finite fault source study of the great 1994 deep bolivia earthquake we apply an empirical green s function method to recordings of teleseismic body waves from the june 9 1994 deep focus bolivia earthquake the far field source time functions reveal that rupture occurred on a sub horizontal fault plane the main rupture began some 25 km to the east of the hypocenter consists of three large subevents and covers a total area of about 60 40 km the static stress drop is quite large more than 150 mpa and the obtained rupture velocity is less than 2 km sec while the total dimensions of the earthquake are large compared to estimates of the width of a metastable olivine wedge at this depth the moment release is confined largely to a narrow band along an ew axis and may be compatible with phase transitions within a thickened slab core

tectonic and metasomatic mixing in a high t subduction zone melange insights into the geochemical evolution of the slab mantle interface abstract the catalina schist california contains an amphibolite grade 0 8 1 1 gpa 640 750 c melange unit consisting of mafic and ultramafic blocks in high mg schistose melange matrix with varying modal proportions of talc chlorite anthophyllite calcic amphibole enstatite and minor phases including zircon rutile apatite spinel and fe ni sulfides this melange unit is interpreted as a kilometer scale zone of tectonic and metasomatic mixing formed within a juvenile subduction zone the study of which may yield insight into chemical mixing processes at greater depths in subduction zones relationships among the major and trace element compositions of the mafic and ultramafic blocks in the melange the rinds developed at the margins of these blocks and the surrounding melange matrix are compatible with the evolution of the melange matrix through a complex combination of infiltrative and diffusional metasomatism and a process resembling mechanical mixing simple linear mixing models are compatible with the development of the melange matrix primarily through simple mixture of the ultramafic and mafic rocks with cr al ratios serving as indicators of the approximate proportions of the two lithologies this conclusion regarding mafic ultramafic mixing is consistent with the field observations and chemical trends indicating strong resemblance of large parts of the melange matrix with rinds developed at the margins of mafic and ultramafic blocks the overall process involved development of metasomatic assemblages through complex fluid mediated mixing of the blocks and matrix concurrent with deformation of these relatively weak rind materials which are rich in layer silicates and amphibole this deformation was sufficiently intense to transpose fabrics progressively disaggregate more rigid block derived materials in weaker chorite and talc rich melange and in some particularly weak lithologies e g chlorite talc and amphibole rich materials intimately juxtapose adjacent lithologies at the sub cm scale approaching grain scale sampled by the whole rock geochemical analyses chemical systematics of various elements in the melange matrix can be delineated based on the cr al based mixing model simple mixing relationships exhibited by al cr mg ni fe and zr provide a geochemical reference frame for considerations of mass and volume loss and gain within the melange matrix the compositional patterns of many other elements are explained by either redistribution local stripping or enrichment at varying scales within the melange ca na k ba and sr or massive addition from external sources si and h2o the latter probably in infiltrating h2o rich fluids that produced the dramatic o and h isotopic shifts in the melange melange formation resulting in the production of high variance ultramafic assemblages with high volatile contents may aid retention of volatiles in this case h2o to greater depths in subduction zones than in original subducted mafic and sedimentary materials the presence of such assemblages i e containing minerals such as talc chlorite and mg rich amphiboles would impact the rheology of the slab mantle interface and perhaps contribute to the low velocity seismic structure observed at near the slab mantle interface in some subduction zones if operative along the slab mantle interface complex mixing processes such as these involving the interplay between fluid mediated metasomatism and deformation also could impact slab incompatible trace element and isotopic signatures ultimately observed in arc magmas producing fluids with geochemical signatures inherited from interactions with hybridized rock compositions

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

the eclogite garnetite transformation at high pressure and some geophysical implications mineral assemblages displayed by morb and alkali poor olivine tholeiites have been investigated over the pressure interval 4 6 18 gpa at 1200 c both compositions crystallize to form normal eclogites between 4 6 and 10 gpa and there is little change in the relative proportions of garnet and pyroxene over this range however the proportion of garnet increases rapidly above 10 gpa as pyroxene dissolves in the garnet structure and pyroxene free garnetites stishovite are produced by 14 15 gpa dependent upon composition the garnetite facies for both compositions possess zero pressure densities of 3 75 g cm3 implying that subducted oceanic crust remains appreciably denser than surrounding mantle to depths exceeding 600 km it is demonstrated that the seismic velocity distributions in the mantle between 400 and 650 km are inconsistent with anderson s hypothesis that this region is of eclogitic composition

determining the origin of ultrahigh pressure lherzolites the occurrence of coesite and diamond in regional ultrahigh pressure metamorphic rocks requires that rocks formed at pressures of 3 to 4 gpa 100 to 120 km below the earth s surface have been transported back to the surface 1 1 l dobrzhinetskaya et al suggest that garnet lherzolite from

dispersion of p waves in subducted lithosphere evidence for an eclogite layer cold subducted lithosphere has relatively fast seismic velocity which leads to early arrivals for some event station paths the effect is very large for events in the tonga kermadec deep seismic zone recorded at certain new zealand stations these particular arrivals are very high frequency 3 hz or greater and sometimes resemble two distinct phases the later arrival appearing at about the time predicted by jeffreys bullen tables data from the digital station snzo in wellington confirm the travel time results of the analog stations and furthermore show frequencies above 5 hz much higher than can be seen on analog records and up to 4 dispersion in the range 1 8 hz energy in the second phase which is often absent at snzo is mainly 1 2 hz the digital data support the idea proposed earlier that the effect is caused by propagation through a thin slab which passes only short wavelength waves the essential features of the wave propagation are modeled by acoustic waves in a one dimensional high velocity slab the waveforms produced by the model are discussed in terms of the leaky modes of the system and calculated by a reflectivity method a very thin 15 km uniform slab provides the required dispersion but the waves are heavily attenuated and would never be observed at teleseismic distances a thicker slab allows the energy through but does not give enough dispersion altering the variation of velocity across the slab provides the required dispersion if a thick high velocity layer with wave speed increasing gradually with height is overlain by a thin lid of even higher velocity for the models considered the lid thickness must lie in the range 6 15 km and be continuous from a depth of about 50 km to the bottom of the earthquake zone the thick layer could arise from the thermal anomaly in the subducted lithosphere the thin lid may be the gabbroic part of the subducted crust that has transformed to eclogite

transition from interplate slip to double seismic zone along the kuril kamchatka arc during subduction the downgoing slab interacts with a strong overriding plate at shallow depths and with the surrounding asthenosphere at intermediate depths thus the transition from earthquakes representing interplate slip to those reflecting intraplate deformation within subducting lithosphere is important for understanding how the mechanics of subduction varies with depth to characterize this transition we studied 44 large to moderate sized earthquakes that occurred between 1964 and 1990 in two regions along the central portion of the kuril kamchatka arc precise earthquake source parameters are determined by analyzing body waveforms recorded at teleseismic distances at depths of 30 50 km our results delineate a transitional thrust zone characterized by earthquakes showing thrust faulting on moderately dipping nodal planes the northwesterly dipping nodal plane dips approximately 40 some 10 15 steeper than the dip of low angle earthquake faulting along the interplate thrust zone at depths shallower than 30 km for earthquakes along the transitional thrust zone secondary features in observed waveforms suggest that either a thick layer of sediments is present in the forearc region or the sources are embedded in materials with crustal seismic wave speeds in the former case the transitional thrust zone is interpreted as the deep seated portion of the plate interface whose dip increases with depth the alternative implies that the transitional thrust zone occurrs below the plate interface within subducted crust of the pacific plate and that the occurrence of earthquakes along the transitional zone may indicate a locked plate interface between depths of 30 and 50 km the inference of a locked plate interface is consistent with the occurrence of compressional earthquakes beneath the outer rise region however a locked interface does not seem to affect the state of strain in the double seismic zone the upper compressional layer of the double seismic zone abuts the transitional thrust zone at a depth of approximately 50 km at this depth the two layers of seismicity in the double seismic zone are separated by as much as 30 40 km too large a distance to be accommodated by models of seismogenesis invoking either the gabbro eclogite transition or dehydration of subducted oceanic crust

observations of body wave multipathing from broadband seismograms evidence for lower mantle slab penetration beneath the sea of okhotsk we present observations of anomalously broad and complex sh waveforms from two deep focus earthquakes in the southern kurils to digital world wide standard seismograph network and regional seismic test network stations in north america by making use of broadband three component seismograms and relying on a variety of phases one can limit the class of possible explanations for these phases in particular it is shown that the anomalous arrivals are not due to source complexity near receiver structure a triplication from a lower mantle velocity gradient a low q zone in the mid lower mantle or shear wave splitting we show that these arrivals correspond to relatively fast paths and have enhanced amplitudes indicating that they are most probably caused by multipathing due to interaction with a high velocity lower mantle feature below the source region we examine the hypothesis that this feature may be a lower mantle extension of the kuril slab ray tracing experiments show that multipathing is indeed expected along the azimuths corresponding to the north american stations used in this study for a realistic slab model observations of reversed polarity arrivals are consistent with such an explanation where it appears that two rays comprising the arrival emerge from different points on the focal sphere one test of this explanation is that slab induced multipathing should not be seen along back of slab oceanic azimuths examinations of seven events for which the stations are in this azimuth range do not exhibit the same broadening and complexity of the sh waveforms identification of slab induced multipathing associated with deep focus events has two important consequences first it appears to require slab penetration beyond the 650 km discontinuity at least 300 400 km into the lower mantle under the sea of okhotsk second these phases may act as a contaminant when attempting to study other seismic phenomena such as discontinuities low q zones near receiver structure or source complexity

