the 9 june 94 bolivian deep earthquake an exceptional event in an extraordinary subduction zone we investigate the physical setting of the bolivian shock based on the history of the subducting nazca plate intraslab seismicity deep seismic moment release and seismic tomography south america has two broad regions of reverse arc curvature subduction constrained to this unique geometry produces slab kinking contortions that may cause unusual slab thickening as they sink to the bottom of the transition zone and encounter resistance to penetration into the lower mantle such contortions are observed at intermediate depths upstream from the slab source regions of both the great 1970 colombian and 1994 bolivian events thickening helps explain how the nazca slab accommodates large seismic source dimensions at depths of 625 650 km

a high resolution p wave model for the top 1200 km of the mantle this work attempts to map aspherical velocities in the top 1200 km of the mantle that are illuminated by the international seismological centre p wave data the processing includes summary ray sorting and hypocentral redetermination to cope with the extreme variation of ray coverage a multicell inversion is used to simultaneously constrain anomalies in overlapping cells the multicell solutions are summed after inversion to form the final model named p1200 at cell size of 1 1 and about 50 km in thickness the lateral resolution of the model is about 1 2 in major subduction zones and about 5 15 in most places major features in the model withstood tests on signal coherency and different reference models the long wavelength parts of the p1200 model correlate well with previous global models especially the s12 model of shear wave among the large features in the transition zone and below there are two fast bands one from canada to south america and another from siberia through east asia and the philippine sea to western australia there are also slow patches under the central pacific and from india to eastern africa though most subduction zones have slab like high velocity anomalies they are sandwiched by broad slow velocities at shallow depths some of the slabs appear to penetrate the 660 km discontinuity and stagnant slabs are also seen in the transition zone plume like slow anomalies exist below some prominent hot spots like hawaii iceland yellowstone and french polynesia

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

two dimensional three dimensional waveform modeling of subducting slab and transition zone beneath northeast asia subduction plays a fundamental role in dynamics of the mantle convection and in material circulations of the earth s interior slabs have been imaged to subduct near horizontally in the transition zone tz beneath the northeast asia in seismic tomography triplication waveform modeling is an effective tool to study the detailed seismic structure in the tz however tz triplication modeling has traditionally relied on 1 d models in this study we use the spectral element method to explore influences of 2 d 3 d slab structure on tz triplication waveforms and to model for the first time the slab and tz structures beneath the northeast asia synthetic tests suggest that for a subduction zone earthquake slab structure can have important influences on tz triplication waveforms and that even in a narrow azimuth range the effects from 2 d 3 d slab structure on the wave propagation can lead to erroneous conclusions with 1 d modeling our data are high quality triplicated sh waveforms at distances of 10 32 from a deep event below 410 km discontinuity in the pacific subducting slab our 2 d 3 d waveform modeling results suggest that a simple model of the subducting slab 5 high velocity anomaly and 100 km thick down to 560 km but normal below 560 km can match most of the observed waveforms remarkably well the bottom of the tz of the sampling region north of the yellow sea contains a patch of very slow anomaly the results indicate that subhorizontal slab above the 660 discontinuity is not everywhere beneath northeast asia and the subducting slab is not everywhere continuous to the bottom of the tz compared with the traditional 1 d modeling our new 2 d 3 d approach provides better fits to the data and allows us to constrain the slab geometry and to separate tz structure from slab structure

earth science tiny triggers deep down the documentation and characterization of remotely triggered earthquakes deep within the earth is an achievement that provides insight into the mechanisms that initiate such events

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

unsuccessful initial search for a midmantle chemical boundary with seismic arrays compositional layering of the midmantle has been proposed to account for seismic and geochemical patterns van der hilst and karason 1999 and inferred radiogenic heat source concentrations kellogg et al 1999 compositional layering would require thermal boundary layers both above and below an interface we construct a minimal 1 d model of a mid mantle boundary consistent with the observed nearly adiabatic compressional velocity structure dziewonski and anderson 1981 and the proposed high heat flow from the lower mantle albarede and van der hilst 1999 kellogg et al 1999 ray tracing and reflectivity synthetic seismograms show that a distinct triplication is predicted for short period p waves although topography on a boundary would cause uncertainty in the strength and the range of the triplication many clear observations would be expected we examine data from the us west coast regional networks in the most likely distance range of 60 to 70 for a 1770 km depth boundary and find no evidence for p wave triplications

seismic properties of the central indian shield we use broadband seismic data at hyderabad to investigate average crustal properties of the central indian shield crustal receiver functions p to sv conversions based on data of excellent quality and azimuthal coverage show essentially no signal on the transverse component indicating laterally homogeneous structures near this station a joint analysis of receiver functions and the dispersion of fundamental mode rayleigh waves result in a simple model for the shear wave speed v s with a moderate gradient of 0 010 0 028 km sec km in the crust the moho is sharp with a contrast of approximately 0 7 km sec in v s across a small thickness of only 0 4 km measured from the middle of the moho region the crust is 30 34 km thick with an average v s of 3 58 0 10 km sec by modeling the timing and waveforms of sp and sspmp phases sv to p conversions we estimate the average crustal poisson9s ratio to be 0 26 0 01 this value is comparable to that found in the tanzanian craton which is also of archean age both values being somewhat lower than the global average of 0 29 0 02 for shield regions that include both proterozoic and archean crust a low poisson9s ratio and a slow v s in the crust indicate that on average archean crust beneath the central indian shield is intermediate to felsic in composition somewhat less mafic than globally averaged precambrian crust

a general test of the hypothesis that transformation induced faulting cannot occur in the lower mantle abstract transformation induced faulting which has been proposed as a mechanism for deep focus 300 km earthquakes requires a thermal runaway associated with an exothermic chemical reaction this shearing instability has been demonstrated for several polymorphic phase transformations including the olivine wadsleyite and olivine ringwoodite reactions however it has not been experimentally tested whether a strongly exothermic decomposition reaction can also lead to this instability as would be the case if olivine were to be carried metastably into the lower mantle and break down to magnesium silicate perovskite magnesiowustite here we report experiments designed to test this possibility using the proxy reaction albite jadeite coesite a highly exothermic reaction with a large v we find no mechanical or microstructural evidence for transformation induced faulting during this reaction applying these results to the reaction olivine ol perovskite pv mw potentially occurring in subducting oceanic lithosphere at the top of the lower mantle suggests that this breakdown reaction cannot trigger earthquakes thus transformation induced faulting as a mechanism of deep earthquakes is consistent with the abrupt cessation of earthquakes at the base of the mantle transition zone but only if subduction zones are colder than currently proposed we discuss other reasons for termination of earthquakes at 700 km and find them also difficult to reconcile with experimental evidence or natural observations

sharpness of upper mantle discontinuities determined from high frequency reflections an understanding of the nature of seismic discontinuities in the earth s upper mantle is important for understanding mantle processes in particular the amplitude and sharpness of these discontinuities are critical for assessing models of upper mantle phase changes and chemical layering so far seismic studies aimed at determining the thickness and lateral variability of upper mantle discontinuities have yielded equivocal results particularly for the discontinuity at 410km depth1 2 here we present short period 0 8 2 0 s recordings of upper mantle precursors to the seismic phase p p pkppkp from two south american earthquakes recorded by the 700 station short period array in california our results show that the 410 and 660 km discontinuities beneath the indian ocean are locally simple and sharp corresponding to transi tion zones of 4 km or less these observations pose problems for mineral physics models3 5 which predict a transitional thickness greater than 6 km for the peridotite to spinel phase transition in contrast to the results of long period studies6 7 we observe no short period arrivals from near 520 km depth

elastic wave velocities of forsterite and its β spinel form and chemical boundary hypothesis for the 410 km discontinuity a method for measuring elastic wave velocities at pressures up to 15 gpa has been developed velocities of forsterite and spinel were measured using a piston cylinder apparatus with a pulse echo overlap method for pressures up to 4 gpa and using a split sphere apparatus with a pulse transmission technique for higher pressures to validate the results volume compression incompressibility and rigidity were estimated from measured travel times and comparison was made between the present estimates and data from x ray brillouin and acoustic measurements the v p and k values for spinet are 3 and 8 higher than previous measurements at 10 gpa respectively velocity jumps in v p and v s due to olivine to spinel transition were about 1 34 and 0 80 km s 1 respectively very different from seismological observations of velocity jumps at the 410 km discontinuity which were concentrated around 0 45 km s 1 for the p wave and 0 22 km s 1 for the s wave velocity jumps in mantle peridotite were calculated using the experimental results and comparison was made between these estimates and seismological observations at the 410 km discontinuity the evidence does not support the phase change hypothesis and a chemical boundary was proposed as the origin of the 410 km discontinuity an eclogitic mineral assemblage might be an alternative for the composition of the transition layer

seismic observations of splitting of the mid transition zone discontinuity in earth s mantle the transition zone of earth9s mantle is delineated by globally observed discontinuities in seismic properties at depths of about 410 and 660 kilometers here we investigate the detailed structure between 410 and 660 kilometers depth by making use of regional stacks of precursors to the ss phase the previously observed discontinuity at about 520 kilometers depth is confirmed in many regions but is found to be absent in others there are a number of regions in which we find two discontinuities at about 500 and 560 kilometers depth an effect which can be interpreted as a splitting of the 520 kilometer discontinuity these observations provide seismic constraints on the sharpness and observability of mineralogical phase transitions in the mantle transition zone

