensitive to changes in the mantle s ambient conditions through the thermodynamic response changes in temperature or composition shift the transformation to a different pressure creating topography on a level discontinuity with this use as an exploratory tool for the mantle in mind we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities reviewing the evidence for and against the phase transition model we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

a global study of s to p and p to s conversions from the upper mantle transition zone summary long period data of the global digital seismograph network gdsn recorded over the three year period from 1984 to 1986 were studied for the occurrence of s p and p s conversions from the upper mantle transition zone that appear as precursors to teleseismic s arrivals conversions of this type were identified on a large number of single station records simple stacking of many records enhanced the appearance of converted phases and demonstrated that no major lateral variations in the nature of the transition zone exist between various tectonic regions s p and p s conversions from the 400 km discontinuity were best observed at distances between 70 and 85 while conversions from the 670 km discontinuity showed up best at distances beyond 87 the analysis of published source mechanisms and comparison with synthetic seismograms suggests that the appearance of converted phases is primarily governed by the earthquake radiation pattern phases that have undergone s p conversions beneath the receiver are best observed from dip slip events that radiate strong sv and weak p waves towards the station p s conversions beneath the source area on the other hand are frequently observed from events that radiate strong p and little sv energy towards the station and also from some strike slip events comparison of observed with synthetic seismograms suggests that the prem model of dziewonski anderson 1981 explains most of the observations observed s p and p s conversions from the 670 km discontinuity however often have larger amplitudes than in the synthetics constructive interference of converted waves with the p wave coda source radiation effects and a velocity contrast across the 670 km discontinuity which is higher than in prem may all contribute to the discrepancy

comments on reflection properties of phase transition and compositional change models of the 670 km discontinuity by alison c lees m s t bukowinski and raymond jeanloz

p wave travel times from deep and intermediate depth earthquakes to local seismic stations and the subducted slab of oceanic lithosphere beneath the tonga island arc to investigate the seismic structure of the tonga subduction zone mobile seismic stations were temporarily operated on the four main islands of the tonga group data from intermediate depth and deep earthquakes were analyzed using p wave arrival times only to improve on single event locations as published in bulletins a joint hypocentral determination method was applied to three groups of deep earthquakes and two groups of intermediate depth earthquakes the resulting p wave travel time residuals observed in the tonga island arc are generally negative and reveal a characteristic pattern a notable feature is the observation that residuals from deep earthquakes located near 18 s are consistently less negative than residuals from deep earthquakes located south of 20 5 s it cannot completely be ruled out that part of this difference is caused by systematic depth errors that may arise as a result of slab propagation effects on the upgoing p leg of pp but only a minor part of the observed difference can be explained in this way the major part is probably caused by two factors first p waves traveling from the southern part of the tonga subduction zone to the northernmost stations located in the islands of vava u and ha apai spend more time in the high velocity slab than p waves originating in the northern part of the tonga seismic zone second ray tracing through a number of models of the subduction zone suggests that p waves from deep earthquakes located south of 20 5 s have their propagation paths close to the high velocity core of the slab whereas p waves from deep and perhaps also from intermediate depth earthquakes located near 18 s are likely to miss parts of the high velocity slab on their way up to the island arc this is most pronounced for the seismic station tga located in the island of tongatapu which is some 30 km further away from the trench than the other stations of the network the magnitude of residual differences 2 3 s at tga observed from different groups of deep earthquakes is best explained by a sharp bend in the dip of the subducting plate in the northern part of tonga near a depth of 200 km ray tracing through such a model shows that p waves from deep earthquakes near 18 s traveling to tga escape the slab at a depth of about 250 km and from there on propagate through the mantle overlying the slab if the interpretation is correct p wave velocities are indicated that are 5 7 higher in the slab above 250 km than in the overlying mantle independent of this interpretation the magnitude of negative residuals from deep earthquakes south of 20 5 s is best matched by velocities in the slab that are uniformly higher by 6 relative to the herrin model in another model where p wave velocities vary linearly over horizontal slab width from maximum values in the seismic zone to the velocities in the herrin model at the slab boundaries the data are equally well fitted if the velocity anomaly reaches a maximum value of 8 relative to the herrin model allowance for a crustal correction term of 0 5 s reduces these estimates by 1 modification of the velocity models to include the case of a velocity anomaly in the slab that decreases linearly with depth to 0 over the range from 400 to 700 km increases the estimates by 1 the data do not allow a proper assessment of the question of whether seismic velocities approach normal mantle values at depths below 400 km or remain substantially higher than in standard models of the earth