seismic thermal and compositional structures of the stagnant slab in the mantle transition zone beneath southeastern china abstract the structure of earth s mantle transition zone mtz contains important information about how oceanic plates are recycled into the mantle however morphologies of stagnant slab in the mtz inferred from seismic studies remain controversial here we conduct combined geodynamic mineral physical and seismologic modeling to infer fine features of the stagnant slab in the mtz beneath southeastern china this method applies mineral physics modeling in combination with geodynamically consistent thermal anomalies to predict seismic velocity structures in the mtz that are used in triplication waveform modeling the synthetic waveforms are systematically compared with observed data using a grid search optimization our best fit model reveals a stagnant slab lying at the bottom of the mtz with a peak temperature reduction of 350 80 k and a thickness of 85 20 km indicating that it underwent little deformation by joint modeling of the mineral physics and thermal history of the slab our approach improves estimates of both the thickness and temperature of the stagnant slab and provides a better understanding of the paleo subduction system beneath southeastern china

olivine spinel transitions experimental and thermodynamic constraints and implications for the nature of the 400 km seismic discontinuity the sequence of high pressure phase transitions in olivine is traditionally used as a model for seismic velocity variations in the 200 to 650 km depth interval in a mantle of peridotitic bulk composition it has been proposed that the observed seismic velocity increase at 400 km depth is too sharp and of too small a magnitude to be attributable to the phase change and that the upper mantle must thus be chemically stratified with the 400 km discontinuity being due either to a combination of phase changes in a layer of pyroxene garnet rich piclogite composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer using available calorimetric thermoelastic and synthesis data and their associated experimental uncertainties we have derived internally consistent high pressure phase relations for the mg2sio4 fe2sio4 join we find that the divariant transition which is generally regarded as occurring over a broad depth interval for mantle olivine compositions is in fact extremely sharp the seismic discontinuity corresponding to the transition in mg0 9fe0 1 2sio4 should occur over a depth interval isothermal of about 6 km at a depth of approximately 400 km the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric thermoelastic and synthesis data in addition we have computed seismic velocity profiles for a model mantle consisting of pure olivine of mg0 9fe0 1 2sio4 composition comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400 km depth is consistent with a mantle transition zone composed of about 60 70 olivine we conclude that there is no need to infer the existence of pyroxene garnet rich compositions such as eclogite or piclogite in the transition zone since an upper mantle of homogeneous olivine rich peridotitic composition is consistent with the available seismic velocity data

seismological evidence for a low velocity layer within the subducted slab of southern taiwan abstract a thin low velocity layer that continues from the surface down to at least 135 km depth has been detected within the subducted slab through the analysis of a three component short period set of digital data from seismic stations in southern taiwan two distinct phases in the first few seconds of the p waves generated by nine intermediate depth earthquakes are observed at one station taw located just at the intermediate up dip of the wadati benioff zone detailed analyses of these phases show the faster one is a refracted wave from the moho of the subducting plate while the subsequent phase is the direct wave propagated within the subducting crust these two distinct phases are not observed at any other stations even at station hen which is only 20 km from the plate boundary these phenomena suggest that the intermediate depth earthquakes occurred in the low velocity layer of the subducting crust and that the thickness of the low velocity layer is less than 15 km the continuity of such a thin low velocity layer down to at least 135 km is hard to explain by the metastable persistence of dry gabbro at depths less than 60 km instead however the low velocity layer might be largely attributed to the metastable persistence of hydro phases and or fluid produced by a series of dehydration reactions during subduction

ultradeep metamorphic rocks the retrospective viewpoint ultradeep or ultra high pressure uhp metamorphic rocks formed from crustal protoliths within the stability field of coesite at pressures 2 5 3 0 gpa corresponding to depths 80 120 km occur locally though regionally distributed in at least five continental areas their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high pressure studies detailed petrographic and microprobe work especially on mineral inclusions in favorable cases allows the derivation of prograde pt paths during subduction and of retrograde ones during exhumation commonly the gneisses adjacent to the uhp rocks do not exhibit signs of ultradeep metamorphism apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower pt conditions during the retrograde path it is also possible however that uhp equilibria were not attained throughout the rock volumes subducted but only along zones of shearing and fluid introduction if it is true that not all uhp metamorphic rocks return to the crustal orogenic belts but some continue to be subducted to greater mantle depths the classical geochemical pattern of a one way mass transfer from mantle to crust throughout the earth s history is at stake the assumed gradual growth of continents may have had a counterpart of continent destruction during collision events most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous mg a1 silicates exist that were not found in nature thus far but may be characteristic minerals in the cold portions of old subduction zones thus extending the water retentivity of subducting slabs to greater and hitherto unexpected depths

elasticity of glaucophane seismic velocities and anisotropy of the subducted oceanic crust abstract upon subduction the oceanic crust transforms to blueschists and eclogites with seismic properties that gradually become similar to those of the surrounding mantle in order to evaluate the anisotropy of blueschists and glaucophane bearing eclogites the elastic constants of glaucophane single crystal plates from the sesia lanzo zone aosta valley western alps were measured using brillouin spectroscopy at ambient conditions the mean p and s wave velocities are 7 8 and 4 6 km s 1 respectively and the anisotropy is high 38 1 av p and 27 3 av s glaucophane develops strong lpo characterized by the 001 axes concentrated sub parallel to the lineation and the 110 poles concentrated sub perpendicular to the foliation in both blueschist and eclogite rocks the measured lpo is in good agreement with viscoplastic self consistent numerical models seismic properties of glaucophane bearing blueschist and eclogite are calculated by combining measured lpo and the single crystal elastic moduli of glaucophane with the other main mineral constituents of the rock mostly epidote for blueschist and omphacite and garnet for eclogite blueschists present stronger anisotropies av p 16 1 and av s 10 3 than eclogites av p 2 9 and av s 1 7 the shear wave splitting and resulting delay times for a 7 km thick layer of eclogite or blueschist are low for the eclogite

seismic detection of the untransformed basaltic oceanic crust subducting into the mantle summary subcrustal earthquakes occur in a slab like zone beneath sw japan in association with the subduction of the philippine sea plate they generate a distinct pair of p and s later phases with anomalously low apparent velocities of about 7 and 4 km s 1 which cannot be explained by a laterally homogeneous velocity structure these later phases are observed for particular source receiver geometries the sources at depths shallower than 50 60 km and the receivers beneath which the subcrustal seismic zone comes in contact with the bottom of the continental crust we examined the nature of these later phases using two methods one is to determine two types of apparent velocities an event common apparent velocity and a station common apparent velocity which may be different from one another for a laterally inhomogeneous velocity structure the other is to trace seismic rays numerically for a laterally heterogeneous velocity model an extensive analysis using these two methods leads to a conclusion that the slab like seismic zone constitutes a low velocity channel in the uppermost mantle with p and s velocities comparable to those of the lower continental crust beneath sw japan the waves trapped within the low velocity channel escape from it through its contact with the continental moho and are observed as a distinct pair of later phases the head waves guided by the bottom boundary of the low velocity channel are observed as weak arrivals of initial phases the thickness 10 km and the velocities vp 7 km s 1 vs 4 km s 1 of the subcrustal seismic zone indicates that it is made up of the untransformed basaltic oceanic crust subducted with the underlying lithospheric mantle and that the transformation of basalt to eclogite does not take place in the subducting slab at least down to depths of 50 60 km we present laterally heterogeneous structural models of the crust and the uppermost mantle along two profiles in sw japan the models are characterized by an apparent thickening of the lower continental crust in particular regions due to a direct contact of the subducting oceanic crust to the continental moho such features are qualitatively consistent with the results of explosion seismology and with the bouguer anomaly pattern in the relevant regions the oceanic lithosphere with the untransformed light oceanic crust at its top might still be buoyant after subduction a factor which must be taken into account in considering the driving mechanisms of plate tectonics the slab geometry and the earthquake related tectonic process

the application of synthetic seismograms to the interpretation of the upper mantle p wave velocity structure in northern australia abstract during the period 1974 to 1977 a long range seismic refraction project was conducted in central australia along a profile extending south from darwin earthquakes from the banda sea region were used as seismic sources for this experiment an analysis by hales and co workers of the resulting data based on travel times and using geometric raytracing techniques has resulted in the construction of an upper mantle velocity model using synthetic seismograms to model amplitudes it is shown that additional constraints can be placed on the derived velocity profile the low velocity zone beneath the 200 km discontinuity is found to have a more abrupt onset than was previously suggested a smaller discontinuity at 325 km depth is now implied the analysis suggests that the 400 km discontinuity is a first order velocity increase whereas all other observed upper mantle discontinuities are more satisfactorily modelled as second order type structures