rupture characteristics of the deep bolivian earthquake of 9 june 1994 and the mechanism of deep focus earthquakes the m w 8 3 deep 636 kilometers bolivian earthquake of 9 june 1994 was the largest deep focus earthquake ever recorded seismic data from permanent stations plus portable instruments in south america show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers rupture proceeded at 1 to 3 kilometers per second along the down dip azimuth of the slab and penetrated through more than a third of the slab thickness this extent is more than three times that expected for a metastable wedge of olivine at the core of the slab and thus appears to be incompatible with an origin by transformational faulting these large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the earth s surface

the effects of selective sorting on factor analyses of heavy mineral assemblages abstract an investigation has been undertaken of the effects of selective grain sorting on the ability of factor analyses to identify source end members within heavy mineral assemblages q mode factor analysis of samples randomly collected from the beach at otter rock oregon yielded two factors one is dominated by augite hornblende and hypersthene and the second consists of garnet plus zircon these factors correspond closely to selective sorting patterns of minerals on this beach determined from a series of samples along a beach profile collected during a period of beach face erosion factor analysis of those profile samples showed that the garnet and zircon are associated with the high density opaques which tend to concentrate at the landward limit of wave run up while the lower densit augite etc are transported to the outer surf zone factor analyses were also performed on a synthetic data set which simulates the along coast mixing of mineral assemblages from four spatially distinct sources analyses involved pure mixtures of source end members mixtures where random errors were introduced to simulate counting uncertainties and mixtures of the end members where systematic changes were added to simulate the effects of local grain sorting the results indicate that sorting can significantly affect the results from factor analyses with new factors introduced and with random irregularities produced in the spatial patterns of the factors

spatial and temporal variability of titanomagnetite placer deposits on a predominantly black sand beach abstract titanomagnetite forms rich placer deposits along the northwest coast of new zealand these deposits were sampled along 3 shore normal transects spaced over the southern 2 km of a dissipative high energy beach on the west coast in 5 field campaigns covering one year the percentage of opaque minerals mainly titanomagnetite was extremely high in the upper 30 m of the beach face extending seaward where these opaque minerals were gradually replaced with variable amounts of lighter augite hornblende and plagioclase the pattern appeared to be divided into two regions a lower seaward and an upper landward region separated by a point where either marine dominated over aeolian processes or where swash dominated over breaking processes in the seaward region the percentage of opaques increased and particle size fined landward as undertow removed the lighter larger particles seaward in the landward region the percentage of opaques and particle size were more constant or even showed the reverse pattern as wind transported the lighter material shoreward or swash asymmetry transported the heavier material seaward the similarity of settling velocities over the whole beach face suggests that sorting by size rather than weight plays a dominant role in separating the mineral assemblages considerable variations existed between transects this could be explained by the spatial changes in surfzone waves and currents that were associated with proximity to the southern headland and various rip current channels that characterised this dissipative site surprisingly the percentage of opaques decreased when the wave conditions of the day of sampling were more energetic in contrast to many other placer deposits these deposits are abundant on the beach face forming an armouring layer during lower wave energy conditions during higher wave conditions the surface layer erodes allowing lighter augite plagioclase and hornblende to be released from the sediments below

propagation of back arc extension into the arc lithosphere in the southern new hebrides volcanic arc new geophysical data acquired during three expeditions of the r v southern surveyor in the southern part of the north fiji basin allow us to characterize the deformation of the upper plate at the southern termination of the new hebrides subduction zone where it bends eastward along the hunter ridge unlike the northern end of the tonga subduction zone on the other side of the north fiji basin the 90 bend does not correspond to the transition from a subduction zone to a transform fault but it is due to the progressive retreat of the new hebrides trench the subduction trench retreat is accommodated in the upper plate by the migration toward the southwest of the new hebrides arc and toward the south of the hunter ridge so that the direction of convergence remains everywhere orthogonal to the trench in the back arc domain the active deformation is characterized by propagation of the back arc spreading ridge into the hunter volcanic arc the n s spreading axis propagates southward and penetrates in the arc where it connects to a sinistral strike slip zone via an oblique rift the collision of the loyalty ridge with the new hebrides arc less than two million years ago likely initiated this deformation pattern and the fragmentation of the upper plate in this particular geodynamic setting with an oceanic lithosphere subducting beneath a highly sheared volcanic arc a wide range of primitive subduction related magmas has been produced including adakites island arc tholeiites back arc basin basalts and medium k subduction related lavas

ultramafic rocks in the centre of the vredefort structure south africa possible exposure of the upper mantle abstract the rocks in the central region of the vredefort structure are interpreted as lower crust that has undergone uplift of 36 km during the vredefort cryptoexplosion event 2 0 ga ago the outcrops in this central region are mainly high grade leucogranulites interbanded with mafic granulites and supracrustals rocks from a borehole located near the centre of the structure indicate that the core region is underlain by ultramafic rocks which probably represent the upper mantle beneath this region of the kaapvaal craton the main lithologies sampled by the borehole are serpentinized hornblende bearing harzburgite with minor bands of pyroxenite and glimmerite the major and trace element chemistry of these ultramafic rocks is similar to that of fertile oceanic lithosphere mineral chemistry and petrographic data suggest that the vredefort ultramafics are metamorphic rocks which have undergone a complex tectonic and metamorphic history sm nd and rb sr isotopic data together with the field relationships provide some evidence that these rocks are archaean but also that they have been chemically and isotopically altered similar hydrated and chemically altered ultramafic rocks occur in the jamestown ophiolite complex in the barberton greenstone belt accordingly we speculate that the crust mantle transition zone in the kaapvaal craton consists of a zone of hydrated archaean oceanic lithosphere

evidence for a metastable olivine wedge inside the subducted mariana slab abstract an anomalous seismic velocity structure is revealed inside the subducted pacific plate at northern mariana near the deepest earthquake foci we observe anomalous later phases in the p wave coda for a small cluster of deep earthquakes and an anomalous differential p wave slowness between a few pairs of the earthquakes belonging to the cluster we interpret these observations with a 5 low velocity wedge which is nearly 25 km wide at the depth of 590 km and is truncated at 630 km the wedge is bounded by two steeply dipping 60 70 transition layers with the thickness of about 4 km the highly localized seismic anomaly inside the slab represents a wedge of metastable olivine embedded in dry ringwoodite deep earthquakes occur across only one of the boundaries of the metastable wedge with some of them outside the wedge the mechanism of deep earthquakes is discussed based on the spatial pattern of seismicity relative to the wedge

subducted lithosphere or 530 km discontinuity tomographic images of the mantle west of japan and south kuril islands suggest that in this region the subducted lithospheric slabs instead of penetrating the lower mantle lie flat on top of it van der hilst et al 1991 fukao et al 1992 to test these models with independent data and techniques we analyse the slowness of the p arrivals of kuril japan seismic events at a seismic network in baikal region relative to iasp91 standard earth model the flat lying slab should generate an additional refraction line at epicentral distances less than 30 the presence of this line is very clear in our data the slope of the line and its position relative to the other lines are qualitatively consistent with the estimates of the slab velocity and thickness in the tomographic models however a similar wavefield can be generated by 530 km discontinuity the presence of which is established by long range refraction profiling in the neighbouring regions of northern eurasia

seismic structure of the upper mantle beneath the western philippine sea abstract we investigate the seismic structure of the western philippine sea using two sets of seismological observations scs reverberations which provide the layering framework for a regional upper mantle model and observations of frequency dependent phase delays for direct s waves surface reflected phases ss ss sss and surface waves r1 g1 which constrain the velocity and anisotropy structure within the layers the combined data set comprising 17 discontinuity amplitudes and layer travel times from the scs reverberation stack and more than 1000 frequency dependent phase delays was inverted for a path averaged radially anisotropic model mineralogical estimates of the bulk sound velocity and density are incorporated as complementary constraints the final model phb3 is characterized by a 11 5 km thick crust an anisotropic lid bounded by a sharp negative g discontinuity at 89 km an anisotropic low velocity layer extending to 166 km a subjacent high gradient region and transition zone discontinuities at depths of 408 km 520 km and 664 km the lid is slower than in a comparable model for the tonga hawaii corridor pa5 but also significantly thicker requiring a compositional variation between the two regions we explore the hypothesis that the thickness of the oceanic lid is controlled by the melting depth at the spreading centers during crust formation and that the thicker crust and lid in the philippine sea results from deeper melting owing to a higher potential temperature and perhaps a higher water content in the upper mantle