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

seismic evidence against a mantle chemical discontinuity near 660 km depth beneath izu bonin we analyze p wave codas from 12 deep izu bonin earthquakes recorded by hundreds of western united states seismograph stations for evidence of small amplitude phases caused by near source mantle discontinuities for nearly every event the dominant phase in the coda is the result of an s to p conversion from a nearly horizontal discontinuity ranging in depth from 650 to 690 km this is interpreted as a thermally depressed spinel to perovskite and magnesiowustite phase transition if the 660 km seismic discontinuity is also associated with a change in chemistry it would be dynamically depressed by a subducting slab we consistently see that there is no nearly horizontal discontinuity between 700 and 1000 km with shear wave velocity contrast exceeding 1 this observation places constraints on the properties of a postulated chemical discontinuity separating the upper and lower mantles

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

long period surface waves and mantle boundary undulations summary we address two questions with synthetic experiments 1 is there sufficient signal in long period surface wave data to provide useful constraints on topography on the internal boundaries in the upper mantle and around the transition zone 2 what is the magnitude and nature of the bias caused by undulations on internal discontinuities on inversions for volumetric structures that do not also include boundary structures the synthetic experiments rest on models of boundary topography on three upper mantle and transition zone discontinuities at 220 400 and 660 km depth the 660 km topographic model is that of shearer masters 1992 we consider the effect of two different 400 km models one is identical to shearer and master s model model a and the other is their model sign flipped model b the 220 km model is ad hoc but has been motivated by the work of revenaugh jordan 1989 1991b who argue that the 220 km discontinuity is not a global feature at all but has a continental affinity for simplicity our 220 km model is global but the jump in material properties across the discontinuity has been reduced relative prem by a factor of two and is based on a continent ocean model which is depressed up to 25 km below continents and ascends 10 km under oceans these resulting boundary models have a significant effect on long period fundamental spheroidal mode structure coefficients with a magnitude on average of 15 30per cent relative to the volumetric models m84a lo2 56 between harmonic degrees 20 l 75 inversion for volumetric structures of the structure coefficients computed with the boundary models alone devoid of volumetric structure shows that simple mantle volumetric structures can be found to fit the boundary structure coefficients nearly perfectly this demonstrates the essential trade off between boundary and volumetric mantle structures using surface wave data alone destructive interference between the effects of the 660km boundary and model b of the 400 km boundary requires a somewhat oscillatory volumetric mantle model to fit the structure coefficients but the resulting volumetric model is no more oscillatory than current models of the upper mantle both boundary models a and b produce on average a 30 per cent volumetric bias relative to m84a lo2 56 from the base of the crust to 1000 km depth the relative nature of the 400 km and 660 km undulations strongly affects the radial distribution of the volumetric bias but has less of an effect on the average size of the bias

thermodynamically consistent decompression implications for lower mantle composition estimates of the composition of the lower mantle are commonly obtained by a process of adiabatic decompression in which zero pressure mantle parameters are determined with a least squares fit of an equation of state to mantle densities we show that this procedure may lead to biased estimates of the zero pressure density and compressibility of the mantle the bias can be eliminated or minimized by subjecting the equation of state fit to a thermodynamic constraint at zero pressure such inversions provide equations of state that are compatible with the lower mantle as well as the thermal properties of selected mineral models we used this method to evaluate assemblages containing mg fe sio3 perovskite and mg fe o with fe fe mg ratios as high as 0 2 thermodynamic systematics were used to place bounds on the anderson gruneisen parameter s for perovskite we find a fairly large range of acceptable models for the composition of the lower mantle they range from uppermantle like compositions to assemblages that are enriched in both si and fe relative to the upper mantle however a pure perovskite lower mantle whether enriched in fe or not seems improbable small fe enrichments of the order of 1 given density contrasts across the 670 km discontinuity that are probably insufficient to prevent convective mixing of the upper and lower mantles we therefore suggest that unless the mantle is homogeneous in its major oxide components the lower mantle is likely to be enriched in fe by at least several percent