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

reflectivity and seismic properties of the deep continental crust the recovery of drill core from a section of upper amphibolite facies terrane located in the inner piedmont of south carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust seismic reflection profiles over the inner piedmont in the vicinity of the drill hole reveal subhorizontal high amplitude reflections the origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamotphic layering probably originating from lower crustal ductile flow fine tuning of the metamotphic layering which is temperature and pressure related can produce bright spots in the deep crust anisotropy resulting from preferred mineral orientation is another important propeny of this sectim velocities are low for vertical wave propagatim whereas horizontal velocities are relatively high horizontal velocities show mly a slight variatim with azimuth which is unlikely to be detected using standard refractim techniques reflectivity decreases with increasing anisotropy critical to this study has been the continuous coring of a major section of high grade metamotphic terrane which has provided the necessary stratigraphic and lithologic cmtrol for the generation of accurate synthetic seismograms

waveform effects of a metastable olivine tongue in subducting slabs we constructed velocity models of subducting slabs with a kinetically depressed olivine and spinel transition and examined the effect that such structures would have on teleseismic p waveforms using a full wave finite difference method these two dimensional calculations yielded waveforms at a range of distances in the downdip direction the slab models included a wedge shaped low velocity metastable olivine tongue moto to a depth of 670 km as well as a plausible thermal anomaly one model further included a 10 km thick fast layer on the surface of the slab the principal effect of moto is to produce grazing reflections at wide angles off the phase boundary generating a secondary arrival 0 to 4 seconds after the initial arrival depending on the take off angle the amplitude and timing of this feature vary with the lateral location of the seismic source within the slab cross section careful analysis of waveforms from earthquakes with depths near 400 km simple sources and adequate station coverage in appropriate geometries will be required to resolve whether moto is present

seismic evidence for widespread serpentinized forearc mantle along the mariana convergence margin 1 seismic imaging of subduction zones can provide constraints on mineral reactions in the slab and surrounding regions we use p to s converted phases from teleseisms recorded at broadband stations in the mariana islands to image the forearc and arc regions of the mariana convergent margin the subducting oceanic crust is observed between 75 and 110 km depth as a thin low velocity zone overlying the subducting moho demonstrating that the basalt to eclogite phase transition must occur at a greater depth a low velocity zone lvz approximately 10 25 km thick whose upper boundary is imaged at about 40 55 km depth is detected in the forearc region of the mantle wedge along the entire margin the anomaly is located too shallow to represent subducted oceanic crust we interpret the lvz as a serpentinized region in the forearc mantle resulting from hydration by slab expelled water the inferred s wave velocity in the lvz of as low as 3 6 km s represents a level of serpentinization of 30 50 corresponding to a chemically bound water content of about 4 6 wt

the thermal influence of the subducting slab beneath south america from 410 and 660 km discontinuity observations summary regional seismic network data from deep south american earthquakes to western united states and western european seismic arrays is slant stacked to detect weak near source interactions with upper mantle discontinuities these observations are complemented by an analysis of earlier work by sacks snoke 1977 who observed s to p conversions from deep events to stations in south america and similar observations from 1994 95 events using the banjo and seda networks observations of the depth of the 410 km discontinuity are made beneath central south america in the vicinity of the aseismic region of the subducting nazca plate these results image the 410 km discontinuity over a lateral extent of up to 850 km perpendicular to the slab and over a distance of 2700 km along the length of the slab away from the subducting slab the discontinuity is mainly seen near its global average depth whilst inside the slab there is evidence for its elevation by up to around 60 km but with significant scatter in the data these results are consistent with the presence of a continuous slab through the aseismic region with a thermal anomaly of 900 uc at 350 km depth this value is in good agreement with simple thermal models though our data are too sparse to accurately constrain them sparse observations of the 660 km discontinuity agree with tomographic models suggesting penetration of the lower mantle by the slab in the north but stagnation at the base of the transition zone in the south the geographical distribution of the data however does not allow us to rule out the possibility of slab stagnation at the base of the transition zone in the north these observations together with the presence of deep earthquakes require more complicated thermal models than previously used to explain them possibly including changes in slab dip and age with depth

seismological constraints on the gabbro eclogite transition in subducted oceanic crust abstract dispersion of seismic waves travelling along the strike of the subducted pacific plate beneath tonga kermadec shows evidence for a high velocity layer of crustal thickness extending to a depth of at least 450 km the opposite dispersion with low frequencies arriving first is observed in japan and has been taken together with inferences from converted phases as evidence of a thin low velocity layer both layers have in previous studies been interpreted as subducted oceanic crust which has been transformed to eclogite but in tonga kermadec the eclogite must have high seismic velocity and beneath japan it must have a low seismic velocity an apparent contradiction therefore exists here we find theoretical estimates of the seismic velocities of eclogite and normal mantle using the data and equations of duffy and anderson and a variety of mineral assemblages and bulk compositions we find a range of both low and high seismic velocities the high end of the range depending mainly on the contents of jadeite pyrope and grossular it is therefore possible in principle to explain both the high velocity layer beneath tonga kermadec and the low velocity layer beneath japan as being consistent with an eclogitic crust provided they have significantly different compositions however we can find no clear reason for such a difference between that part of the pacific plate currently subducting beneath japan and that beneath tonga kermadec

density and lithospheric structure at tyrrhena patera mars from gravity and topography data the tyrrhena patera highland volcano mars is associated with a relatively well localized gravity anomaly and we have carried out a localized admittance analysis in the region to constrain the density of the volcanic load the load thickness and the elastic thickness at the time of load emplacement the employed admittance model considers loading of an initially spherical surface and surface as well as subsurface loading is taken into account our results indicate that the gravity and topography data available at tyrrhena patera is consistent with the absence of subsurface loading but the presence of a small subsurface load cannot be ruled out we obtain minimum load densities of 2960 kg m 3 minimum load thicknesses of 5 km and minimum load volumes of 0 6 106 km3 photogeological evidence suggests that pyroclastic deposits make up at most 30 of this volume such that the bulk of tyrrhena patera is likely composed of competent basalt best fitting model parameters are a load density of 3343 kg m 3 a load thickness of 10 8 km and a load volume of 1 7 106 km3 these relatively large load densities indicate that lava compositions are comparable to those at other martian volcanoes and densities are comparable to those of the martian meteorites the elastic thickness in the region is constrained to be smaller than 27 5 km at the time of loading indicating surface heat flows in excess of 24 mw m 2

metastability of enstatite in deep subducting lithosphere olivine and mg fe sio3 pyroxene the most abundant minerals in the earth s upper mantle are believed to transform to high pressure phases at 400 km depth1 5 the possible metastable persistence of olivine to greater depths in some subduction zones with consequences for the origin of deep focus earthquakes and the dynamics of subduction has been discussed extensively6 14 but the role of other mantle minerals has not been considered we report here an experimental study of the kinetic behaviour of the magnesian pyroxene enstatite mgsio3 at upper mantle conditions we find that whereas forsterite mg olivine mg2sio4 transforms rapidly to phase at 1 200 c and 16 gpa the transformation of enstatite to phase plus stishovite on the same timescale requires much higher temperatures at lower temperatures enstatite transforms directly to the ilmenite structure but only at pressures greater than 20 gpa enstatite should therefore persist metastably to greater depths than olivine in subduction zones transforming directly to the ilmenite structure the enstatite ilmenite transformation is accompanied by a large decrease in volume which should increase the stresses in subducting slabs change the buoyancy forces that drive subduction and might also contribute to the origin of deep focus earthquakes

on melting of the subducted oceanic crust effects of subduction induced mantle flow abstract based on petrological and geochemical arguments it is possible that arc magma is derived from subducted oceanic crust in this paper regional thermal models have been constructed to study the feasibility of melting cold subducted oceanic crusts at shallow depth i e at depths of about 100 km by a dynamic mantle calculated results suggest that plate subduction will generate an induced flow in the wedge above the subducting slab this current continuously feeds hot mantle material into the corner and onto the slab surface a high temperature thermal environment can be maintained in the vicinity of the wedge corner immediately beneath the over riding plate our regional models further demonstrate quantitatively that production of local melting is possible just about 30 km down dip from the asthenosphere wedge corner additional geological processes such as reasonable amounts of shear heating and minor dehydration which will lower the local melting temperature will further increase the probability of melting a cold subducted oceanic crust at shallow depth

upper mantle anisotropy a preliminary model seismic anisotropy in the upper mantle can be explained by crystallographic mineral alignment achieved through dislocation motion the physical mechanism of mineral alignment requires upper mantle shear flow which reorients and aligns minerals by dislocation glide and climb governed by the dominant glide system of each mineral the dominant glide systems are assumed to be 100 0l0 for olivine and 001 100 for the pyroxenes these yield a predominantly orthorhombic fabric with the olivine 100 and the pyroxene 001 axes aligned in the upper mantle flow direction and the olivine 010 and the pyroxene 100 axes aligned normal to the upper mantle flow plane these glide systems have a threshold temperature of enhanced mobility of 1100 1200 k which yields a solid state thermally defined lithosphere asthenosphere boundary in an olivine pyroxene mantle consistent with recent seismic determinations of the thickness of the lithosphere mantle anisotropy due to mineral alignment is then actively maintained below this boundary asthenosphere and mesosphere and is a fossil state above this boundary lithosphere we use the dominant glide systems to establish the crystallographic orientation of olivine and pyroxene in calculating the maximum seismic anisotropy of two petrologic models pyrolite and piclogite for the upper mantle by an extrapolation of single crystal anisotropic mineral elastic properties to a depth of 400 km the real earth seismic anisotropy will be bound by the limits of maximum anisotropy from perfect mineral alignment and minimum anisotropy isotropy from random mineral alignment the seismic anisotropy of the upper 220 km is best represented by the pyrolite model which reduces to quasi hexagonal symmetry with the unique axis in the direction of mantle flow the lehmann discontinuity is conjectured to be due to a change in composition from pyrolite to piclogite and therefore may represent a change in anisotropy the piclogite model has orthorhombic symmetry distinctly different from that of the pyrolite the piclogite anisotropy model can appear quasi isotropic when measured by transverse isotropy parameterization if the mantle flow is mainly horizontal with a horizontal shear plane that is the shear flow gradient duh dr dominates