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

systematics of deep focus earthquakes along the kuril kamchatka arc and their implications on mantle dynamics subduction along the kuril kamchatka arc has been proposed as a primary example in which subducted lithosphere plunges steeply into the lower mantle with little deformation the occurrence of several unusual earthquakes along this arc during the last few years places new constraints on the configuration and the state of strain of the down going slab we systematically determined the source parameters of 27 large to moderate sized earthquakes mb 5 5 that occurred between 1963 and 1990 at depths greater than 200 km along this subduction zone to our surprise our results show that the configuration and strain due to earthquakes in subducted lithosphere are highly variable along both the strike and dip of the arc approximately 75 km farther west of the deepest earthquakes in the northern portion of the arc the earthquake of august 14 1988 occurred at a comparable depth of 636 17 km thus at least part of the slab has been displaced in a direction perpendicular to the overall strike of the wadati benioff zone earthquakes in the central part of the arc delineate shoaling of the subducted slab at a depth of approximately 570 km reminiscent of the configuration of subducted lithosphere along the tonga arc both observations indicate that the subducted slab is deforming probably in response to resistance encountered near the 670 km discontinuity for all earthquakes north of 48 n focal mechanisms of all events deeper than 200 km show a remarkably consistent pattern of downdip compression strain in the southern kuril the sakhalin island earthquake m0 8 1019n m of may 12 1990 is one of three large deep focus earthquakes that have occurred over 150 km farther west of the well defined wadati benioff zone the relationship between the source region of these events and the rest of the subducted slab is equivocal due to the lack of background seismicity nonetheless for earthquakes south of 48 n net downdip extension dominates between depths of 200 450 km before switching to downdip compression below 540 km thus the strain field within the slab is segmented both along strike and dip if the slab in southern kuril kamchatka can serve as a stress guide down to depths of 615 km the changeover from extension to compression occurs at an unusually large depth within the transition zone of the upper mantle alternatively the deep focus events west of the island of sakhalin could have occurred in a detached piece of lithosphere as a whole the configuration and state of strain of the subducted slab along the kuril kamchatka arc are highly variable close to a microcosm of variations among different deep subduction systems observed on a global scale in particular the subducted lithosphere is apparently undergoing considerable deformation as it approaches the base of the upper mantle our results do not preclude the possibility of subducted lithosphere ever reaching the lower mantle nevertheless it seems unlikely that the down going slab along the kuril kamchatka arc penetrates into the lower mantle unhindered

spatial variations in heavy minerals and patterns of sediment sorting along the nile delta egypt abstract sand samples were collected along 34 beach profiles spanning the 240 km length of the nile delta in order to examine the sorting patterns of heavy minerals that develop during cross shore and alongshore sediment transport factor analysis of the heavy mineral contents shows that two mineral factors or groups result from sorting due to the contrasting densities and sizes of the grains one factor is dominated by augite hornblende and epidote while the second factor consists of opaques garnet zircon rutile and monazite minerals of factor 1 are of lower densities and coarser sizes and represent the grains that are selectively entrained by waves and currents in areas of beach erosion and preferentially transported to zones of beach accretion where they are deposited the higher density minerals of factor 2 tend to become concentrated in areas of beach erosion accounting for the formation of black sand placers where the erosion has been greatest the spatial distributions of these two factors reflect the grain sorting patterns of the heavy minerals which coincide with the general trends of shoreline erosion versus accretion that exist along the length of the delta there is also a distinctive pattern of cross shore sorting with factor 2 containing the denser minerals being best developed within the inner surf zone generally with a maximum at the shoreline its development corresponds to the formation of black sand concentrates on the beach face where erosion has been greatest implying that grain sorting has taken place mainly within the swash zone there is a general decrease in factor 2 relative to factor 1 toward the offshore often with oscillations that are congruent with the beach topography factor 1 with the lower density minerals is greatest over offshore bars where deposition has prevailed

multiplicity of the 660 km discontinuity beneath the izu bonin area abstract the relatively simple subducting slab geometry in the izu bonin region provides a valuable opportunity to study the multiplicity of the 660 km discontinuity and the related response of the subducting slab on the discontinuity vertical short period recordings of deep events with simple direct p phases beneath the izu bonin region were retrieved from two seismic networks in the western usa and were used to study the structure of the 660 km discontinuity after careful selection and pre processing 23 events from the networks forming 32 pairs of event network records were processed related vespagrams were produced using the n th root slant stack method for detecting weak down going sdp phases that were inverted to the related conversion points from depth histograms and the spatial distribution of the conversion points there were three clear interfaces at depths of 670 710 and 730 km these interfaces were depressed approximately 20 30 km in the northern region in the southern region only two layers were identified in the depth histograms and no obvious layered structure could be observed from the distribution of the conversion points

discordant contrasts of p and s wave speeds across the 660 km discontinuity beneath tibet a case for hydrous remnant of sub continental lithosphere abstract using high resolution triplicate shear waveforms recorded by broadband seismic arrays we show that a corresponding anomaly of high s wave speed v s is absent where an anomaly of high p wave speed v p was recently recognized in the transition zone of the mantle tz beneath central tibet a likely cause of the discrepancy between anomalies in v p and v s is a minor amount of water in nominally anhydrous polymorphs of olivine prior to thickening by continent continent collision the tibetan mantle was part of a mantle wedge which has been hydrated during past episodes of subduction hydration of the sub continental lithospheric mantle sclm provides a natural mechanism to facilitate ductile deformation so rapid thickening of the sclm which would have been hindered by advection of cold materials can take place convective instability would then lead to removal and sinking of thickened cold sclm leaving a remnant in the tz detectable only in v p this interpretation not only is consistent with previous reconstruction of the sclm in tibet based on several types of independent data but also provides a new pathway for water to enter and be stored in the tz

thermochemistry and phase equilibria of hydrous phases in the system mgo sio2 h2o implications for volatile transport to the mantle calorimetric phase equilibrium and in situ equation of state data for antigorite talc and dense hydrous magnesium silicate dhms phases have been obtained to construct a self consistent thermodynamic data set that ensures internal consistency well behaved extrapolation and physically reasonable values of h s v k k and for talc antigorite and hydrous phase a used in conjunction with temperature profiles of subducting oceanic lithosphere the calculated phase equilibria show that there is no barrier to subducting substantial amounts of water to depth of 400 600 km in colder slabs since the slab can remain in the stability field of hydrous phases throughout its descent once water is brought to these depths it can probably diffuse into hotter mantle regions via hydrous mg2sio4 or by other mechanisms indicating that the transition zone is unlikely to be dry

evidence for a large scale remnant of subducted lithosphere beneath fiji we combine spatial variations of p and s wave speeds 1000 fault plane solutions and 6600 well determined hypocenters to investigate the nature of subducted lithosphere and deep earthquakes beneath the tonga back arc we show that perplexing patterns in seismicity and fault plane solutions can be accounted for by the juxtaposition of a steep dipping wadati benioff zone and a subhorizontal remnant of slab that is no longer attached to the actively subducting lithosphere the detached slab may be from a previous episode of subduction along the fossil vitiaz trench about 5 to 8 million years ago the juxtaposition of slabs retains a large amount of subducted material in the transition zone of the mantle such a configuration if common in the past would allow the preservation of a primordial component in the lower mantle

sh velocity and compositional models near the 660 km discontinuity beneath south america and northeast asia 1 we constrain sh wave velocity structures near the 660 km discontinuity beneath south america and northeast asia using triplicated phases near the discontinuity recorded in the epicentral distance range of 10 35 for three deep events we then explore mineralogical and compositional models appropriate for explaining the inferred seismic structures between the two regions sh velocity structures near the 660 km discontinuity are found to be different in the two regions beneath south america the velocity gradient above the 660 km discontinuity is larger than that of preliminary earth reference model prem while the velocity jump across the discontinuity is the same as prem beneath northeast asia the velocity gradient above the 660 km discontinuity is the same as that of prem while the velocity jump across the discontinuity is larger than prem both regions are characterized by a large velocity gradient extending about 80 km deep below the 660 km discontinuity the different velocity structures require different mineralogical models in the transition zone in the two regions the larger velocity gradient above the 660 km discontinuity beneath south america requires existence of the ilmenite phase in the bottom of transition zone while that beneath northeast asia can be explained by the temperature and pressure dependence of elastic properties of the major mantle mineral assemblages the observed large velocity gradients in the top of the lower mantle can be explained by gradual transformation of garnet to perovskite persisting to greater depths the velocity jump across the 660 km discontinuity beneath south america in the presence of the ilmenite phase in the bottom of the transition zone requires a bulk composition of more garnet than the pyrolite model while a larger velocity jump across the discontinuity beneath northeast asia requires a larger fraction of garnet transforming to perovskite across the discontinuity than what is required for explaining prem these different mineralogical models can be caused by different mantle temperature or composition especially the aluminum content in mantle composition the presence of garnet 80 km below the 660 km discontinuity in the two regions may be explained by a uniform composition in the lower mantle with an aluminum content of 3 4 the existence of ilmenite in the bottom of the transition zone beneath south america and the absence of ilmenite beneath northeast asia can be explained by either a difference in mantle temperature of about 100 c with that beneath south america being lower between the two regions assuming a uniform mantle composition or alternatively a difference in aluminum content of about 1 with that beneath south america being lower between the two regions without invoking a temperature difference between the two regions we also discuss conditions of mantle composition and temperature that double discontinuities may occur near the 660 km depth as well as the depth separation and velocity jumps of the double discontinuities for the inferred mantle temperature and composition beneath south america and northeast asia the maximum separation of the double discontinuities is 20 km and cannot be resolved by the sh wave data

