global p wave tomography on the effect of various mantle and core phases abstract in this work many global tomographic inversions and resolution tests are carried out to investigate the influence of various mantle and core phase data from the international seismological center isc data set on the determination of 3d velocity structure of the earth s interior our results show that when only the direct p data are used the resolution is good for most of the mantle except for the oceanic regions down to about 1000 km depth and for most of the d layer and pp rays can provide a better constraint on the structure down to the middle mantle in particular for the upper mantle under the oceans pcp can enhance the ray sampling of the middle and lower mantle around the pacific rim and europe while pdiff can help improve the spatial resolution in the lowermost mantle the outer core phases pkp pkikp and pkkp can improve the resolution in the lowermost mantle of the southern hemisphere and under oceanic regions when finer blocks or grid nodes are adopted to determine a high resolution model pp data are very useful for improving the upper mantle structure the resulting model inferred from all phases not only displays the general features contained in the previous global tomographic models but also reveals some new features for example the image of the hawaiian mantle plume is improved notably over the previous studies it is imaged as a continuous low velocity anomaly beneath the hawaiian hotspot from the core mantle boundary cmb to the surface implying that the hawaiian mantle plume indeed originates from the cmb low velocity anomalies along some mid oceanic ridges extend down to about 600 km depth our results suggested that later seismic phases are of great importance in better understanding the structure and dynamics of the earth s interior

topography of the 410 and 660 km discontinuities beneath the korean peninsula and southwestern japan using teleseismic receiver functions topography of the 410 and 660 km seismic upper mantle discontinuities beneath the korean peninsula and southwestern japan were determined using teleseismic receiver functions p receiver functions were migrated from delayed times to corresponding piercing conversion points of p to s converted phases using one dimensional 1 d and three dimensional 3 d models receiver functions were then stacked using common conversion point ccp techniques to enhance signal to noise ratios and thereby reduce uncertainty noise the 410 and 660 km discontinuities were clearly imaged as positively valued amplitude peaks of ccp stacked receiver functions in the study area topographic variations were roughly consistent with the low temperature of the subducting pacific plate however the complex structure of the subducting pacific plate produced distinct changes of upper mantle discontinuities which cannot be explained by temperature variations alone depression of the 410 km discontinuity observed in a wide region extending from the korean peninsula to kyushu island may be related to trench rollback history furthermore the topography of the 660 km discontinuity varies significantly with latitude at latitudes higher than 38 n its depth remains unchanged despite the presence of the stagnant slab while significant depression has been observed at latitudes below 36 n this may have been caused by differences in the angles of subduction of the japan slab and the izu bonin slab however heterogeneity of the water content of slabs may also have contributed to this topographical difference

lithospheric and upper mantle structure of southern tibet from a seismological passive source experiment fifteen seismic stations were operated with about 20 km spacing in southern tibet across the zangbo suture the collision zone between india and asia between may and october 1994 as part of the international deep profiling of tibet and the himalaya project indepth ii for wide angle recording of the controlled source experiment and for passive earthquake recording in addition a dense deployment 4 km spacing of stations within the german depth profiling of tibet and the himalayas gedepth project also recorded a number of teleseismic earthquakes the third data source used in this study is the records of the permanent broadband station at lhasa teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from p to s converted phases important results are as follows the moho at 70 80 km and a second discontinuity at 50 60 km depth are observed over the entire profile south and north of the zangbo suture the data from the gedepth dense array enable the detection of inclined structures penetrating the crust at the zangbo suture a pronounced low velocity zone exists north of the zangbo suture at about 10 20 km depth the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model iasp91 kennett 1991 over a large region of the himalaya and southern tibet

factors influencing cavity detection in karst terrain on two dimensional 2 d direct current dc resistivity survey a case study from the western part of thailand to create a three dimensional 3 d cave and cavity network model in karst terrain from two dimensional 2 d direct current resistivity dcr profiles we first need to recognize their effects on the inversion results from a number of special cases 2 d and 3 d cavity models are created to demonstrate effects from 1 depth of the cavity 2 size and shape 3 distance to the edge of the cavity 4 orientations inclination and declination of the cavity and 5 several smaller cavities lying in an array for other complicated models combinations of these factors can be used to infer the results two parallel dcr profiles from the western part of thailand were first inverted with the 2 d inversion for interpretation knowledge gained from the numerical modeling on these special cases is helpful to constrain the interpretation and also to extend the interpretation to 3 d this gives us a picture of the 3 d cave and cavity networks beneath the area of study two main connected cavities between these two profiles can be inferred from the data along with several local cavities near the surface

a complex 660 km discontinuity beneath northeast china abstract we present a detailed seismic study of the 660 km discontinuity beneath northeast china using the receiver function technique we use seismic data collected from 24 broadband stations in northeast china analysis of these data shows that the 660 km discontinuity is locally depressed in the region from longitude 128 0 e to 130 5 e and latitude 40 0 n to 44 0 n and then it splits into multiple discontinuities in the surrounding regions the complexity of the 660 km discontinuity beneath the subduction zones is probably attributable to the interaction between the upper mantle and subducted slab we speculate that the flattening of the subducted lithosphere near the bottom of the upper mantle causes the multiple discontinuity structure and that the slab penetrating the 660 km discontinuity into the lower mantle causes the narrow 660 km depression area

