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

the effect of water on the 410 km discontinuity an experimental study 1 in order to evaluate the effect of h on the pressure and transition interval of the olivine wadsleyite transformation synthesis experiments were carried out in hydrous and anhydrous peridotite compositions at 1400 c under hydrous near saturated conditions the pressure of the transition is 1 0 0 5 gpa lower than under anhydrous conditions in addition the two phase interval appears to broaden from about 0 4 gpa 12 km in the anhydrous system to as much as 1 3 gpa 40 km in the hydrous system under hydrous conditions the spatial distribution of olivine and wadsleyite appears to be controlled by h diffusion extrapolating previously measured diffusion coefficients for h in olivine and in liquids it appears that h diffusion driven by gravitational equilibration may serve to narrow the transition interval between olivine and wadsleyite

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

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

upper mantle seismic discontinuities in a subduction zone japan investigated from p to s converted waves abstract the lateral variations of the upper mantle discontinuities in a subduction context are investigated by the use of broadband wave data provided by the geoscope network at the inuyama station japan differential arrival times of p and p to s conversions on the discontinuities are used these converted phases though of weak amplitude can be observed by stacking many seismograms a stack of 50 data with high signal to noise ratio is performed by different techniques the dominant period is around 7 10 s in this study and the lateral resolution is estimated to be of the order of 300 km this is an intermediate scale between short wavelength studies which can resolve very small details and long wavelength studies for which the fresnel zone is wider than about 1000 km the first stacking technique allows to retrieve the mean depth of the discontinuities below the station the main result is that the well known discontinuity close to 660 km is depressed down to about 700 km while the 410 km discontinuity is observed close to its usual depth and that a discontinuity close to 520 km is visible however in such a heterogeneous region the different traces are not coherent and it is useful to account for the variability of arrival times of converted phases using alternative stacking techniques we show that the 410 km discontinuity is elevated in the slab area of about 60 km and the 660 km discontinuity is depressed by more than 50 km this depression of more than 50 km found for the 660 km discontinuity yield to a value of 3 gpa k 1 of the clapeyron slope for the phases change olivine magnesiowustite perovskite although the 520 km discontinuity clearly shows up with all the techniques used it is difficult to reliably access its depth variation in the slab area the evidence for a 520 km discontinuity in this period range while it is not observed at shorter periods suggest that this discontinuity has a thickness between 15 and 25 km the large elevation of the 410 km discontinuity suggests that the thermally controlled depth of the phase change may affect the dynamic behaviour of the slab

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

ps conversions from the upper mantle transition zone beneath the canadian landmass broadband teleseismic p waveforms recorded on the canadian national seismograph network have been processed to isolate near receiver ps conversions originating from upper mantle discontinuities at approximately 410 and 660 km depth a simultaneous deconvolution procedure which removes source effects and exploits the move out characteristics of the weak ps phases is used to raise signal above the noise levels in the p coda large amplitude arrivals exhibiting times and slownesses consistent with an interpretation as ps converted phases from the 410 and 660 km discontinuities are observed at 18 and 20 of the 24 stations examined respectively the arrival times corrected for crustal heterogeneity show strong correlation with upper mantle structure above 410 km depth as determined in previous studies stations in the cordillera above low velocity mantle are characterized by residuals up to 3 3 s slow relative to the iasp91 model whereas sites overlying the high velocity root below the canadian shield are advanced by as much as 1 9 s the difference between the p660s and p410s times provides information on the structure of the transition zone beneath canadian stations the trend of the data indicates that topography on the discontinuities is at or below resolvable levels 5 km over most of the region in general accord with a previous study of the transition zone beneath continental stations but in contrast with results for the pacific basin

seismic discontinuities and subduction zones abstract the seismic discontinuities at 410 and 660 km depth in the mantle are generally believed to be due to phase transformations in its major component olivine an increasing observational base of short period regional seismic network data supports this view here we analyse cross sections of discontinuity topography in the northwest pacific subduction zones to infer the thermal and chemical state of the mantle in a subduction environment most of the data is from the southern part of the izu bonin subduction zone penetration of the lower mantle by the steeply dipping subducting slab is clearly indicated by depression of the 660 km discontinuity elevation of the 410 km discontinuity in this region by up to 60 km implies a thermal anomaly of approximately 900 1000 c relative to the ambient mantle observation of the elevated 410 in the seismically active part of the slab indicates that the olivine wadsleyite phase transformation occurs under essentially equilibrium conditions there which argues against the transformational faulting hypothesis as the mechanism for deep earthquakes upgoing p wave reflections from the 410 km discontinuity indicate that it is sharp in the mantle far from subducting slabs reflection coefficient variability in and near slabs indicates either a chemical effect or the consequences of topographic focusing