deep earthquakes in the southwest pacific a tectonic interpretation this paper examines the spatial distribution of deep earthquakes in the tonga fiji new hebrides region of the southwest pacific our interpretation emphasizes the complex cenozoic tectonics of the pacific indo australian plate boundary as a primary control on the distribution and deformation of subducted lithosphere most deep earthquakes in the interarc region are associated with the contorted pacific plate lithosphere subducted at the tonga trench however anomalous groups of deep earthquakes located west of the tonga zone are unrelated to the present plate configuration tectonic reconstructions of the region to 8 m y b p provide circumstantial evidence that 1 the anomalous events west of the tonga zone occur in two pieces of detached lithosphere subducted at the vitiaz and proto new hebrides trenches during the late miocene 2 the flattening of the inclined seismic zone in northernmost tonga is related to the rapid opening of the lau basin since 4 m y b p and 3 the sharp westward curvature of the tonga seismic zone in this area coincides with a preexisting bend in the late miocene vitiaz arc the sharpness of its present curvature appears to be a secondary effect of shear flow in the lower mantle and compression between detached vitiaz and attached tonga lithosphere thus much of the contortion of the subducted lithosphere beneath tonga fiji may be produced by local tectonic interactions rather than collision of the slab with an impenetrable boundary in the midmantle

seismic evidence for very deep roots of continents abstract a major problem in geodynamics with seismic data is discussed how deeply do the continents penetrate into the mantle differential travel times of underside reflections from mantle discontinuities that appear as precursors to ss in large parts of the globe show a clear correlation with oceans and continents they are significantly larger beneath the asian and north american continents than underneath the neighbouring pacific from this observation we conclude that the asian and north american continents affect the mantle well below 410 km changes in the thickness of the transition zone can explain our observations which are in agreement with the hypothesis of petrological phase changes causing the 410 km and 660 km seismic discontinuities average thickness of the transition zone underneath continents is about 14 kim thicker than beneath oceans moreover our findings imply temperature variations about 100 200 k in the mantle transition zone weak reflections from 520 km depth corresponding to an impedance contrast of about 2 can be observed only in some areas of the earth while observations from other locations definitively show no signal from this depth therefore we propose that the 520 km reflector is only a regional feature

lateral variations in sh velocity structure of the transition zone beneath korea and adjacent regions 1 using seismic profiles of triplicated waveforms we show significant lateral variations in the sh velocity vs structure of the transition zone tz beneath korea and adjacent regions beneath sakhalin we detected a high vsanomaly 2 limited to middle regions of the tz mid tz and a largevsjump across the 660 km discontinuity a similar jump invs also occurs beneath the northern portion of the north china craton ncc beneath korea a high vs anomaly 2 in the lower tz is inferred accompanied by a relatively small vsjump across the 660 km discontinuity which is depressed by about 15 20 km the deep structure under the eastern part of northeast china nec also includes a slightvsanomaly 1 in the lower tz but does not exhibit significant depression of the 660 km discontinuity compared with previous study our observations reveal strong regional variations of the tz structure on a relatively short scale these variations most likely reflect the geometrical distribution of the subducting northwest pacific plate our results suggest that the subducting slab dips across the mid tz under sakhalin and becomes flattened atop of the 660 km discontinuity beneath korea while only the tip of the slab reaches the lower tz beneath the nec the tz beneath the ncc does not show evidence of the slab stagnation

effects of slab mineralogy on subduction rates although velocities of subducting slabs should be controlled primarily by their negative thermal buoyan cies their mineralogy can also have signicant eects we explore this by using thermo kinetic modeling to predict mineralogy and compare the resultant buoyant driving force to the opposing viscous drag phase transitions of mg fe 2sio4 in subducting slabs depend on thermal struc ture in two ways first equilibrium phase boundaries should be deflected causing local buoyancy anomalies whose sign depends upon that of the clapeyron slope as slabs rst enter the transition zone negative anomalies should acceler ate them but positive anomalies that arise when they fully penetrate the transition zone should slow them such ef fects may induce geologically abrupt changes in plate mo tions second olivine that persists metastably in slabs will form regions of positive buoyancy which should reduce slab velocities the coldest and fastest slabs should be slowed more greatly thus narrowing the range of feasible subduc tion rates decreased descent rates however allow slabs to warm and metastable wedges to thermally erode such neg ative feedback mechanisms may serve to regulate subduction rates

elasticity of superhydrous phase b mg10si3o14 oh 4 abstract we have used first principles simulation based on density functional theory to calculate the equation of state and elasticity of superhydrous phase b mg 10 si 3 o 14 oh 4 the pressure volume results for superhydrous phase b is well represented by a third order birch murnaghan formulation with k 0 161 8 0 2 gpa and k 0 4 4 0 01 the calculated full elastic tensor at 0 gpa is in good agreement with brillouin scattering results with the compressional elastic constants c 11 329 5 gpa c 22 294 9 gpa c 33 306 8 gpa the shear elastic constants c 44 99 8 gpa c 55 98 gpa and c 66 99 gpa the off diagonal elastic constants c 12 82 5 gpa c 13 84 6 gpa and c 23 98 7 gpa at the depths corresponding to the mantle transition zone the aggregate sound wave velocities for superhydrous phase b is slower compared to dry ringwoodite which is the dominant mineral phase however hydrous ringwoodite bulk sound velocities are comparable to that of superhydrous phase b majoritic garnet the second most abundant mineral in the transition zone has bulk sound wave velocities slower than superhydrous phase b an assemblage consisting of hydrous ringwoodite superhydrous phase b and majorite garnet could account for the low velocities observed in certain subduction zone settings at depths corresponding to the base of the transition zone and upper mantle superhydrous phase b exhibits moderate single crystal elastic anisotropy with av p 3 and av s 5 at the base of the transition zone single crystal elastic anisotropy of other dense hydrous magnesium silicate phases phase such as hydrous phase d is significantly larger at these conditions and might play a major role in explaining the observed mid mantle seismic anisotropy

the march 9 1994 m w 7 6 deep tonga earthquake rupture outside the seismically active slab we investigate the rupture process of the march 9 1994 m w 7 6 deep tonga earthquake and its relationship to the background seismicity of the subducted tonga slab variations in observed p and s wave pulse duration indicate that the rupture propagated to the nne and extended well beyond the background seismicity we inverted 47 p and sh waveforms including regional broadband waveforms from the southwest pacific seismic experiment using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane the results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10 20 km beyond the bounds of the previous seismic activity a significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region rupture along the near vertical nne striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near horizontal nodal plane this result combined with the planar alignment of aftershocks and the general nne directivity of the waveforms provides strong evidence that the rupture occurred on the near vertical plane thermal modeling of the tonga slab indicates that the rupture terminated in material about 200 c warmer than the temperature that normally limits the occurrence of smaller earthquakes additionally aftershocks seem to be suppressed in the outer regions of the rupture which contain about half of the moment release but only 1 of the 15 well located aftershocks we suggest that slabs may be composed of an inner cold core where seismic rupture initiates and small earthquakes occur and a thermal halo of warmer material which can sustain rupture and only a few aftershocks the mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core nucleation may occur through a process limited to the cold core such as transformational faulting and propagation through the warmer material may occur through ductile faulting or plastic instabilities isolated deep earthquakes in other subduction zones such as the 1994 bolivia event may occur almost completely within the warmer zone accounting for the lack of background seismicity and the dearth of aftershocks

evidence of upper mantle processes related to continental rifting versus oceanic crust in the gulf of california summary receiver functions from teleseismic events recorded by stations around the gulf of california are used to map the upper mantle seismic discontinuities we observe a mean transition zone thickness comparable to the global average for most of the region a low velocity layer is detected above the 410 discontinuity that varies in thickness along the gulf of california the 660 discontinuity shows complex waveforms south of latitude 30 n as a result of the phase change of garnet to perovskite within the transition zone a complex behaviour of the receiver functions is observed mainly at the southern end of the gulf the north south variations of this zone are likely associated with a slab window at the northern gulf resulting from the cessation of subduction of the farallon plate 12 ma and the subduction of the guadalupe and magdalena microplates at the southern end resulting in a hydrated upper mantle our results suggest that change in rifting styles occurring along the gulf of california mirrors deeper processes in the upper mantle

seismic evidence for a metastable olivine wedge in the subducting pacific slab under japan sea abstract we apply a forward modeling approach to high quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting pacific slab beneath the japan sea our results show that a finger like anomaly exists within the subducting pacific slab below 400 km depth which has a p wave velocity 5 lower than the surrounding slab velocity or 3 lower than that of the normal mantle suggesting the existence of a metastable olivine wedge mow in the slab the mow top and bottom depths are 400 and 560 km respectively the mow is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary at 560 km depth the mow is located at about 25 km below the slab upper boundary most of the deep earthquakes are located in the mow our results favor transformational faulting as the mechanism for deep earthquakes