upper boundary of the pacific plate subducting beneath hokkaido japan estimated from scsp phase abstract three dimensional geometry of the upper boundary of the pacific plate subducting beneath hokkaido japan was obtained using the scsp phase the phase converted from scs s wave reflected at the core mantle boundary to p wave at the plate boundary taking the advantage of a dense seismic network hi net recently deployed across the japanese islands we applied several seismic array analyses to the recorded waveform data for a large nearby deep earthquake in order to enhance very weak scsp signals in the original records at first we set up five blocks for the region in plate dip directions after aligning the travel times of scs and stacking seismograms among stations in a given sub block perpendicular to each dip direction we searched for the optimal plate model i e two dimensional geometry of the upper boundary for each block the model was parameterized by seven depth grids and seismograms were stacked based on the travel time of scsp as a time lag of each sub block so that the optimal model would yield the maximum spectral energy of scsp after stacking this model parameter search was conducted using ray tracings of scsp with a reference velocity model and a non linear inversion scheme neighbourhood algorithm the optimal model of each block was combined each other by cubic spline interpolation in order to construct an overall three dimensional geometry of the upper boundary of the plate next we performed the frequency wavenumber f k spectral analysis to refine the above result assuming each station as a reference point we made beam output from records of its adjacent stations as a function of wavenumber vector k x k y and frequency the peak of its power spectrum was considered to represent the wavenumber vector of scsp that is azimuth of arrival and slowness so that we can estimate the position and depth of the corresponding scs scsp conversion in the frequency range from 0 5 to 1 5 hz we could estimate the conversion points for 21 stations or hypothetical arrays and revised the geometry of the upper boundary obtained by the non linear stacking approach in the previous step the final plate model was compared with the distribution of intraplate earthquakes in the pacific plate this comparison clearly reveals that the upper seismic zone merges with the lower from 150 to 200 km in depth deviating systematically away from the upper boundary where the boundary is slightly bumped in a convex manner

fossil flat slab subduction beneath the illinois basin usa abstract the illinois basin is one of several well studied intracratonic sedimentary basins within the north american craton whose formational mechanisms and subcrustal structure are not well understood we study the s velocity structure of the upper mantle beneath the illinois basin and its surrounding area through seismic tomography we utilize continental scale waveform data of seismic s and surface waves enhanced by regional earthquakes located near the illinois basin our 3d tomographic model il05 confirms the existence of a slow s velocity structure in the uppermost mantle beneath the illinois basin region this anomalously slow region exists from the base of the crust to depths of 90 km and is slower than the north american cratonic average by about 200 m s this anomalous uppermost mantle beneath the illinois basin is underlain by a faster lithosphere typical of the surrounding craton to depths of 200 km excluding the formation of the reelfoot rift this area of north american has been stable for over 1 0 gy thus we do not expect thermal anomalies from before that time to persist into present day s velocity anomalies and we consider a delamination origin as an explanation of illinois basin subsidence unlikely we cannot rule out that the slow mid lithosphere beneath the illinois basin is caused by an uppermost mantle enriched by a deep but weak plume we attribute the slow mid lithosphere to the presence of either oceanic hydrous crust or a relatively cool mantle wedge with preserved hydrous minerals in the illinois basin s uppermost mantle related to a fossilized flat subduction zone

high resolution image of the oceanic moho in the subducting nazca plate from p s converted waves short period p s converted waves outline the oceanic moho of the descending nazca plate in northern chile at 24 s comparison with hypocentral locations obtained with temporary local seismic networks show that earthquakes inside the subducting oceanic lithosphere of the nazca plate are located predominantly in the oceanic crust at depths between 70 and 120 km and probably in the upper mantle at depths below 120 km the results suggest that dehydration embrittlement as mechanism for intermediate depth earthquakes is active both in the former oceanic crust and upper mantle of subducting lithosphere

dispersion of regional body waves at 100 150 km depth beneath alaska in situ constraints on metamorphism of subducted crust phase delays at high frequencies are observed in body waves that travel in the alaska slab along its strike at 100 150 km depth the delays between 2 6 hz energy and the direct 0 5 1 hz arrival are 0 5 1 5 s for p waves and 1 5 4 s for s waves such dispersion suggests a waveguide structure that parallels the slab perhaps near its top a channel that is 2 6 km thick and 2 5 5 slower than surrounding mantle can explain the observations the thickness of the layer is comparable to that of subducted oceanic crust or somewhat thinner the layer may be crust that is slow at these depths the required velocity anomaly is too small to be due to a continuous layer of metastable gabbro yet too large to represent an eclogite layer it may indicate a mixture of the two or persistence of hydrated mineral assemblages to depth

evolution of the downgoing lithosphere and the mechanisms of deep focus earthquakes summary the thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated an alternating direction implicit finite difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries these models with the appropriate spreading rates and dip angles are compared with different island arc systems temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent the depth to which temperature anomalies persist is generally about 700 km or less the thermal results are used to construct the seismic velocities and ray paths density anomalies and the resulting stress distribution comparing the theoretical stress distribution and the focal mechanism studies of intermediate and deep focus earthquakes indicates the importance of both the mantle s rheology and the temperature dependence of the slab s elastic properties the intermediate and deep focus earthquakes are located along the coolest region of the slab the theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs

a hypothesis for the seismogenesis of a double seismic zone summary the seismogenesis of a double seismic zone in particular the lower layer of a double seismic zone has not been adequately explained in the literature on the basis of seismic data and geothermal structures along three well studied cross sections in the kuril kamchatka and japan subduction zones we investigate the temperature pressure conditions associated with seismogenic structures of the double seismic zones the corresponding tip loci seem to suggest that earthquakes observed in the lower layer and in the lower part below approximately 130 20 km of the top layer of a double seismic zone were caused by metastable phase transition a mechanism similar to that responsible for deep focus earthquakes only at lower temperature pressure conditions under this hypothesis the wedge shaped configuration of a double seismic zone is interpreted to represent the loci of the kinetic boundary of the phase transition according to theoretical experimental studies and the constraints imposed by our observations a likely candidate for such a phase transition is the metastable al rich enstatite decomposing into the assemblage of al poor enstatite plus garnet earthquakes in the upper part of the top layer were most probably due to conventional mechanisms such as dehydration of subducted materials and or facies change from basalt to eclogite that the top layer involves more than one seismogenic mechanism is also implied by the distinct behaviour of seismicity in the vicinity of 130 f 20 km because the presence of deviatoric stress is critical to the reaction rate of a metastable phase transition it is inferred that single seismic zones are also caused by the same mechanisms except that the implicit layer of a supposed double seismic zone is missing due to the insufficient amount of appropriate metastable minerals or to the lack of appropriate deviatoric stresses in the source region

subduction zones observations and geodynamic models abstract this review of subduction and geodynamic models is organized around three central questions 1 why is subduction asymmetric 2 are subducted slabs strong or weak 3 how do subducted slabs interact with phase transformations changes in mantle rheology and possibly chemical boundaries in the mantle based on laboratory measurements of the temperature dependence of olivine one would conclude that the core of a subducting slab is at least 10 000 times more viscous than ambient mantle however there are a number of complementary but independent observations that suggest that slabs are much weaker than this slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone the lack of a clear correlation between the observed dip angle of deep slabs and plate velocity rate of trench migration and slab age in modern subduction zones is consistent with hypothesis that subduction is a time dependent phenomenon both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon but that slabs bend thicken stretch and change dip through time this is at odds with the assumptions used in steady state slab thermal models where slab deformation is not considered

structure of the subducting nazca plate beneath peru arrival times from intermediate depth 110 150 km earthquakes within the region of flat subduction beneath the subandean zone and foreland basins of east central peru provide constraints on the geometry and velocity structure of the subducting nazca plate hypocentral locations and origin times for these events were determined using observations from a 15 station digitally recording locator array deployed in the epicentral region of eastern peru observed p wave arrival times for coastal stations in peru some 3 6 from the epicenters are up to 4 s early relative to predicted arrival times based on the best fit velocity depth model used for hypocenter locations these large negative time residuals appear to be the result of propagation paths which have long segments in the colder higher velocity subducting plate p wave travel times were modeled for the effects of the slab using three dimensional 3 d ray tracing computed ray paths show that travel times to coastal stations for the eastern peru events can be satisfactorily modeled with average velocities relative to the surrounding mantle 6 lower within the uppermost slab assumed on the basis of other studies to be unconverted basaltic oceanic crust 6 km thick and 8 higher within the cold uppermost mantle of the slab ray tracing for this plate model shows that p wave shadow zones can occur if the source slab receiver geometry results in seismic rays passing through regions in which the dip angle of the slab changes significantly such geometries exist for seismic waves propagating to some coastal stations from intermediate depth earthquakes located east of the andes observed first arrival times for such cases do in fact have smaller negative residuals than those for geometries which allow for direct paths at similar distances modeling such arrivals as internally reflected waves propagating through the high velocity part of the plate produces a significant improvement in the travel time residuals for the slab velocities given above we obtain a model thickness of approximately 36 km for the cold slab interior and a slight northwest component of dip in the region of subhorizontal subduction

a detailed receiver function image of the upper mantle discontinuities in the japan subduction zone abstract we have imaged the upper mantle discontinuities in a 30 20 large region at the active continental margin of the japan subduction zone and neighboring areas using p to s converted phases from teleseismic records of permanent broadband stations the 410 km discontinuity is detected within 10 km of its global average position an interesting exception in its observation is a gap near 135 e very close to the slab penetration of the 410 km discontinuity and in an area where we have rather high data density the 660 km discontinuity reaches 700 km depth at two places where it is hit directly by the slab these data generally show good agreement with tomographic results both the 660 km discontinuity depression and the p velocity anomalies suggest about 400 500 k temperature deficit within the slab however no downward bending of the 660 km discontinuity is observed in eastern china where the flat lying slab is imaged by tomography this suggests that the slab does not cool the 660 km discontinuity in this region therefore the positive buoyancy required to keep the slab lying flat cannot have been provided by the negative clapeyron slope of the spinel perovskite phase transition another mechanism is needed which could possibly be metastable olivine in the cold core of the slab we have also imaged the shallower portion of the slab down to about 150 km underneath some seismic stations likely because metastable gabbro is still existing to this depth providing a sufficient velocity contrast a strong negative converted phase is observed at 150 200 km depth underneath a volcanic region in japan if real observed at one station only the zone which produces this conversion and which may begin exactly above the slab would require fluid fed melting