the structure and sharpness of mg fe 2sio4 phase transformations in the transition zone abstract to calculate accurately the pressure interval and mineral proportions i e yields across the olivine to wadsleyite and wadsleyite to ringwoodite transformations requires a detailed knowledge of the non ideality of fe mg mixing in these mg fe 2 sio 4 solid solutions in order to constrain the activity composition relations that describe non ideal mixing fe mg partitioning experiments have been conducted between magnesiowustite and mg fe 2 sio 4 olivine wadsleyite and ringwoodite as a function of pressure at 1400 c using known activity composition relations for magnesiowustite the corresponding relations for the three polymorphs were determined from the partitioning data in all experiments the presence of metallic iron ensured redox conditions compatible with the earth s transition zone the non ideality of the mg fe 2 sio 4 solid solutions was found to decrease in the order w wadsleyite femg w ringwoodite femg w olivine femg these partitioning data were used along with published phase equilibria measurements for the mg 2 sio 4 and fe 2 sio 4 end member transformations to produce an internally consistent thermodynamic model for the mg 2 sio 4 fe 2 sio 4 system at 1400 c using this model the pressure interval of the olivine to wadsleyite transformation is calculated to be significantly smaller than previous determinations by combining these results with fe mg partitioning data for garnet the widths of transition zone phase transformations in a peridotite composition were calculated the olivine to wadsleyite transformation at 1400 c in dry peridotite was found to occur over a pressure interval equivalent to approximately 6 km depth and the mineral yields were found to vary almost linearly with depth across the transformation this transformation is likely to be even sharper at higher temperatures or could be significantly broader in wet mantle or in regions with a significant vertical component of mantle flow the entire range of estimated widths for the 410 km discontinuity 4 35 km could therefore be explained by the olivine to wadsleyite transformation in a peridotite composition over a range of quite plausible mantle temperatures and h 2 o contents the wadsleyite to ringwoodite transformation in peridotite mantle was calculated to take place over an interval of 20 km at 1400 c this transformation yield was also found to be near linear

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

the 410 km depth discontinuity a sharpness estimate from near critical reflections the abrupt increase in seismic velocity and density about 410 km below the surface of the earth occurs at the depth where a phase change in olivine is expected akaogi et al 1989 katsura and ito 1989 ringwood 1969 we have assembled recordings from 22 short period seismometers in washington and british columbia of 35 nuclear explosions in nevada these data reveal relatively noise free reflections from the 410 discontinuity most of the transition is completed within a depth interval of less than 10 km under the western united states this result adds to the accumulating seismic evidence of a fairly sharp 410 transition

the effect of trace elements on the olivine wadsleyite transformation multianvil experiments were conducted at 1400 to 1600 8c on olivine and peridotite starting compositions to determine the partitioning of ti al cr ni ca and na between coexisting olivine and wadsleyite all of these elements occur as minor amounts in mantle olivine our experiments indicate that all except ca partition preferentially into wadsleyite relative to olivine the order of preference for wadsleyite is ni na cr ti al with d of about 2 for ni 3 for na and between 5 and 8 for cr ti and al we observe a wad ol tr strong negative correlation between the si and cr 1al contents of wadsleyite indicating a coupled substitution of 2cr 31 for mg 21 1 si 41 modeling the influence of the trace elements on the olivine wadsleyite transformation in the mantle indicates broadening effects on the order of 1 3 km much smaller than that predicted to arise from mantle concentrations of h2o therefore effects of these trace elements on the properties of the 410 km seismic discontinuity are considered negligible the maximum solubility of tio2 in wadsleyite about 0 6 is consistent with the suggestion that olivines containing about 1 vol fetio3 could be inverted wadsleyite