sound velocities and elastic properties of fe bearing wadsleyite and ringwoodite the sound velocities and single crystal elastic moduli of phase wadsleyite and phase ringwoodite of mg fe 2sio4 with fe fe mg ratios of 0 075 and 0 09 respectively have been determined at ambient conditions by brillouin spectroscopy both compressional and shear wave aggregate velocities decrease with increasing fe content in both phases but the magnitude of this decrease is different for the two phases the adiabatic bulk modulus ks of fe bearing mg2sio4 ks 170 2 gpa is indistinguishable from that of the mg end member within experimental uncertainty whereas ks of mg fe 2sio4 increases rapidly with increasing iron content the shear moduli of both phases decrease with increasing fe content our measurements indicate that the velocity and impedance contrasts between olivine and mg fe 2sio4 are independent of fe content for mg rich compositions but the contrast for the mg fe 2sio4 transition increases significantly with increasing fe content the new data support a previous estimate of 40 10 for the olivine content of the upper mantle and suggest that less than 50 mg fe 2sio4 is sufficient to account for the observed impedance contrasts at depths of both 410 km and 520 km unless the effect of fe on elastic properties is accounted for it is difficult to account for both the 410 and 520 km discontinuities with a single olivine content

upper mantle sh and p velocity structures and compositional models beneath southern africa abstract we constrain sh and p wave velocity structures in the upper mantle beneath southern africa using triplicated phases recorded in the epicentral distance range of 11 28 for one shallow event we then explore thermal and compositional models appropriate for explaining the inferred seismic structures in the region both sh and p wave data suggest presence of a low velocity zone with velocity reductions of at least 5 for s waves and 2 for p waves beneath a 150 km thick high velocity lithospheric lid seismic observations also suggest that the p s ratio is larger 1 88 in the transition zone than in the lithospheric lid 1 70 the inferred p wave velocity jump across the 660 km discontinuity is small

small 660 km seismic discontinuity beneath tibet implies resting ground for detached lithosphere 1 using seismic profiles comprising of high resolution triplicate waveforms across apertures of over 1 000 km we show that because of high p wave speed vp near the bottom of the mantle transition zone the contrast in vp across the 660 km discontinuity beneath central tibet is small only about 70 of that beneath the northern indian shield this subhorizontal anomaly of high vp is most likely a remnant of detached mantle lithosphere that recently sank to depth thus providing key evidence for a direct connection between continental collision near the surface and deep seated dynamics in the mantle

the 2018 fiji mw 8 2 and 7 9 deep earthquakes one doublet in two slabs abstract the cold fiji tonga subduction zone accounts for 75 of cataloged deep earthquakes but none of the largest ten in the last century on 19 august 2018 and 06 september 2018 a deep earthquake doublet with moment magnitude m w 8 2 and 7 9 struck the fiji area providing a rare opportunity to interrogate the behaviors of great deep earthquakes in cold slabs while the aftershock productivity of the 2018 m w 8 2 event is similar to the 2013 okhotsk m w 8 3 event in a cold slab the inferred compact rupture dimensions of both the m w 8 2 and 7 9 events appear to be similar to the 1994 bolivia earthquake in a warm slab this seems to contradict the traditional view that slab temperature controls deep earthquakes however we find that neither event was confined within the cold tonga slab core the m w 8 2 ruptured mostly in the warmer rim of the tonga slab and the m w 7 9 occurred in a warm relic slab leaning on top of the tonga slab the fiji doublet demonstrates local slab temperature as the critical factor for deep earthquakes and reveals complex interaction of subducted slabs in tonga

deep earthquakes beneath the fiji basin sw pacific earth s most intense deep seismicity in stagnant slabs abstract previous work has suggested that many of the deep earthquakes beneath the fiji basin occur in slab material that has been detached and foundered to the bottom of the transition zone or has been laid down by trench migration in a similar recumbent position since nowhere else in the earth do so many earthquakes occur in slabs stagnated in the transition zone these earthquakes merit closer study accordingly we have assembled from historical and modern data a comprehensive catalogue of the relocated hypocenters and focal mechanisms of well located deep events in the geographic area between the bottoms of the main vanuatu and tonga wadati benioff zones two regions of deep seismogenesis are recognized there i 163 deep shocks have occurred north of 15 s in the vityaz group from 1949 to 1996 these seismological observations and the absence of other features characteristic of active subduction suggest that the vityaz group represents deep failure in a detached slab that has foundered to a horizontal orientation near the bottom of the transition zone ii another group of nearly 50 outboard deep shocks occur between about 450 and 660 km depth west of the complexly buckled and offset western edge of the tonga wadati benioff zone their geometry is in the form of two or possibly three small circle arcs that roughly parallel the inferred motion of tonga trench migration earthquakes in the southernmost of these arcs occur in a recumbent high seismic wavespeed slab anomaly that connects both to the main inclined tonga anomaly to the east and a lower mantle anomaly to the west van der hilst r 1995 complex morphology of subducted lithosphere in the mantle beneath the tonga trench nature vol 374 pp 154 157 both groups show complexity in their focal mechanisms the major question raised by these observations is the cause of this apparent temporary arrest in the descent of the tonga slab into the lower mantle we approach these questions by considering the effects of buoyant metastable peridotite in cold slab material that was detached and rapidly foundered or was buckled segmented and laid out in the transition zone

global de correlation of the topography of transition zone discontinuities analysis of long period shear wave underside reflections from upper mantle discontinuities shows the shape of the 410 km discontinuity to be neither globally nor locally correlated with that of the 660 km discontinuity the long wavelength pattern of 410 topography is dominated by degree 1 spherical harmonics and has a reasonable correlation with the seismic velocity perturbations in the mantle above it the global topography of 660 dominated by degree 2 spherical harmonics is strongly correlated with shear wave velocity variations in the transition zone the poor correlation between the observed topography of the 410 and 660 km discontinuities is inconsistent with a simple model of large scale whole mantle flow thus implying a more complex pattern which includes at least temporary accumulation of subducted material in the transition zone ringwood and irifune nature 331 1988 131 136 tackley et al nature 361 1993 699 704 the modest seismic discontinuity at 520 km is well observed when the reflections are under mid age oceans but is not observed when they are under continental shields interpretation of this observation in terms of olivine depletion in the deep continental mantle indicates the possible existence of continent ocean chemical heterogeneity at depths exceeding 500 km

rupture of the large mw 7 8 deep earthquake of 1973 beneath the japan sea with implications for seismogenesis the earthquake of 29 september 1973 beneath the western edge of japan sea is the largest deep earthquake along the western pacific m w 7 8 focal depth 575 km the source region is near the tip of the japan wadati benioff zone where a continuous belt of seismicity clearly defines a unique shallow dipping 30 slab down to depths about 600 km this unique geometry of slab predicts a limited vertical extent available for earthquake rupture in direct conflict with previous reports of a large subvertical fault for the 1973 earthquake using a comprehensive data set of nearly 20 p and sh waveforms recorded at teleseismic distances we investigate rupture associated with this event in some detail by inversion of waveforms in particular we use wave trains with all relevant depth phases pp sp and ss that are crucial for resolving the vertical extent of rupture in addition to a northward component of rupture as in previous reports we find clear evidence for a significant component of eastward rupture propagation that cannot fit within a subvertical north south striking nodal plane meanwhile all regions of major slip are at essentially the same depth thus the true fault plane appears subhorizontal with the entire region of slip extending only about 15 6 km across the slab9s thickness the spatio temporal distribution of slip is well explained by a simple circular rupture front propagating at a speed of 3 4 km sec a comparison among the largest deep earthquakes shows that most rupture planes are more or less dimensionally equal with high aspect ratios ranging approximately from 0 5 to 1 while some events have fairly restricted source regions others seem to extend more than 30 km into the thickness of the slabs

when slabs collide a tectonic assessment of deep earthquakes in the tonga vanuatu region an unusual group of more than 100 earthquakes is located 600 km beneath the north fiji basin in the southwest pacific ocean these earthquakes are attributed to seismicity within detached segments of the subducted australian plate one of these detached slab segments has collided with and now impinges on the subducted pacific plate at a depth of 500 km the region in the mantle where the two slab segments meet is also characterized by an unparalleled abundance of large magnitude mw 7 0 earthquakes for this depth the folded shape of the pacific slab beneath fiji as well as the abundance of earthquakes is interpreted to result from deformation and deformation enhanced phase transformations as the two slabs collide and settle on the 660 km discontinuity detachment of the slab segments is interpreted to have occurred at ca 5 ma but collision between the eastern segment and the west dipping pacific slab is interpreted to have occurred at ca 4 ma coincident with initial opening of the overriding lau basin

concave slab out board of the tonga subduction zone caused by opposite toroidal flows under the north fiji basin abstract an alternative scenario is proposed for the origin of a concave ne facing slab under the north fiji basin between the tonga subduction slab and vanuatu arc during rollback of the australian plate vanuatu arc rotated clockwise whereas fiji platform rotated counterclockwise from 12 10 ma until 1 5 ma ago thereafter only vanuatu arc rotated until the present during the period of opposite rotations toroidal flows entered the mantle around the northwest slab edge of vanuatu arc and from the northeast slab edge of fiji platform the latter lies close to the northern end of the tonga slab where arc parallel flows are shown by volcanic geochemistry and mantle anisotropy opposite toroidal flows with upwelling and downwelling components generate the concave form of the combined vanuatu fiji platform slab match its extent to mapped deep seismicity explain its position overlying the tonga slab provide a mechanism for high heat flow in the north fiji basin as well as enriched morb and oib basalts in the northern nfb and obviate slab collisions invoked to produce slab curvature