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

implications of slab mineralogy for subduction dynamics phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs both under equilibrium and disequilibrium metastable conditions such perturbations yield anomalies not only in seismic velocities but also in density the buoyancy forces arising from these density anomalies may exert several important effects they contribute to the stress field within the slab in a fashion consistent with observed patterns of seismicity they may affect subduction rates both by inducing time dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions they may affect slab morphology possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating which may generate adiabatic shear instabilities capable of triggering deep seismicity 2001 elsevier science b v all rights reserved

seismic waves guided by untransformed oceanic crust subducting into the mantle the case of the kanto district central japan a model of the subduction of oceanic crust is established through an analysis of the upper mantle earthquakes occurring beneath the kanto district central japan a distinct pair of later p and s phases xp and xs commonly observed at specific stations for these events in a depth range of 40 60 km and is considered to take place in association with the subducting motion of the philippine sea plate the xp phase is found from its particle motions to be a longitudinal seismic wave a principal component analysis of wavelets including the initial p and the xp phases shows that the latter arrives at the earth s surface with a slightly shallower angle relative to the former close agreement between the relative arrival time ratio of xs s xp p and the vp vs ratio of crustal or mantle rocks suggests that both xp and xs travel along the same path as longitudinal and tangential waves respectively one can clearly observe these later phases at stations located where the upper surface of the philippine sea plate is apparently in direct contact with the bottom of the crust of the honshu island apparent velocities of the xp and the xs phases are comparable to the seismic velocities of the main layer of the oceanic crust or the lower continental crust which are never expected for an upper mantle earthquake if the velocity structure is laterally homogeneous travel time curves for several earthquakes indicate that these apparent velocities can be directly related to the seismic velocity in the source region the hypocentral distribution of the events related to the subducting motion of the philippine sea plate does not form such a thin seismic zone as observed in southwest japan but extends over the entire thickness of the philippine sea plate the earthquakes from which the later phases are observed occur only in a shallower portion of this thick and contorted seismic zone the deeper portion does not generate such events these characteristics are best explained if these later phases are regarded as the seismic waves guided by the subducted portion of the gabbroic oceanic crust at the top of the philippine sea plate taking the result of our earlier work into account we conclude that the subducting oceanic crust remains in a gabbroic phase without transformation to eclogitic rocks at depths down to about 60 km along the entire northern boundary of the philippine sea plate

velocity structure in the izu bonin seismic zone and the depth of the olivine spinel phase transition in the slab in this study a method is presented for determining the in situ elastic wave velocities of slabs simultaneously with the relative relocations of earthquakes the method is an extended arrival time difference atd scheme that incorporates a realistic model several assumptions are made about the characteristics of ray paths in and around the slab in order to take advantage of the realism of the model without completely sacrificing the simplicity inherent in the atd technique the errors introduced by these approximate rays are estimated by comparing them with more exact rays calculated with a three dimensional ray tracing routine in general the comparisons suggest that these errors are usually much less than 0 1 s the inverse method is tested on the same set of hypothetical data the results of these tests show that the parameters that influence the arrival time differences the most are the amplitude of the slowness anomaly within the slab and the azimuths and takeoff angles for rays leaving the master event the arrival time differences are relatively insensitive to the other parameters describing the slab strike dip half width and the position of the master event which implies that the scheme is fairly insensitive to the particular choice of analytic function to describe the slowness variation within the slab convergence of the inverse routine is rapid in the tests usually less than five iterations are required before a stable solution is reached it appears that coupling between solutions for slab parameters and ray directions is insignificant unlike the standard atd approach solutions for hypocenters and velocities are not directly coupled in this scheme although well located events are required to insure that the hypocenters are not unduly biased by variations in structure once the velocity structure of the slab is determined that information can be used routinely by this scheme to relocate less well recorded events this method was applied to arrival time data from earthquakes located between 200 and 600 km depth in the izu bonin subduction zone structure in the 200 300 km depth range could not be resolved and results in the 500 600 km depth range proved to be too sensitive to noise data from events between 27 n and 31 n show that p wave velocities within the slab from 400 to 500 km depth are 4 6 higher than those of the ambient mantle between 300 and 400 km depth the slab structure varies regionally north of about 32 5 n the p wave velocity within the slab is 3 4 higher than ambient from 180 to 375 km depth in contrast the velocities south of 32 5 n increase from 6 7 higher than ambient in the 375 410 km depth range to 8 10 higher in the 325 375 km depth range and appear to be 9 higher than ambient in the 270 325 km depth range the determination of the high velocities in this region between 325 and 375 km depth proved to be independent of the selection of data and of the choice of starting model above 325 km depth however the velocity determinations are somewhat dependent on the choice of starting model as a result of this dependence no upper bound on the depth of the high velocities could be resolved a plausible explanation for the exceptionally high p wave velocities within the slab south of 32 5 n is that the olivine spinel phase change boundary is elevated in this part of the slab while north of 32 5 n it is not significantly elevated it is not known if the phase change is elevated south of 31 n but the velocities between 400 and 500 km depth in this region imply that it is not depressed the high velocity zone south of 32 5 n lies in an area that marks the beginning of a seismic gap increasing in breadth to the south between deep and shallow seismicity the dip of the slab also becomes gradually steeper to the south of this region in contrast the lower velocities north of 32 5 n occur in a region where the seismicity is fairly continuous and is aligned along a shallow dipping plane if the seismic gap south of 32 5 n represents a region of high velocity then the increasingly vertical dip of the slab in the south could be due to a substantial gravitational force provided by the excess mass extending above the ambient olivine spinel phase boundary the decreasing width of the gap to the north suggests that the evolution of the izu bonin seismic zone has been governed by a propagating instability from the south

the non double couple component of deep earthquakes and the width of the seismogenic zone it has recently been proposed that all deep earthquakes 400 km depth are caused by a shear instability associated with the transformation of metastable olivine to its high pressure polymorphs within the cold core of subducting slabs green and burnley 1989 kirby et al 1991 if so then the seismogenic zone would narrow with depth below 400 km following slab isotherms because the ratio of fault slip to length does not increase markedly with depth and stress orientations indicate that deep fault planes are not parallel to the subducting slab a fault plane large enough to generate a very large deep earthquake would not fit within a narrow seismogenic zone and multiple fault planes would be needed if the fault planes are not parallel due to spatial variations in stress a deviatoric non double couple component volume preserving non shearing part of the seismic moment tensor is likely to result thus if the seismogenic zone narrows with depth one would expect an increasing non double couple component 1 for large earthquakes as depth increases from 400 to 700 km and 2 for very deep earthquakes below 550 km depth as seismic moment increases data from the harvard cmt catalog augmented by moment tensor inversions for the two largest deep earthquakes during the past thirty years are examined for these features although significant scatter is present the data roughly follow the trends described above suggesting a relationship between the non double couple component of deep earthquakes and the geometry of a seismogenic zone narrowing with depth

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

a thermodynamic model for subsolidus equilibria in the system caomgoal2o3sio2 this study integrates phase equilibrium and calorimetric data for the following phases in the cao mgo al2o3 sio2 system clinopyroxene orthopyroxene garnet fomente anorthite spinel kyanite sillimanite corundum and quartz the internally consistent data set includes mixing properties for clinopyroxene orthopyroxene and garnet solid solutions and enables calculation of any equilibrium involving the components of the above 10 phases under any desired p t conditions major conclusions of the study apart from the data set are 1 1 spinel appears to have an enthalpy of mg al exchange on the order of 20 kcal or more substantially above the calorimetrically measured value 2 2 caal2sio6 pyroxene has virtually complete al si disorder 3 3 enstatite has an enthalpy of formation from oxides at 1000 k of about 16840 cal mole substantially greater than the calorimetric value of 17620 340 cal 4 4 anorthite has about 0 6 cal k entropy of al si disorder 5 5 cmas garnets of between pyrope100 and pyrope80grossular20 composition have almost zero excess entropy 6 6 nonstoichiometry in clinopyroxene has a moderate but predictable influence on reaction boundaries in sio2 saturated compositions

experimental determination of elastic properties of talc to 800 c 0 5 gpa calculations of the effect on hydrated peridotite and implications for cold subduction zones abstract we have measured compressional wave velocity v p in talc as a function of temperature at 0 5 gpa v p falls from 5 27 0 23 km s 1 at 25 c to 4 35 0 15 km s 1 at 800 c combining these results with a previously published bulk modulus k and k p gives a poisson s ratio of 0 268 and a shear modulus g of 22 6 gpa under atmospheric conditions assuming that is independent of temperature gives a k t value of 19 3 0 64 mpa k 1 at 0 5 gpa we used the experimental data in combination with published data to calculate bounds on the elastic moduli of a peridotite hydrated with talc just above cold subducting lithosphere we find that an assemblage hydrated by only 0 9 wt h 2 o has elastic wave velocities calculated from the voigt reuss hill average lower than the anhydrous rock by an average of 7 6 for v p and 9 7 for shear waves v s at 700 c these results are consistent with observations of converted phase behavior in the vicinity of the surface of a cold subducting slab and may place the low velocity layer observed in the lowermost mantle wedge not the uppermost slab as has previously been suggested