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

upper mantle seismic discontinuities and the thermal structure of subduction zones the precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western united states the boundary near 410km depth is locally elevated that near 660 km depressed the depths of these boundaries which mark phase transitions provide an in situ thermometer in subduction zones the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth in addition a reflector near 210 km depth may mark the bottom of the aesthenosphere

sharpness of the mantle discontinuities seismic estimates of sharpness of the mantle discontinuities are important for constraining models of composition and temperature in the deep earth but these data are difficult to obtain we explore a possibility to determine sharpness of the major mantle discontinuities those at depths around 400 and 650 km from the broad band records of phases converted from p to s underneath the receiver our estimates are obtained from a comparative analysis of waveforms of the converted phases and those of the p waves in the teleseismic records of brv kazakhstan grf germany nre0 norway and ykw canada the data indicate that the shape of the pulse converted at the 400 km discontinuity is close to that of the p wave whereas the pulse of the 650 km conversion is substantially broader we infer from this comparison that the 400 km discontinuity is sharp width of the transition zone is less than 5 7 km the 650 km transition is modelled by a linear gradient zone 20 30 km thick

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

effect of water in depleted mantle on post spinel transition and implication for 660 km seismic discontinuity abstract we have determined the post spinel transition boundary in anhydrous and hydrous mg2sio4 in a temperature range from 1173 to 2023 k at 19 3 25 4 gpa using synchrotron in situ x ray diffraction measurements the phase boundary in mg2sio4 is located at 22 gpa and 1800 k and 22 1 gpa and 1500 k which is slightly lower 0 3 0 5 gpa than that determined in the previous in situ measurements using the same pressure scale e g katsura et al 2003 post spinel transition in mg2sio4 determined by high p t in situ x ray diffractometry phys earth planet inter 136 11 24 the clapeyron slope of mg2sio4 was found to be gentle i e between 0 4 and 0 7 mpa k which is also consistent with previous in situ measurements but inconsistent with diamond anvil cell experiments and theoretical estimations the phase boundary in mg2sio4 2 wt h2o which is relevant to fe free depleted harzburgitic composition is located between 23 4 and 23 6 gpa and 1500 k which shifts the hydrous boundary to the higher pressures relative to anhydrous mg2sio4 from 1 3 to 1 0 gpa the result for hydrous mg2sio4 shows steeper clapeyron slope between 3 2 and 3 1 mpa k compared with anhydrous mg2sio4 and hydrous pyrolite system the present data suggest that water has a strong influence on 660 km discontinuity and the depressions observed at this boundary in several regions especially related to subduction zones can be explained by the presence of water in depleted harzburgite component

seismic discontinuities in the mediterranean mantle abstract layering in the upper and lower mantle across the mediterranean has been determined using p to s converted phases identified through receiver function analysis conversion of the receiver function s time axis to depth was based on local 1d s velocity models the 1d velocity model for each station was extracted from a three dimensional s velocity model for the mediterranean region eav03 enhanced with crustal structure derived from receiver function analysis under the mediterranean region we observed a mantle transition zone thickness of 261 10 km on average which agrees with a dominance of high velocities imaged in tomographic models at these depths a thick mantle transition zone 270 km was observed in regions with ongoing or past subduction eastern spain southern italy southern greece and the north western african coast conversions from the 410 km and 660 km discontinuities were clearly observed for most stations amplitudes of both discontinuities are anomalously large throughout the mediterranean region 4 9 and 5 7 respectively we observed significant conversions from the 520 km discontinuity the lower mantle showed strong lateral variations resulting in spatially incoherent converted phases from discontinuities near 860 km between 900 km and 1200 km and 1320 km depth

visualising active volcanism with high spatial resolution satellite data the 1991 1993 eruption of mount etna we present seven landsat thematic mapper tm images that document the period before during and after the 1991 1993 eruption of mount etna this episode remains the most volumetrically significant to occur at the volcano within the past 300 years we show how tm data can be used to identify major changes in the lava flow regime specifically the transition between channel and tube fed flow conditions by mapping changes in the radiant properties of the flow surface the high spatial resolution of tm 30 m pixels allows point sources of radiance such as skylights and tumuli to be distinguished from linear radiant features such as surface lava flows using this information we illustrate the spatial and temporal development of a major tube system that developed within the flow field detailed field maps of the tube system compiled by calvari and pinkerton 1998 allow us to validate the accuracy of our interpretations and lead us to conclude that tm can be used to infer the position of tubes within the active flow field to a reasonable level of accuracy this indicates that similar high spatial resolution data could be used as a stand alone tool to provide timely information regarding tube formation at future eruptions useful information given that tubes can be a major factor in determining for a given effusion rate how far lava flows when draped over a digital elevation model the thermal data not only portray the nature of the relationship between flow surface structure and regional topography but also provide a unique visualisation of how the flow field advanced and threatened to inundate the town of zafferana before ultimately being diverted