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

dynamical and thermal structure of a subduction zone influence of slab geometry on the convective state of the earth s upper mantle preliminary results received 29 march 1996 accepted 3 july 1996 abstract the aim of this study is to investigate the influences of plate kinematics and of the downgoing slab s geometry on internal dynamics and thermal structure in the vicinity of a subduction zone a systematic study depending on these parameters has been carried out based on a two dimensional finite differences code in a convective domain representing the upper mantle plate displacements are prescribed by kinematic boundary conditions in particular the subducting plate moves and dips at a constant velocity under the slower overriding one typical subduction dips 30 45 and 70 have been studied for a realistic rayleigh number of about l05 and for plate velocities around a coupling velocity of 2 3 cm year 1 this limit value was inferred from the balance between surface forces and buoyancy forces for any subduction dip and plate velocities lower than this coupling velocity plate kinematics variations induce significant effects on thermal and mineralogical patterns at greater plate velocities these patterns stabilize for plate velocities of the order of the coupling velocity a subduction dip decrease of 40 involves a heating of the downgoing plate of 100degrees and a depth increase of 20 km of the olivine spinel phase transition in this plate in the particular case of a non moving overriding plate the balance between this plate and underlying internal dynamics requires a moving plate velocity lower than 2 3 cm year j for a lower dip moreover a shallow subduction enhances a lateral large scale convection under the upper plate even if this plate is motionless in contrast to the case of a steep subduction this is consistent with observed back arc extension at the surface which is often associated with steep subductions our model suggests a spreading zone of between 500 and 1000km in the upper plate for a subduction velocity lower than 2 3 cm year i and a rayleigh number of about l05 1 introduction the earth s mantle undergoes a rayleigh benard thermal convection coupled to oceanic lithospheric plate movements rigid and moving lithospheric corresponding author plates form the thermal and mechanical limit bound aries of the earth s mantle convection cells one of the concerns of numerical modelling is to account for a real dynamic convection model coupled to plate tectonics to force the movement of rigid plates one may either model a dynamic plate by prescribing a lower viscosity at plate margins or impose the horizontal surface velocity davies 1989 0031 9201 97 17 00 copyright 1997 elsevier science b v all rights reserved ph s0031 9201 96 03209 8

elastic properties of hydrous ringwoodite γ phase in mg2sio4 abstract single crystal elastic properties of hydrous ringwoodite are reported for samples that were synthesized at 19 gpa and 1300 c the mg si ratio is determined to be 1 95 by epma which is slightly lower than that of ideal anhydrous ringwoodite the h 2 o content determined by sims is 2 2 wt resulting in the stoichiometry mg 1 89 si 0 97 o 4 h 0 33 the lattice parameter a 8 0786 4 a and elastic constants were determined from a single crystal with x ray diffractometry and brillouin scattering spectroscopy the crystal structure is cubic fd3m with a unit cell volume which is 0 51 larger than that of the anhydrous ringwoodite the adiabatic single crystal elastic moduli of hydrous ringwoodite in gpa are c 11 281 6 c 44 117 4 c 12 92 5 the isotropic properties of hydrous ringwoodite are k vrh 155 4 and g vrh 107 3 which are about 16 and 10 smaller than those of anhydrous ringwoodite thus hydrous ringwoodite if present drastically reduces the seismic velocity in the mantle transition zone relative to anhydrous ringwoodite in addition our results show that the anisotropy is enhanced relative to anhydrous ringwoodite using the present results we discuss the 520 km seismic discontinuity in a wet mantle transition zone

the fate of descending slabs oceanic lithosphere the stiff plate formed by the cooling of chemically diffcrentiated material that rises and mclts under mid ocean ridges and spreads laterally in the process called sea floor spreading has an ephemeral residence at the surface of the earth typically within 100 200 ma densi fication of the old oceanic lithosphere leads to a dynamic instability causing it to sink into the interior descending as a layered slab with highly anom alous thermal and chemical characteristics relative to the surrounding ambient mantle the slab heats as it sinks but the warming is sluggish slow to overcome the effect of more than 100 ma of cooling at the surface and the negative buoyancy of the slab continues to drive its descent increasing pressure and temperature with depth drive various chemical reactions in the slab including expulsion of water and other volatiles accumulated at the surface and destabilization of associated hydrous phases suites of mineralogical phase transformations occur within the slab s crustal and depleted mantle components because the slab has been processed through the melting zone at the ridge partially hydrated and cooled significantly the phase equilibria within the slab differ from those of the surrounding mantle near 400 km depth an exothermic phase transformation occurs with mg fe 2si04 olivine in the slab converting to the high pressure modified j spinel structure the spinel transition helps to drive the slab onward although a central core of metastable olivine may persist to greater depths with eventual transformational faulting

single crystal elasticity of γ mg0 91fe0 09 2sio4 to high pressures and to high temperatures sound velocities and elastic moduli have been measured on mg0 91fe0 09 2sio4 ringwoodite single crystals to 16 gpa and to 923k by brillouin spectroscopy a third order finite strain fit to the data yields pressure derivatives of 4 1 2 and 1 3 1 for the adiabatic bulk modulus ks and shear modulus respectively mg rich and mg fe 2sio4 have identical values of ks p and of p within the experimental uncertainties ks t and t for mg fe 2sio4 are 0 021 and 0 016 gpa k respectively our results indicate that the shear and compressional impedance contrasts associated with mg fe 2sio4 transition are sufficient to produce a visible discontinuity at 520 km even with a moderate 30 50 amount of olivine the generally low visibility of the 520 km discontinuity is likely due to its width and large amplitude topographic variations

contrasts in seismic wave speeds and density across the 660 km discontinuity beneath the philippine and the japan seas 1 using triplicate waveforms we construct 34 seismic profiles east of taiwan to investigate lateral variations of p and s wave speeds vp and vs near the mantle transition zone data from broadband and densely spaced short period arrays are combined to achieve high resolution we find correspondingly high vs in the back arc of izu bonin where the northern philippine sea anomaly npsa was previously delineated in the lower portion of the transition zone by an average vp that is 1 5 0 5 higher than the surrounding regions high vs of the npsa reduces the contrast in vs across the 660 km discontinuity vs to about 3 9 consistent with the notion that npsa reflects a subhorizontal remnant of cold pacific slab resting immediately above the lower mantle the aseismic npsa contrasts the fiji tonga region where many outboard earthquakes low vp and vs and seismic anisotropy mark a large scale petrologic anomaly the contrasting pattern implies that the npsa is no longer cold enough to preserve materials such as metastable olivine or volatiles that can trigger earthquakes reduce vp and vs and cause anisotropy in regions surrounding the npsa vs is well constrained to be 5 9 0 5 this precise result and known impedance contrast 7 under the japan sea lead to a density contrast of about 8 circumventing the trade off between and vs the magnitude of is higher than recent estimates 5 without independent control on vs but close to longstanding global models implying considerable resistance to slab penetration into the lower mantle north of the npsa

earthquakes and strain in subhorizontal slabs 1 using an extensive database of fault plane solutions and precise locations of hypocenters we show that the classic patterns of downdip extension dde or downdip compression ddc in subduction zones deteriorate when the dip of the slab is less than about 20 this result is depth independent demonstrated by both intermediate focus depths from 70 to 300 km and deep focus depths greater than 300 km earthquakes the absence of pattern in seismic strain in subhorizontal slabs also occurs locally over scales of about 10 km as evident from a detailed analysis of a large mw 7 1 earthquake sequence beneath fiji following the paradigm that a uniform strain of dde ddc results from sinking of the cold dense slab as it encounters resistance from the highly viscous mantle at depth breakdown of dde ddc in subhorizontal slabs reflects waning negative buoyancy slab pull in the downdip direction our results place a constraint on the magnitude of slab pull that is required to dominate over localized sources of stress and to align seismic strain release in dipping slabs under the condition of a vanishing slab pull eliminating the only obvious source of regional stress the abundance of earthquakes in subhorizontal slabs indicates that a locally variable source of stress is both necessary and sufficient to sustain the accumulation of elastic strain required to generate intermediate and deep focus seismicity evidence is growing that the process of seismogenesis under high pressures including localized sources of stress is tied to the presence of petrologic anomalies