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

topography of the 660 km seismic discontinuity beneath izu bonin implications for tectonic history and slab deformation we analyze the p wave codas of 65 paths from deep northwestern pacific earthquakes recorded by arrays of stations in germany the western united states india and turkmenistan we identify a phase resulting from a near source s to p conversion at a nearly horizontal discontinuity ranging in depth from 650 to 730 km which we interpret as a thermally depressed spinel to perovskite and magnesiowustite phase transition we migrate these data along with 39 more from wicks and richards 1993 accounting for three dimensional ray bending by the sloping discontinuity to produce a high resolution topography map of the 660 km discontinuity in the izu bonin region assuming an equilibrium phase transition we interpret the discontinuity depth in terms of local temperatures the slab if defined by a thermal anomaly greater than 400 k is only about 100 km thick near 28 n suggesting the slab is penetrating into the lower mantle with little or no advective thickening farther to the north however cold material appears spread out over a wide region consistent with the slab having been laid down flat on the 660 km discontinuity as the trench retreated 2000 km eastward both the narrow slab to the south and the flat lying slab to the north are consistent with recent high resolution tomographic images the depression to 745 km along the arc is consistent with a maximum thermal anomaly of about 1100 k along the entire arc the depression occurs directly beneath the deepest earthquakes even where seismicity is dipping at 45 and stops at 450 km depth suggesting that the slab steepens to a vertical dip at the deepest seismicity this change to a vertical orientation suggests that the slab loses strength temporarily through a physical process which causes the seismicity to increase dramatically and then abruptly cease

guided waves propagating in subducted oceanic crust 1 we use guided waves traveling updip along the surface of the nazca slab to image subducted oceanic crust at the chile peru subduction zone observed p onsets of intermediate depth events near 21 s in northern chile reveal waveguide behavior with growing focal depth low frequency energy 2 hz becomes more and more dominant and higher frequencies arrive delayed sometimes resembling two distinct phases to explain the observations we employ two dimensional finite difference fd simulations of complete p sv wave propagation along an updip profile of the subduction zone the fd calculations shed some light on several basic issues regarding crustal waveguides the development of guided waves dependent on event focal depth is simulated further we show that the observed guided wave energy must decouple from the waveguide near 100 km depth to reach the deployed stations and that the decoupling process is related to variations in subduction angle simulations also yield constraints on source locations relative to the low velocity structure finally the frequency content of p onsets is used to constrain the thickness of the waveguide the results indicate that a structure of 4 5 km average width and 7 low velocity remains seismically slow compared to the surrounding mantle down to a depth of at least 160 km the layer is interpreted as the unaltered lower part of the subducted oceanic crust suggesting that complete eclogite transformation in the chile peru subduction zone is unlikely to take place until beyond the volcanic front

imaging upper mantle discontinuity topography using underside reflection data this paper presents a method to invert underside reflection p d p or s d s arrivals data for lateral depth variations of upper mantle discontinuities combining traveltime and amplitude data the method greatly improves the resolution of small scale undulations obtained by existing imaging methods and does not suffer from the long wavelength biases that are likely to be present in currently available models existing inversion methods account for the large size of the fresnel zone of underside reflections but not for its complexity arising from the mini max traveltime nature of pp and ss related waves this neglect results in long wavelength artefacts from small scale undulations of the discontinuities obscurring true long wavelength depth variations the inversion method presented in this paper uses a complex valued sensitivity kernel derived from the representation of underside reflections through a kirchhoff integral formulation the sensitivity kernel accounts for the varying sensitivity of the waveforms to discontinuity structure over the fresnel zone the method is applied to a large synthetic data set the data set consists of p d p amplitudes and traveltimes the results show that the new inversion method resolves depth variations on a lateral scale that is smaller than the size of the fresnel zone of individual underside reflections but larger than the dominant wavelength retaining the resolution of large scale variations the results presented here suggest that the discontinuity depth variations induced by slab penetration of the 670 discontinuity could be resolved by current broad band p670p data sets

receiver function imaging of upper mantle complexity beneath the pacific northwest united states abstract small scale topographic variations on the upper mantle seismic discontinuities provide important constraints on the thermal influences of upwellings and downwellings in geodynamically complex regions subduction of the juan de fuca plate and other tectonic processes dominating the pacific northwest united states in the cenozoic involve massive thermal flux that likely result in an upper mantle that has strong 3 d temperature variations we address the interaction of such processes in the region using receiver functions to image the upper mantle seismic discontinuities at 410 and 660 km we utilized over 15 000 high quality receiver functions gathered from 294 teleseismic earthquakes recorded at 277 regional broadband seismic stations primarily those of the earthscope usarray transportable array we find the average depths of the discontinuities to be 412 km and 658 km respectively with no obvious 520 km discontinuity detected the peak to peak range is greater on the 410 than the 660 suggesting the possibility of more significant regional dynamic processes at upper mantle depths our results are not consistent with a mantle plume below central oregon in the high lava plains region our observation of a thinner transition zone beneath the western snake river plain region however is consistent with a regional increase in mantle temperatures perhaps due to either asthenospheric flow from beneath and around the southern edge of the juan de fuca plate or to vertical flow in the form of regional mantle upwelling related to the snake river plain yellowstone hotspot track further our results are not consistent with a simple subducting juan de fuca slab morphology but rather suggest similar levels of significant complexity in slab structure found by recent regional tomographic studies we find evidence for a thickened and therefore cooler mantle transition zone beneath the wallowa idaho batholith region consistent with tomographic models which suggest broad scale increased seismic velocities to transition zone depths we speculate that this region may be the location of mantle downwelling at larger scales than previously proposed

the p velocity within the tonga benioff zone determined from traced rays and observations p waves with travel time residuals between 0 s and 12 s are observed at regional stations in samoa afi and raoul island rao for 39 earthquakes in tonga with focal depths between 70 km and 300 km these anomalously large residuals apparently are produced because seismic phases travel along the strike of the tonga benioff zone within the high velocity subducted lithosphere for up to 1200 km before arriving at afi and rao to eliminate erroneous residuals caused by poor event locations we selected 11 stations and reread the available p times at these stations for the 39 events these arrivals and pp p intervals were used to relocate the events using a variant of the joint hypocenter determination method then the pattern of residuals at afi and rao not used in the relocation was compared to the pattern of residuals expected for various models of the subducted lithosphere as determined by ray tracing the observed pattern of residuals at afi is consistent with the ray traced models if some of the first arrivals are produced by rays traveling directly along the strike of the subducting lithosphere and if others are produced by rays which reflect once off the upper surface of the subducting lithosphere before arriving at afi the observed residuals can be explained by a model where the p velocity in the subducted lithosphere is 8 higher than the velocity in the herrin model the residuals are fit even better by a layered slab model in which the seismic velocity is about 6 higher than the herrin velocity at the upper surface of the subducted lithosphere and about 9 higher at the bottom of the slab these velocity contrasts could be produced if the temperature in the slab was 700 c cooler than the surrounding mantle and if there were no partially melted material within the slab the existence of these anomalously large residuals suggests that the high velocity region in the upper 300 km of the mantle beneath tonga must be fairly continuous over distances of 1000 km and more

theoretical prediction of phase relationships in planetary mantles calorimetric and phase equilibrium data have been used to establish an internally consistent thermodynamic data base for the silica undersaturated part of the system na2o feo cao mgo al2o3 sio2 this data base has been used to calculate the stability fields of assemblages involving the phases olivine ol orthopyroxene opx clinopyroxene cpx spinel sp plagioclase plag and garnet gt at high pressures and temperatures in the six component system all these phases are solid solutions their mixing properties were also obtained from phase equilibrium and calorimetric data in the model system cao mgo al2o3 sio2 cmas the stable assemblages are ol opx cpx plag at pressures less than 8 kb at 1300 c and less than 6 kb at 900 c above these pressures spinel replaces plagioclase until pressures of 16 kb at 1300 and 14 kb at 900 c are reached at higher pressures garnet lherzolite ol opx cpx gt is stable stable phase assemblages have also been calculated for sodium and iron bearing compositions postulated as possible martian mantles the mcgetchin smyth composition would not contain orthopyroxene and a low pressure assemblage of plag ol cpx sp would be replaced by gt ol cpx sp at pressures above 13 kb in contrast the morgan anders composition has stability fields of plagioclase iherzolite p 8 kb and a small field of spinel iherzolite at about 6 kb at temperatures below 900 c

subduction products according to experimental prediction knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products the phase relationships do provide important constraints for testing geophysical and petrogenetic models especially when considered along with geochemical constraints physical questions such as the behavior of partially molten rocks are intimately related to the compositions of magmas from source to eruption site tentative predictions and conclusions include 1 andesite is not a primary magma from oceanic crust 2 it is unlikely that liquids from subducted oceanic crust yield andesites by fractionation except by melting of amphibolite in an especially warm subducted crust 3 it is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle 4 andesite magmas can be generated in peridotite only under exceptional circumstances 5 h2o undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation 6 phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels 7 andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite derived from crust most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250 c at 100 km as proposed by marsh 1979a this demonstrates the dependence of experimental predictions on geophysical models