seismic evidence for water deep in earth s upper mantle water in the deep upper mantle can influence the properties of seismic discontinuities in the mantle transition zone observations of converted seismic waves provide evidence of a 20 to 35 kilometer thick discontinuity near a depth of 410 kilometers most likely explained by as much as 700 parts per million of water by weight

erosion rates of alpine bedrock summit surfaces deduced from in situ 10be and 26al we have measured the concentration of in situ produced cosmogenic 10be and 26al from bare bedrock surfaces on summit flats in four western u s mountain ranges the maximum mean bare bedrock erosion rate from these alpine environments is 7 6 3 9 m my 1 individual measurements vary between 2 and 19 m my 1 these erosion rates are similar to previous cosmogenic radionuclide crn erosion rates measured in other environments except for those from extremely arid regions this indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments even though frost weathering should be intense in these areas our crn deduced point measurements of bedrock erosion are slower than typical basin averaged denudation rates 50 m my 1 if our measured crn erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion then relief in the mountain ranges examined here is probably increasing we develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady state erosion model to episodically eroding outcrops our simulations show that interpreting measurements with the steady state erosion model can yield erosion rates which are either greater or less than the actual long term mean erosion rate while errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples the mean value of many steady state erosion rate measurements provides a much better estimate of the long term erosion rate

the effects of phase transition kinetics on subducting slabs the effects of kinetics on the non equilibrium aspects of the olivine to spinel transition in a descending slab have been studied numerically a one dimensional model consisting of the kinetic equations and the heat diffusion equation with latent heat release has been constructed numerical results show that the position and sharpness of the kinetic phase boundary is determined by the surface tension and the activation volume for slow slab velocities less than 6 cm yr near equilibrium conditions are found finger like structures emanating from the phase boundaries are obtained in this regime these phase boundary prolusions may cause earthquakes for slab velocities of around 10 cm yr the metastable olivine region may be pushed down to a depth of about 600 km with a sharp phase boundary a direct transition from olivine to perovskite may be feasible

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

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

global variability of transition zone thickness 1 we analyze recordings from more than 3000 shallow earthquakes and measure s410s s660s differential travel times on a global scale these differential times are obtained using a novel cross correlation technique that eliminates effects associated with a priori assumptions of crustal thickness and velocity variations above the upper mantle transition zone our measurements show absolute perturbations up to 8 s from the global average 242 km which imply transition zone thickness variations of 20 km on a length scale of 2000 km the transition zone under major subduction zones e g the western pacific and south america is significantly thicker than the global average this could be associated with large scale depressions of the 660 km discontinuity due to the accumulation of cold oceanic lithosphere at the base of the upper mantle a narrow transition zone is observed under the central pacific and northern atlantic oceans we also observe a modest anticorrelation between s410s s660s times and the delay times of shear waves predicted using s12 wm13 su et al 1994 in the transition zone which suggests considerable thermal influence on the large scale topography of the 410 and 660 km discontinuities the lack of a stronger anticorrelation suggests that 1 compositional heterogeneities may be present and 2 a significant fraction of thermal anomalies near the transition zone do not extend through the transition zone and influence both the 410 and the 660 km discontinuities a regional analysis shows that the transition zone is on average 6 8 km thicker under continents than under oceans this implies a temperature difference of 50 70 c however we do not observe a clear correlation between thickness and the age of the crust

modeling the tonga slab can travel time data resolve a metastable olivine wedge we present the results of detailed modeling of the tonga slab with the goals of determining whether high resolution travel time data 1 can be fit by simple thermal and petrological slab models and 2 can resolve a metastable olivine wedge at depths greater than the equilibrium olivine spinel phase boundary we model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers obs and island broadband seismic stations extending from the lau backarc basin across the tonga trench and onto the pacific plate the data consist of 388 local p wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3 month obs deployment in late 1994 we locate the events using both local and teleseismic arrival times and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference earth model the modeling consists of grid searches using a three dimensional finite difference algorithm to compute local first arriving travel times for equilibrium and metastable p wave velocity models constructed from thermal mineralogical and morphological constraints the travel time anomalies are well fit by standard slab thermal models and p velocity temperature derivatives of 0 4 to 0 3 ms 1 c 1 forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low velocity region wedge velocity models provide a slightly better fit to the data than equilibrium models but f tests indicate the improvement is not significant at the 95 level our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary