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

can sharp seismic discontinuities be caused by non equilibrium phase transformations a long posed problem of the seismic discontinuities at the depths of 410 and 660 km in earth s mantle is whether these discontinuities are caused by phase transitions alone or together with chemical layering among several tests the sharpness of the boundary is one of the most crucial recent data suggest that the transition region thickness is less than 4 km for both the 410 and the 660 km discontinuities this may be smaller than predicted by phase equilibria we suggest that if there is a sufficiently large nucleation barrier for phase transformation and if the transformation front cannot move faster than the convective flow the transformation in rising or descending material does not take place until the metastable overshoot becomes equal to the nucleation barrier an avalanche like transformation following this overshoot occurs in a very narrow region and can be the cause of a sharp seismic discontinuity in this case the topography of the phase boundary is also substantially modified

thermally induced phase changes lateral heterogeneity of the mantle continental roots and deep slab anomalies pressure induced solid solid phase changes are responsible for most of the increase of density and seismic velocity with depth in the upper mantle lateral variations in temperature cause a similar effect abrupt changes of density and seismic velocity due to phase changes are superposed on smaller changes associated with thermal expansion temperature induced isobaric phase changes are as important in explaining various recent geophysical data as are the more familar pressure induced phase changes in cold slabs the equilibrium mineral assemblage contains high density high velocity phases which are not stable in hotter mantle in particular the ilmenite form of mgsio 3 and the spinel form of mg 2sio 4 have broad stability fields in cold mantle which increase the density and velocity of deep slabs to values in excess of those which have been used in geoid and seismic travel time modeling recent arguments for slab penetration into the lower mantle and whole mantle convection are based on thermal models of the slab which ignore the large density and seismic velocity anomalies associated with temperature induced phase changes when these effects are taken into account the geoid and seismic anomalies associated with subducted slabs are consistent with slab confinement to the upper mantle and layered models of mantle convection the seismicity cutoff and evidence for slab thickening at 670 km also favor this style of convection mantle seismic velocities between 200 and 400 km depth in tectonic and young oceanic regions are lower than in shield regions and this is due to the presence of a melt phase and lower velocity high temperature phase assemblages deep 200 km long lived continental roots differing in chemistry from normal mantle are not required when isobaric phase changes are taken into account high velocity subshield mantle is closer to normal subsolidus mantle than is suboceanic mantle which is affected by the presence of high temperature phase assemblages the whole mantle convection thick continental root and deep slab penetration hypotheses are not supported by seismic and geoid data when isobaric phase changes are included in the analysis phase changes are more effective in changing density and seismic velocity than are lateral variations in temperature and composition the lack of correlation of the geoid with ridges shields heat flow and upper mantle velocity variations suggests a low geoid sensitivity to the upper mantle consistent with layered convection

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

the upper mantle discontinuities in western canada from ps conversions we have investigated variations in the travel times ofps converted phases from the upper mantle 410 and 660 km discontinuities recorded on the western stations of the canadian national seismograph network using a variant of the technique introduced byvinnik 1977 clear and unambiguous signals for both discontinuities are observed at 8 of the 11 stations considered and exhibit variations which correlate well with regional tectonic setting stations located in regions which are currently tectonically active are characterized by largerps arrival times relative to directp than those situated on the north american craton in addition the difference in arrival times between the 410 and 660ps phases suggest that most of the variation is the result of structure above the transition zone but below the moho stations located in the cascadia subduction zone generally exhibit poorer signal quality than those elsewhere a feature that may result from upper mantle velocity heterogeneity discontinuity topography or a combination of both a detailed investigation of possible lateral variations in discontinuity topography associated with subduction awaits the compilation of a more comprehensive data set which will permit the monitoring of an azimuthal dependence in signal

horizontal shear flow in the mantle beneath the tonga arc in the tonga subduction zone a systematic shear deformation in which deep material moves relatively to the south is superimposed on the mode of down dip shortening which is characteristic of the region we interpret this observation together with a tectonic reconstruction in terms of the interaction between the subducting lithospheric slab and a horizontal shear flow in the mantle