t waves from the great 1994 bolivian deep earthquake in relation to channeling of s wave energy up the slab we report the observation of t phases recorded at a number of pacific island sites following the great bolivian deep earthquake of june 9 1994 by studying their arrival times at the various stations we unravel the process of conversion from seismic to acoustic energy at the south american coastline we find that the maxima of amplitude of the phase are incompatible with propagation along the great circle from source to receiver but rather require the generation of the acoustic wave at a common point at the arica bight for all stations in the south pacific and even farther south for stations in hawaii and the bonin islands the timing of these conversions corresponds to the arrival of regional s waves at the conversion points the high frequency nature of the t phase requires the channeling of the s wave from the seismic source to conversion point through low attenuation material which in turns adds supportive evidence for the mechanical continuity of the downgoing slab under that section of south america despite the observed gap in seismicity between 300 and 600 km

water flow to the mantle transition zone inferred from a receiver function image of the pacific slab variations in seismic velocity near the subducting slab provide constraints on the thermal structure and the distribution of hydrous minerals and water in a subduction zone we investigated the seismological structure in the upper mantle beneath japan using both radial and transverse receiver functions rfs radial rf can image horizontal layers of velocity contrast while transverse rf can image dipping layers the combination allows us to simultaneously image both horizontal and dipping velocity discontinuities we investigated the records of 45 teleseismic events observed with tiltmeters at 678 hi net stations the frequency band we used is from 0 02 to 0 16 hz the resultant rf image shows an elevated 410 km discontinuity and more importantly the top surface of the pacific slab down to below the 410 km discontinuity with forward modeling we determined that the mantle wedge is about 8 slower in shear wave speed than the subducting slab at depths deeper than 200 km the seismic velocity contrast is presumably caused by a sequence of hydrous minerals at the base of the mantle wedge which receives water released by dehydration reactions in the oceanic crust we employed numerical simulations to determine the distribution of water in and around the subducting slab the result suggests that hydrous minerals are continuously stable above the subducting slab in relatively cool conditions and carry water to the mantle transition zone

interslab earthquakes and phase changes in subducting lithosphere at 0033 ut on 9 june 1994 a great deep earthquake occurred 635 km beneath the amazonian rain forest of northern bolivia the deep rupture of this record setting magnitude 8 3 shock lasted about a minute harvard university centroid moment tensor catalogue the seismic strain release of this single event as indicated by its seismic moment was greater than the total strain release for all deep earthquakes in the harvard cmt catalogue during the previous 17 years of its coverage of global seismicity incredibly ground motion was felt as far north as seattle washington and toronto canada evidently it occurred within the subducting nazca plate that is descending beneath south america this earthquake reminds us of the sometimes grand scale of deep seismic failure and of the need to consider more closely the physical processes that accompany slab descent and in particular how such earthquakes may reflect the inner workings of slabs

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

slab mantle interactions 1 sheared and refertilised garnet peridotite xenoliths samples of wadati benioff zones abstract most sheared and deformed garnet peridotite facies xenoliths found in kimberlites worldwide have been formed from initially depleted peridotite protoliths which have subsequently experienced varying degrees of refertilisation ti fe ca al na p zr and y have been introduced to various degrees by silicate melts and the protoliths thereby refertilised these melts were not kimberlitic they possessed basaltic 100 mg mg fe mole ratios of 65 70 and contained several wt tio2 and were often capable of precipitating ilmenite equilibration temperatures recorded by sheared refertilised garnet periodtites are usually above those of a steady state shield geotherm it is proposed that these rocks acquired their high equilibration temperatures refertilised character and highly deformed fabrics along wadati benioff zones in regions of active subduction beneath cratons bodies of former serpentinite in the deeper layers of the basaltic crust of the subducted lithosphere or in underlying harzburgite may transport water to depths of several hundred kilometers water released by the breakdown of subducted former serpentinite migrates upwards into hotter regions of the eclogitic former oceanic crust causing partial melting to occur particularly in the depth interval between 150 and 300 km these slab derived melts are episodically injected into peridotite immediately overlying the former oceanic crust hybridisation occurs in this boundary layer with ti fe ca al na p zr and y being transferred from the slab into this region thereby rendering it fertile episodic and repeated injection of small volumes of slab derived melts into adjacent pre stressed peridotite would also contribute to local weakening accompanied by catastrophic shear failure at high strain rates the deformed fabrics of most sheared peridotites may have been produced in this manner local transient heating may also accompany displacements of this kind thus accounting for the seemingly elevated equilibration temperatures recorded by these rocks

two types of deep seismicity in subducting slabs the relation between variation of seismic activity with depth and temperature structure in the subducting slabs in five subduction zones kuril ne japan izu bonin mariana and tonga is investigated when the temperature of the core of the slab at depths of 300 350 km is higher than the kinetic cut off temperature of the olivine spinel phase transformation there is no seismicity maximum in the slab below that depth range when the temperature of the core of the slab at 300 350 km depths is lower than the kinetic cut off temperature there is a depth range of relatively high seismic activity in the slabs moreover estimated temperature in the depth range of relatively high seismicity is near the kinetic cut off temperature it is inferred that the phase transformation of meta stable olivine triggers deep earthquakes and causes high seismic activity below the depths of 300 350 km in relatively low temperature slabs when the phase transformation is the cause of deep earthquakes in relatively high temperature slabs however it is considered that the phase transformation occurs in nearly equilibrium conditions and meta stable olivine does not exist other mechanisms in addition to the phase transformation are required to explain observed deep seismicity especially in relatively high temperature slabs

seismic evidence for a steeply dipping reflector stagnant slab in the mantle transition zone studies of seismic tomography have been highly successful at imaging the deep structure of subduction zones in a study complementary to these tomographic studies we use array seismology and reflected waves to image a stagnant slab in the mantle transition zone using p and s sh waves we find a steeply dipping reflector centred at ca 400 km depth and ca 550 km west of the present mariana subduction zone at 20n 140e the discovery of this anomaly in tomography and independently in array seismology this paper helps in understanding the evolution of the mariana margin the reflector stagnant slab may be the remains of the hypothetical north new guinea plate which was theorized to have subducted ca 50 ma

seismic evidence for continuity of the deep slab beneath central and eastern peru seismograms of peru brazil deep focus earthquakes recorded on the nearby carnegie broadband station at cuzco peru cus are characterized by an anomalous large amplitude p wave arrival that follows about 1 5 s after direct p the anomalous arrival is interpreted to be an underside wide angle reflection from the upper surface of the descending nazca plate somewhere in the depth range 150 400 km and in a region where the slab is wholly aseismic particle motion analysis of the cus seismograms shows that the direct p wave has arrival angles as predicted from simple ray tracing the large impulsive anomalous phase termed pr also arrives as a p wave but is of opposite polarity and about 1 5 times the amplitude of the direct p arrival the back azimuth of pr is about 20 clockwise from the path of direct p and its angle of emergence is significantly larger forward modeling calculations indicate that the seismic observations are consistent with a model in which pr originates as a p wave that travels up inside the slab from the source and is then reflected at wide angle from the steeply dipping upper plate boundary results from modeling lead to the following conclusions 1 the descending nazca plate exists in and is probably continuous through the aseismic region between 150 and 500 km depth 2 the plate is very steeply dipping 70 below depths of about 150 or 200 km and 3 the velocity discontinuity at the reflection point must be relatively sharp probably less than about 10 km in thickness

a seismological constraint on the depth of basalt eclogite transition in a subducting oceanic crust the oceanic crust primarily has a basaltic composition1 2 which must transform into eclogite on subduction of the oceanic crust beneath continents3 4 there has been a considerable debate about the role of the basalt eclogite transformation in driving lithospheric plate motion5 and in the thermal regieme of subducting plates6 how important this transformation is in tectonics critically depends on the depth of transformation which is affected by factors such as temperature7 8 bulk rock chemical composition2 water content9 and perhaps stress environment becuase it would be practically impossible to evaluate all these factors correctly we attempt here to constrain the transformation depth by a more direct seismological method

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

patterns of deep seismicity reflect buoyancy stresses due to phase transitions thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near 400 km depth down dip compression over 400 690 km peaking at 550 km and transition to rapidly fading tension below 690 km such features even when complicated by olivine metastability are consistent with observed patterns of deep seismicity that such a simple model neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs it also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release

dry intrusions lagrangian climatology and dynamical impact on the planetary boundary layer abstractdry air intrusions dis are dry deeply descending airstreams from the upper troposphere toward the planetary boundary layer pbl the significance of dis spans a variety of aspects including the interaction with convection extratropical cyclones and fronts the pbl and extreme surface weather here a lagrangian definition for di trajectories is used and applied to ecmwf interim reanalysis era interim data based on the criterion of a minimum descent of 400 hpa during 48 h a first global lagrangian climatology of di trajectories is compiled for the years 1979 2014 allowing quantitative understanding of the occurrence and variability of dis as well as the dynamical and thermodynamical interactions that determine their impact dis occur mainly in winter while traveling equatorward from 40 50 latitude dis typically reach the lower troposphere with maximum frequencies of 10 in winter in the storm track regions as well as over the mediterranean sea arabian sea and eastern north pa

the effect of small scale structure on normal mode frequencies and global inversions presently very little is known about the characteristic length scales of the structures in the upper mantle in generating synthetic seismograms and in large scale inversions one usually assumes that the upper mantle is smooth so that ray theory can be used however it is known that structures in the mantle exist on a scale of a few hundred kilometers notably near subduction zones and other tectonic features in this paper a model of all the subduction zones and spreading ridges is used to investigate the effect of small scale structures on normal mode frequency shifts and on global inversions the normal mode frequency shifts for this model have the same characteristics as observed normal mode frequency shifts furthermore the global models obtained from inversions of synthetic data for the model of subduction zones and spreading ridges are similar in character to the models obtained from global inversions of real data for a realistic set of wave paths the model obtained from the synthetic inversions differs appreciably from a smoothed version of the true model this is due to an uneven path coverage and to a lesser extent due to neglecting scattering effects this means that the models obtained from global inversions of seismological data may contain appreciable artifacts