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

computation of seismic profiles from mineral physics the importance of the non olivine components for explaining the 660 km depth discontinuity the recent increasing number of experimental works leads us to review the elastic properties and mineralogical transformations of mantle minerals the updated data set is used to compute seismic profiles for two petrological models along three adiabatic temperature profiles these profiles are chosen to stress out the influence of non olivine minerals on seismic parameters and to represent cold and horizontally averaged temperature profiles of the earth s mantle in a first part starting compositions of pyrolite and piclogite and a single layer convection are assumed the results clearly point out the importance of the non olivine part of the mineralogy two scenarios are found to explain the 660 km depth discontinuity

experimental determination of the effect of h2o on the 410 km seismic discontinuity abstract due to the high solubility of h 2 o in the mineral wadsleyite it has been recognized that the presence of h 2 o in the mantle could influence the pressure and pressure interval of the olivine to wadsleyite phase transformation that is generally considered to be the cause of the 410 km seismic discontinuity we have performed multianvil experiments in the mg 2 sio 4 h 2 o system in order to quantify the expansion of the wadsleyite stability field as a result of the presence of h 2 o at 1200 c we find that the wadsleyite stability field expands to lower pressures by approximately 1 gpa under h 2 o rich fluid saturated conditions at 1400 c the expansion is approximately 0 4 gpa and no expansion can be detected at 1600 c a thermodynamic model based on recent h 2 o solubility measurements and crystallographic observations predicts an expansion of the wadsleyite stability field that is in good agreement with the experimental results when these results are combined with phase relations in the mg 2 sio 4 fe 2 sio 4 system it can be shown that significant effects on the pressure and pressure interval of the olivine to wadsleyite transformation i e on the depth and width of the 410 km discontinuity are only expected at lower than ambient mantle temperatures 2 o concentrations that are substantially greater than 0 2 wt areas of the mantle where the 410 km discontinuity appears to occur over a depth interval of over 20 km can only be explained if the temperature in these regions is at or below 1200 c and water concentrations are close to the level where olivine would become h 2 o saturated i e 0 5 wt the presence of ferric fe in the mantle however may also play a role in the broadening of the 410 km discontinuity that is as yet unquantified

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

reflections of p p from upper mantle discontinuities beneath the mid atlantic ridge summary precursors to p p observed in japan from a large deep earthquake mb 6 4 in the fiji islands are analysed to investigate reflection properties of upper mantle discontinuities beneath an area close to the mid atlantic ridge mar in the equatorial atlantic ocean traveltime analysis indicates that the precursors are interpreted as underside reflections of p p from discontinuities at depths of 655 and 415 km in the mantle the latter reflection is much weaker and more intermittent than the former slant stacking and beam forming are attempted to measure the slowness and incidence azimuth of p p and its precursors the obtained slownesses are 3 4 and 2 8 s deg 1 for p p and p p respectively little deviations of the ray incidence off the diametral azimuth are found for both phases amplitude ratio p dp p p here d 655 or 415 is studied as a function of the wave period and the distance of reflection point of p p to the axis of mar the overall trend of the spectral amplitude ratio for p 655p is consistent with the theoretical value for the 670 km discontinuity of the prem model dziewonski anderson 1981 p 415p signals were very weak and it was difficult to obtain an unbiased estimate of p 415p p p the amplitude ratios p 655p p p of filtered seismograms with a passband of 0 5 4 0 s have a mean value of 0 13 and a standard deviation of 0 03 this amplitude ratio is in good agreement with those of previous studies on the reflections from the 650 km discontinuity beneath antarctica and surrounding seas when the reflection points of p p are calculated under the assumption of a spherically symmetric earth the observed p 655p p p seems to decrease as the points become close to the axis of mar thus it could be inferred that some changes in the reflectivity of the discontinuity might occur at or near the spreading center another interpretation may also be possible the low velocity beneath the mar region would cause focussing of p p rays