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

a petrologic anomaly accompanying outboard earthquakes beneath fiji tonga corresponding evidence from broadband p and s waveforms 1 using high resolution triplicate body waveforms we investigate strong lateral variations in p and sh wave speeds vp and vs beneath the tonga subduction zone where cold lithosphere is subducting rapidly at 200 mm yr 1 under these conditions the lack of prominent anomalies of high vp and vs in the lower mantle implies that a large amount of recently subducted material remains above the lower mantle this inference is also evident from a subhorizontal swath of outboard earthquakes in the transition zone that is up to 1000 km away from the wadati benioff zone beneath the back arc we find a sharp east west trending demarcation north of which outboard earthquakes occur and vp and vs abruptly drop by 3 thus the effect of cold temperature to raise vp and vs as indicated by the presence of outboard earthquakes must be counteracted by petrologic variations we interpret that outboard earthquakes occur where impounding of subducted metastable olivine or volatiles reduces seismic wave speeds and triggers deep earthquakes considering the density of these candidates and the strong effect of partial melt on vs volatiles in the form of hydrous phases or melt induced by dehydration are not consistent with observed reductions in vp and vs meanwhile metastable olivine remains a viable candidate as a cold slab passes below the 410 km discontinuity olivine becomes metastable and makes the tip of the slab buoyant such a mechanism could explain why the leading end of a fast subducting slab may be successively sequestered in the transition zone

seismic anisotropy in the mantle transition zone beneath fiji tonga 1 we demonstrate a rare unequivocal case of anisotropy in the transition zone tz evident from many horizontal polarized shear waves sh that arrive up to 3 s earlier than vertical polarized shear waves sv the anisotropic region correlates with deep earthquakes away from the active subduction zone and known boundaries of lateral heterogeneities in compression and shear wave speeds we interpret the anisotropy as strong fabric associated with a large petrologic anomaly in the core of remnant lithosphere which is buoyant and stagnates in the tz fast subduction of cold slab actually favors such a configuration so slab penetration into the lower mantle does not uniformly occur

synthetic seismogram images of upper mantle structure no evidence for a 520 km discontinuity seismological data used by shearer 1990 1991 to infer the existence of a seismic discontinuity at 520 km depth are compared with complete long period bodywave seismograms calculated for the international association of seismology and physics of the earth s interior 1991 iaspei9l earth model the iaspei9l model does not contain a seismic discontinuity at or near 520 km depth the observed p and sh multiples caused by topside reflections and ss precursors caused by underside reflections from the 410 km and 660 km discontinuities are well reproduced by the synthetic stacks the synthetics exhibit clear phase extrema between these reflections that correlate well with similar features in the observed waveform stacks

phase transition buoyancy contributions to stresses in subducting lithosphere the sequence of phase transitions undergone by minerals with increasing depth in earth s mantle is perturbed within subducting lithospheric slabs by their thermal structure such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab while other factors contribute to overall slab stresses thermal and phase transition effects largely control the structure of the stress field within the slab the resulting maximum in down dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity furthermore metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity these observations are independent of the variety of failure mechanisms proposed for deep seismogenesis

formation of a metastable olivine wedge in a descending slab we present a thermal model of a descending slab in which the transformation of olivine to spinel is controlled by pressure and temperature dependent reaction kinetics two different formulations of the kinetics are considered with the main discriminant being the temperature range over which olivine converts to spinel at pressures of about 15 gpa about 500 515 c and 560 650 c we use a finite element method to solve the coupled heat conduction perpendicular and parallel to the dip of the slab and kinetics equations and we include the latent heat of the phase transformation latent heat release together with heat conduction parallel to the dip of the slab reduces significantly the length of the metastable olivine wedge and results in a very thin 5 km two phase region we employ the thermal parameter sin is the velocity of the descending slab is the age of the slab and is the dip of the slab to the horizontal to interpret the results for the length of the metastable wedge for values of the thermal parameter smaller than about 4000 and 7000 km depending on the model of the kinetics no metastable olivine wedge exists the critical value of 7000 km for the thermal parameter corresponds to the kinetics model with the lowest transition temperature range the length of the metastable olivine wedge is also found to be very sensitive to the model of the kinetics and to the effects of adiabatic heating if the occurrence of deep earthquakes is related to the transformation of metastable olivine to spinel then data on earthquake depth versus thermal parameter require that the onset of the reaction takes place at temperatures of about 550 575 c in this case the slab thermal parameter must be larger than 10 000 km for the metastable olivine wedge to extend down to 660 km depth but deep earthquakes occur near 660 km depth in slabs with thermal parameter as small as about 5000 km south america for example either some deep earthquakes are unrelated to olivine metastability or our knowledge of olivine spinel reaction kinetics is incomplete

time dependent interaction between subduction dynamics and phase transition kinetics summary numerical subduction models are presented that account for the kinetics of the olivine spinel transformation and the equilibrium transformation of spinel into perovskite and magnesiowustite temperature depth and stress dependent rheology is used buoyancy due to temperature and the olivine spinel and magnesiowustite perovskite phases leads to free subduction into the transition zone and retarded penetration into the lower mantle a dynamic feed back mechanism is found between the different buoyancy forces the kinetics of the phase transition and the release of latent heat this mechanism modulates the parachute effect of the metastable olivine mo in a subducting slab and leads to characteristic temporal variations of the depth of mo and with a time delay variations of the subduction velocity time periods are of the order of 3 4 myr this mechanism allows to explain variations of the depths of the deepest earthquakes in otherwise similar subduction zones further conclusions are as follows the maximum depth of mo increases roughly linearly with lithospheric thickness and the maximum depth may reach as deep as 720 750 km different subduction zones may represent different stages of velocity depth phase loops of mo

olivine flotation in mantle melt abstract molten komatiite and peridotite have been compressed in an octahedral multi anvil device up to 10 gpa densities of the melts were measured at pressure intervals in the range 7 to 10 gpa by observing sinking and floating san carlos olivines and synthetic forsterite marker spheres the multi anvil results for komatiite when combined with piston cylinder measurements done at 4 to 6 gpa and a calculated reference density at 105 pa yield a birch murnaghan isothermal bulk modulus of k1900c 26 gpa and pressure derivativek 4 25 the pressure of neutral buoyancy for olivine in komatiite is confirmed to be near 8 gpa as predicted in earlier work olivine flotation in the experimental komatiite commences at a pressure close to where the liquidus phase changes from olivine to denser garnet leading to the possibility of density driven crystal sorting during fractionation molten peridotite klb 1 shows an isothermal compression 2000 c of 0 065 g cm 3 gpa 1 in the interval 105 pa to 8 2 gpa the olivine liquid peridotite density crossover is predicted to lie between 9 and 11 gpa indicating that olivine flotation can operate at depths of 300 500 km in a molten peridotitic mantle

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

detection of metastable olivine wedge in the western pacific slab and its geodynamic implications abstract seismic tomography and numerical simulations show that the western pacific slab bends horizontally when it reaches the boundary between the upper mantle and lower mantle beneath northeast asia it is expected that a metastable olivine wedge mow exists in the cold core of the slab because of a delayed phase transition from olivine to its high pressure polymorphs however it is still debated whether the mow actually exists or not and even if it exists its physical properties such as seismic velocity and density are still unclear in this work we use high quality arrival time data of 17 deep earthquakes occurring within the pacific slab under northeast asia to study the detailed structure of the slab the deep earthquakes are relocated precisely by applying a modified double difference location method to arrival time data recorded at both chinese and japanese stations based on the precise hypocentral locations a forward modeling method and differential travel time residuals data are used to estimate seismic velocity within the deep source zone which can decrease or remove the influence of ambient velocity heterogeneities our results show that the mow does exist within the pacific slab under northeast asia and the mow has a mean velocity anomaly of 7 9 lower than the iasp91 earth model the existence of mow in the slab has important geodynamic implications it can reduce the speed of slab subduction and affect the generation of deep earthquakes

variations in p wave speeds and outboard earthquakes evidence for a petrologic anomaly in the mantle transition zone we investigate the nature of laterally varying p wave speeds vp and their relationship to a group of unusual deep earthquakes beneath the backarc of tonga this subhorizontal swath of outboard earthquakes occurs within the transition zone of the mantle but is several hundred kilometers away from the inclined wadati benioff zone surprisingly the seismogenic region is not associated with fast vp it follows that the effect of cold temperature as implied by the presence of large earthquakes must have been counteracted by other factors such as compositional or mineralogical effects our results are based on a sequence of seismic profiles in a fan shot geometry for a given azimuthal sector we seek the simplest model of radially varying vp that explains observed broadband waveforms and travel times the lateral extent of anomalous vp is then constrained by comparing the results from a wide range of azimuths this approach fully utilizes information from triplicate p arrivals that are sensitive to seismic wave speeds at the depths of interest our results identify prominent anomalies of fast vp 3 over average earth models both in the transition zone and in the lower mantle beneath the tonga backarc however the region of fast vp in the transition zone is offset by 300 km to the south of that in the lower mantle and the region of offset coincides with the outboard earthquakes a straightforward interpretation is that a large flux of subducted pacific lithosphere lingers in the transition zone before descending into the lower mantle within this broad remnant of subducted lithosphere the outboard earthquakes occur in a region where impounding of subducted material is most significant a condition favorable for the accumulation of met astable olivine or volatiles such materials reduce vp and are candidates for triggering earthquakes thus accounting for the absence of fast vp in the source zone of deep earthquakes

a graveyard for buoyant slabs geoscientists have been debating whether convection deep inside earth involves the whole mantle or is divided into two layers separated at 660 km depth compromise models suggest delayed penetration of the transition zone due to a phase change in his perspective green highlights a study by chen and brudzinski who have found evidence for a buoyant cold slab in the transition zone between the upper and the lower mantle the slab appears to be retained in the transition zone because of mineralogical differences between its interior and its surroundings