study of the structure of the slab mantle interface using reflected and converted seismic waves summary seismic waves reflected and converted at the interface between a downgoing slab and the mantle above it can provide important constraints on the velocity contrast at the interface and hence on the properties of the downgoing lithosphere modelling studies suggest that different physical mechanisms may contribute to the velocity contrast at different depths along the interface to explore the possible mechanisms we examine the amplitudes of seismic waves reflected and converted at the slab mantle interface in japanese subduction zones slab face p reflection amplitudes suggest a velocity contrast ranging upwards to 10 per cent at depths of between 300 and 400 km pis amplitude ratios from scs to p conversions at the slab mantle interface depend on the depth of conversion and are large 0 14 and 0 12 at stations shk and mat the size of the velocity contrasts and amplitude ratios suggests that a model of the interface more complicated than one involving merely temperature and or compositional effects is required we use these observations to constrain layered models of the slab mantle interface using thomson haskell matrix modelling comparing observations with calculated amplitude ratios the results suggest that the crustal layer of basaltic composition persisting to depth during subduction plays a role in the generation of the shallow converted waves the deeper conversions appear to require layering as well perhaps involving eclogite at the slab surface in addition to oriented olivine in the overlying mantle and the a p and a y phase transformations in olivine within the slab

waveform effects of a high velocity subducted slab creager and jordan 1986 propose that penetration of subducting slabs under the kurile islands and other subduction zones to depths of at least 1000 km is necessary to explain travel time anomalies of deep earthquakes such penetration would also be expected to affect the amplitudes and waveforms of the body waves from earthquakes synthetic seismograms appropriate for a record section in a plane perpendicular to the strike of the slab are presented using a coupled finite difference and kirchhoff method our shear wave version of the compressional wave velocity structure of creager and jordan 1986 produces an amplitude decrease up to a factor of four and waveform broadening up to 20 seconds for sh arrivals with a take off angle pointing straight down the slab slabs that extend only 300 km below the earthquake but are half as thick and twice as anomalously fast as creager and jordan s 1986 velocity model will roughly preserve the travel time variation pattern and show less waveform broadening but produce first arrivals that are emergent slabs that become thicker with depth show less waveform broadening reconciliation of the amplitude waveform distortion and timing of body waves from deep events is necessary to understand the geometry of slabs in the upper mantle and near and below the 650 km discontinuity

scsp observed on north island new zealand implications for subducting plate structure seismic phase conversions provide important constraints on the layered nature of subduction zone structures recordings from digital stations in north island new zealand have been examined for converted scs to p scsp arrivals from deep 150 km tonga kermadec earthquakes to image layering in the underlying hikurangi subduction zone consistent p wave energy prior to scs has been identified from stations in eastern and southern north island where the subducted plate interface is at a depth of between 15 and 30 km two scs precursors are observed ray tracing indicates that the initial precursor scsp1 corresponds to conversion from the base of an 11 14 km thick subducting pacific crust the second precursor is interpreted as a conversion from the top of the subducting plate the amplitude ratio scsp1 scs increases from 0 10 to 0 19 from northern to southern north island this is within the range expected from a simple first order velocity discontinuity at an oceanic moho a 1 2 km thick layer of low velocity sediment at the top of the subducting plate is required to explain the remaining scsp waveform our results imply that the abnormally thick hikurangi chatham plateau has been subducting beneath new zealand for at least 2 9 myr thus explaining the high uplift rates observed across eastern north island

seismic constraints on mechanisms of deep earthquake rupture 1 observations of the depth distribution of seismicity suggest that several mechanisms cause deep earthquakes cold subducting slabs exhibit a broad peak in seismicity between 300 and 530 km depth that is best explained by transformational faulting of metastable olivine cumulative seismic moment release increases sharply at 530 km depth in both cold and warm slabs implying that events deeper than this are controlled by an equilibrium rather than a metastable phase change which would be temperature dependent this together with the observation that deep seismicity predominates near the garnet rich upper boundary of the subducting plate former oceanic crust suggests that the transformation of garnet to perovskite which may release considerable water is integral in the cause of earthquakes deeper than 530 km where most deep earthquakes occur

subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab mantle interface seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested these studies employing waves reflected and converted at the slab mantle interface suggest that a velocity contrast of 5 10 occurs over a distance of 10 20 km to depths of at least 350 km the authors have investigated whether such sharp contrasts can result form thermal structure compositional differences phase changes or other effects such as preferred mineral orientation using a mantle and slab temperature model a petrologic model of mantle and slab mineralogy and a data base of elastic properties of mantle and slab phrases theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints at 9 6 gpa and 1000c conditions modeled at the slab mantle interface at approximately 280 km depth cooler temperatures in the slab interior produce at constant composition only approximately a 1 75 v sub p increase into the slab in any of the peridotitic and eclogitic mineralogies investigated variation in v sub p among the different bulk compositions is only approximately 0 5 these contrasts are substantially lower than those obtained from the seismological studies andmore indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts they propose that phase transformations in mantle and slab mineralogies notably elevation of the olivine spinel phase change normally occurring at 400 km depth are important factors in these sharp contrasts other contributing effects may include preferential mineral orientation near the slab surface and at shallower depths the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface less

regional variations of amplitudes of scsp phases observed in the japanese islands summary scsp phases are examined with special reference to their amplitude variations over seismograph networks in the japanese islands scsp of rela tively large amplitude are observed along the pacific side of the japanese islands amplitude ratios of the scsp with respect to scs are explained in terms of scs to p conversion at the upper boundary of a descending pacific plate which dips at about 30 and whose seismic wave velocity is between 5 and 10 per cent higher than that of the surrounding mantle large amplitude scsp are observed at stations nearest the kurile or japan trench the ampli tude ratios of scsp scs are found to decrease rapidly with distance inland from the trench an estimate is made of the effects of sharpness of the con version interface the incident angle of scs upon it and lateral variations of anelasticity within the asthenosphere overlying the descending pacific plate upon the scsp scs ratios we show that the sharpness of the conversion inter face is the dominant factor this result implies that the nature of the scs to p conversion interface varies with distance from the trench or equivalently with depth the presence of a low velocity zone in the vicinity of the conversion interface is inferred from the relative polarity between scsp and scs observed at the stations on the outermost side of north eastern japan scsp arrivals from shallower conversions at depths less than about 80 km observed in central and south western japan are also examined to elucidate the structure of the philippine sea plate descending beneath this region a discussion is made on the correlations between dipping subcrustal seismicity and the scs to p conversion interfaces except for the chugoku region the scs to p conversion interfaces correlate well with the dipping subcrustal seismicity our scsp observations indicate the presence of a dipping dis continuity in the uppermost mantle beneath the chugoku region where no subcrustal earthquakes have been detected

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

gross errors in upper mantle discontinuity topography from underside reflection data summary currnet models of upper mantle discontinuity topography derived from traveltime variations of long period underside reflections pdp or sds waves may contain gross errors due to the use of geometrical optics in relating observed traveltimes to depth variations of the discontinuity near the reflection point the use of geometrical optics is not valid if variations in the depth of the discontinuities exist on a lateral scale smaller than the size of the fresnel zone in the data geometrical optics does not take into account the large size and the complex structure of the traveltime surface of underside reflections and may introduce spurious structure when used in inversions examples of synthetic long period pdp waveforms for reflections near subducting lithosphere show that scattering from expected small scale depth variations of the discontinuities inside slabs causes complex waveform variations for a 15 20s pdp wave these small scale depth variations with a scale length of 4 are smaller than the fresnel zone roughly 20 but larger than the pdp wavelength at 420 km depth about 150 200 km the synthetic waveforms are processed to obtain pdp traveltimes which are in turn converted to apparent discontinuity depth at the reflection point using geometrical optics the apparent discontinuity depth variations are not only of the same order of magnitude as those obtained from real data but also exhibit artefacts that closely resemble features observed in current models it is shown that large scale wavelengths of the order of 1000 km downwarping of the 670 km discontinuity near subduction zones derived from underside reflection data can be explained as the long wavelength manifestation of a single small scale wavelengths of several hundreds of kilometres deflection of the discontinuity inside and near subducting slabs

tomographic imaging of lithospheric structures under taiwan abstract tomographic images of the crustal and mantle velocity structures under taiwan are obtained by simultaneous inversion of local earthquake p wave arrival times for hypocenters and p wave velocity structures in northern taiwan a high velocity zone coinciding with the wadati benioff zone can readily be identified as the subducted philippine sea plate the imaged zone dips toward the north at an angle of 40 from a depth range of 20 55 to 100 130 km an upper mantle low velocity wedge ranging from 40 to 80 km in depth exists above the subducted slab above this wedge is the ilan plain of northern taiwan which lies at the west end of the okinawa trough a well recognized back arc basin the crustal velocities under the plain are also relatively low the well defined high velocity zone and the low velocity wedge provide some constraints on the thermal structures of the subduction system under northern taiwan in tomographic images across the central section of taiwan thickening of the crust and up arching of the lower crustal materials under the central range are commonly observed the crust under the western foothills region is clearly thinner and the near surface low velocity layers are well developed the structures under the central range show that although the taiwan orogeny is quite young a root deeper in the north and shallowing to the south has formed the results of our tomography show that a significant portion of the lithosphere is involved in the taiwan orogeny