the metastable olivine wedge in fast subducting slabs constraints from thermo kinetic coupling abstract the effects of a strong non linear thermo kinetic coupling in the olivine spinel transition are investigated with a 2d model for fast subducting slabs the thermo kinetic coupling problem is solved in the lagrangian framework by the differential algebraic method and by using experimental input parameters narrow regions with metastable olivine are found in fast and cold slabs down to the 660 km discontinuity while the thermo kinetic coupling reduces the depth and width of the zone where olivine and spinel coexist in contrast to recent kinetic models both latent heat release and heat diffusion place important constraints on the structure of the metastable wedge both processes trigger the ongoing phase transformation in the cold slab interior a thermal instability over a short timescale 10 4 10 5 yr can occur due to a positive thermo kinetic feedback this effect is enhanced at greater depths the depth of the metastable wedge is found to be extremely sensitive to certain geophysical and thermodynamic parameters such as the lithospheric basal temperature slab width and the activation energy for diffusion variations in these parameters within the experimental uncertainties can cause significant changes in the depth of the metastable wedge hence predictions about the exact position of the metastable olivine wedge are rather difficult we propose that deep focus earthquakes may occur due to a thermal runaway caused by shear heating while the thermal instability is enhanced by the latent heat release associated with the olivine spinel phase transition

short period observation of the 520 km discontinuity in northern eurasia short period three component recordings of the seismic wave field of peaceful nuclear explosions pnes along several long range profiles in northern eurasia are analyzed to study the structure of the upper mantle we use the data to search for evidence of the disputed 520 km discontinuity in the mantle transition zone here we present short period evidence for the existence of this proposed mantle discontinuity on three long range profiles in siberia we observe in the distance range from 2400 to 2600 km a typical change of the apparent velocity of the first arrivals and the appearance of two characteristic travel time branches we interpret these phenomena as being caused by the proposed upper mantle discontinuity to determine the optimum one dimensional p wave velocity model synthetic seismograms were calculated with the reflectivity method by comparing them with the observed data we estimate the p wave velocity contrast across the 520 km discontinuity to be 0 25 0 05 km s while the velocity gradient below the boundary is 0 001 0 0005 1 s this result indicates that in the mantle transition zone both phase transitions and variations in chemical composition can occur

mineralogic evidence for the remnant sebennitic promontory on the continental shelf off the central nile delta abstract heavy mineral compositions of closely spaced bottom samples off the central nile delta are evaluated to determine provenance dispersal patterns and sediment sources of a former branch the sebennitic q mode factor analysis applied to heavy mineral data of 83 samples yields two factors factor 1 augite and hornblende and factor 2 opaques garnet zircon monazite rutile epidote the two factors correlate negatively with each other the observed mineralogic pattern is attributable to selective transport processes of the sediments related to a former subdelta they probably originated from extensive progradation of the remnant promontory of the extinct sebennitic branch which flowed across the subaerially exposed continental shelf during the last holocene sea level low stand grain sorting patterns of heavy minerals provide additional proof for the presence of a large sebennitic channel extending across the continental shelf off the central nile delta

rates and mechanisms of shoreface progradation and retreat downdrift of the amazon river mouth field surveys of the 350 km shoreline adjacent to the amazon river mouth reveal three distinct types erosional mud accretionary sand and accretionary mud formation of these zones is controlled by the delivery of amazon suspended sediment mediated by the hydrodynamic regime erosional mud shorelines extend from rio araguari near the amazon river mouth northwestward 280 km to 3 5 n near cabo cassipore shoreface 5 m water depth retreat annually yields 1 4 106 tons of fine grained sediment deposited during an earlier phase of shoreface progradation sandbodies up to 5 m thick overlie erosional mud shorefaces for 10 30 km downdrift of small rivers the sand is supplied from these rivers and released by shoreline retreat of the river mouth areas amazon river sand is restricted in the coastal zone from its mouth north to the cabo norte shoal area where it mixes with sand carried by rio araguari from the guiana highlands north of 3 5 n mud aggradation and progradation is taking place on underconsolidated low gradient tidal flats backed by mangrove swamps 210pb and 14c geochronology of vibracores from the mudflats indicate that sediment accumulation is rapid 0 24 2 0 cm yr landward of the 2 m isobath supplied from a thick 50 150 cm seasonal surface layer shoreface progradation is episodic separated by decadal hiatuses fine grained suspended sediment flux from the amazon and minor amounts of sand and mud from the local rivers supply sediment to the mudflats shore normal tidal currents and solitary waves rework the surface mud layer preferentially transporting available sand landward into the mangrove fringe and producing very fine grained accumulation on the tidal flat 10 12 mean grain size lateral accretion of features 10 100 km long termed mudcapes produces tens of kilometers of seaward coastal plain addition along the northern coast similar features are identified downdrift along the guianas coast as far as the orinoco river 1600 km the northern amapa shoreface deposits are a locus of modern sediment accumulation which progrades over subaqueous deltaic strata

a broad 660 km discontinuity beneath northeast china revealed by dense regional seismic networks in china 1 we examined the p wave velocity structure around the 660 km discontinuity at the tip of the subducting pacific slab beneath northeastern china by forward modeling waveform triplication data a total of 742 broadband seismograms were recorded by dense regional seismic networks in china from a deep earthquake that occurred near the border of east russia and northeast china providing an unprecedented density of ray coverage near the front edge of the subducting pacific slab multiple p waves were observed on single seismograms in the distance range of 14 29 the p wave triplication shows the following two features 1 the direct arrival traveling above the 660 km discontinuity ab branch extends as far as 29 approximately 6 further than the prediction from velocity model of international association of seismology and the physics of the earth s interior iasp91 2 the refracted wave propagating through the lower mantle cd branch appears at a distance a few degrees greater than that of the iasp91 synthetics forward waveform modeling suggests that they are best explained by a high velocity transition zone underlain by a 50 70 km thick 660 km discontinuity the broadened discontinuity is likely caused by multiple phase transitions associated with the dissolutions of olivine and garnet components

p and sh velocity structure in the upper mantle beneath northeast china evidence for a stagnant slab in hydrous mantle transition zone abstract using high dense regional body waves for three deep earthquakes that occurred around russia china border we investigate both s and p wave velocity structures in the mantle transition zone beneath northeast china and northern part of north china craton where the northwestern pacific plate is imaged to subhorizontally lie above the 660 km discontinuity we observe an increasing trend of s p travel time residuals along the epicentral distance within a distance range of 11 16 5 indicating a velocity anomaly in mtz we seek the simplest model that explains the observed broadband waveforms and relative travel times of triplication for a confined azimuth sector both sh and p data suggest a 140 20 km high velocity layer lying above a slightly depressed and broad 660 km discontinuity shear velocity reduction of 2 5 in the deeper part of the transition zone is required to compensate for the significantly large relative time between ab and cd triplicate branches and the increased trending of s p travel time residuals as well the mtz as a whole is featured by low shear velocity and high v p v s ratio a water rich mantle transition zone with 0 2 0 4 wt of h 2 o may account for the discrepancy between the observed v p and v s velocity structures our result supports the scenario of a viscosity dominated stagnant slab with an increased thickness of 140 km which was caused by the large viscosity contrast between the lower and upper mantles the addition of water and eastward trench retreat might facilitate stagnation of the subducting pacific slab beneath northeast china

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

mapping the tonga slab we applied residual sphere analysis to 25 intermediate and deep focus earthquakes in the tonga subduction zone and mapped variations in slab strike and dip depth extent and deep lateral advection at all latitudes along the arc as in the northwest pacific travel time data from intermediate and deep focus events are well modeled by simple thermal slab structures and a thermal coefficient of velocity p t of 0 5 0 2 m s k i e peak velocity perturbations of about 3 7 in all regions where the seismicity extends to depths of 600 km or more the shortest thermal slab models that cannot be rejected at the 90 confidence level reach depths of 750 km northern tonga to 850 km southern tonga these models however require a value of p t 1 5 m s k 3 times the value suggested by laboratory studies if the magnitude of p t is constrained to be less than 1 0 m s k the minimum penetration depth varies from 800 km to 900 km the travel time data are equally consistent with slab structures that deform at the base of the upper mantle thickening by a factor of 3 and models that contain no advective thickening inferred slab structure varies considerably throughout the subduction zone and the nearly ubiquitous match between the observed seismicity and detailed undulations in slab strike and dip is evidence that residual spheres do reflect near source heterogeneity travel times in southern tonga 21 5 s 27 s are consistent with and in some cases require slab models that bend to a steeper dip at depth this phenomenon is also observed in parts of the northwest pacific but is not required in central 19 s 21 5 s and northern 13 s 19 s tonga the residual spheres of both intermediate and deep focus events suggest a sharp bend in slab strike at 25 s this feature coincides with a kink in strike observed at all depths in the seismicity and occurs at the intersection of the trench and the louisville ridge northern tonga is the locus of considerable complexity in deep slab structure seismicity strain rate and travel time data are consistent with although they do not uniquely require a slab structure in northern tonga which curves sharply to the west at the base of the upper mantle but which strikes to the north at shallower depths and in the lower mantle the contorted segment would have begun subducting roughly 10 m y ago during a major plate reorganization in northernmost tonga a decrease in the maximum depth of seismicity and inferred slab structure provides evidence for southward shear of deep material relative to the shallow slab compared to data from the northwest pacific where travel time observations also suggest slab penetration below the deepest earthquakes tonga seismicity and residual spheres show more complexity at depth and may indicate more deformation internal to the slab these observations argue against convective models which are rigorously stratified at 650 km depth but they are consistent with a discontinuity in mantle properties such as an increase in viscosity or a small percent variation in chemistry if it merely resists and deforms the descending slabs without acting as a complete barrier to flow