regional seismic characteristics of the 9 october 2006 north korean nuclear test we investigate the regional seismic signature of the 9 october 2006 north korean nuclear test broadband regional data for the nuclear test and a group of earthquakes close to the test site were obtained between december 2000 and no vember 2006 epicentral distances from the stations to the test site are between 371 and 1153 km we first use these regional events to calibrate the lg wave magnitude in the network then the network is used to calculate mblg 3 93 for the north ko rean nuclear explosion using a modified fully coupled magnitude yield relation the yield of the north korean nuclear test is estimated to be 0 48 kt because of large uncertainties in the source depth the estimate is preliminary the p s type spectral ratios pg lg pn lg and pn sn are calculated for the nuclear explosion and a group of earthquakes close to the test site at frequencies above 2 hz the network averaged p s spectral ratios clearly separate the 9 october 2006 explosion from the regional earthquakes our result indicates that a single blast explosion in the north korea re gion shows different seismic characteristics from an earthquake any well coupled single blast explosion detonated in this region with yield similar to that for the north korean nuclear test has a large probability of being identified by a regional seismic network such as the one adopted in this study

cratering mechanics on venus pressure enhancement by the atmospheric ocean the mass per unit area of the venusian atmosphere is 106 kg m2 equivalent to a 1 km deep water ocean an important effect of this ocean on cratering is pressure enhancement due to multiple reverberations during the compression stage we consider impactors with diameters d less than a scale height 15 km such impactors drive before them bow shocks with standoff distances of 0 05 d into the lowermost atmosphere reflection of this shock at normal incidence from the basalt surface raises the pressure in the atmosphere and surface but initially not as much as a direct impact would several shock reverberations should occur however as the shock crossing time in the trapped compressed atmospheric layer is much less than either the shock crossing time in the impactor or the time for the impactor to penetrate the atmospheric layer with each reverberation we estimate new hugoniots for shocked atmosphere impactor and target the first is treated as an ideal gas of 1 2 1 3 and the latter are modeled with murnaghan equations of state and gruneisen s 1 the asymptotic pressure enhancement for a 20 km s carbonaceous chondrite impactor modeled as serpentine striking basalt at 20 km s relative to the atmosphereless 1 d impedance match pressure 550 gpa is a factor of 1 4 this maximum enhancement factor increases with velocity reaching 1 6 for serpentine moving at typical venus impacting cometary velocities 65 km s and is greater for more compressible impactors comets each reverberation sends a shock pulse into the surface or impactor that overtakes the previous ones hence at the end of the compression stage both impactor and target are shocked to a higher pressure than would occur in the absence of the atmosphere the impactor is effectively denser or staffer than in the atmosphereless case but as the energy and momentum delivered are essentially the same entering efficiency should be unaffected to first order however fusion and vaporization efficiency should be enhanced for impactors whose impact velocities are below whether initially or due to atmospheric drag the threshold for complete melting or vaporization respectively in the absence of an atmosphere

receiver function migration image of the deep structure in the bohai bay basin eastern china 1 we apply a receiver function poststack migration method to image the crustal and upper mantle structures beneath the bohai bay basin eastern china the thick sediments of the basin appear to have a substantial influence on the migrated image of the deep structure by using the surface reflected ppps multiples instead of the commonly used ps converted phases a curved moho is coherently imaged showing a thinned crust of 30 km in the basin area compared with the 40 km thick crust in the west mountain range by incorporating the sedimentary velocity model into migration both the 410 km and the 660 km discontinuities present a simple structural feature with relatively flat topography within the basin s interior although possible local structural complexities may exist at the base of the upper mantle around the boundary areas of the basin the mantle transition zone is on average 10 15 km thicker than the global average reflecting a cold lower upper mantle environment that is distinct from the warm shallow upper mantle of the region

implications of a simple mantle transition zone beneath cratonic north america many areas of old continental interiors have thick roots that extend to depths of 250 km or more in contrast to the oceans and younger continents whose lithospheric thickness is less than 100 km these cratonic roots might perturb temperatures in their surrounding mantle though the net result could be either cooling or heating both of which may lead to small scale convective flow around the root we show here using new data from a study of the seismic structure of the canadian shield that the relative positions of the 410 km and 660 km seismic discontinuities are unperturbed beneath one of the deepest and broadest cratonic roots on the earth differential arrival times and internal discontinuity structure are remarkably uniform and simple varying by little more than 0 5 s over the root s 3500 km lateral extent this implies that the root has no significant thermal effect on the underlying mantle 50 k and any small scale convection or cold mantle downwelling associated with the large free air gravity anomaly beneath the shield must be confined to the upper mantle our observations are also consistent with phase changes solely in the olivine system mg fe 2sio4 with the caveat that our method is dominantly sensitive to s wave velocity jumps alone

detailed moho variations under northeast china inferred from receiver function analyses and their tectonic implications abstract in this study we investigate detailed moho variations beneath northeast china by applying the arithmetic mean back projected and fresnel zone migration imaging methods to a total of 169 602 high quality p wave receiver functions from seismograms of 2903 teleseismic events recorded at 127 necessarray stations and 321 china earthquake administration stations our results show that the moho depth variations are correlated with the surface geology in the study region the moho is deeper 34 0 42 0 km under the great xing an range the lesser xing an range the zhangguangcai range and the changbaishan mountain whereas it is shallower 26 0 32 0 km under the songliao basin our results also reveal obvious moho variations across the north south gravity lineament the moho offsets up to 5 0 km are clearly observed under the nenjiang balihan yilan yitong and dunhua mishan faults indicating that they are lithospheric scale faults a deeper moho is revealed under the volcanoes such as the jingpohu wudalianchi and changbaishan volcanoes and abaga and halaha volcanic groups in particular the moho under the changbaishan volcano reaches 40 0 km depth and the observation varies with the teleseismic back azimuths suggesting a complicated magma system in the crust in addition the moho under the songliao basin varies significantly from 26 0 km depth in the east to 32 0 km depth in the west which could be related to the lithosphere extension and thinning all these results suggest that there exists a hot and wet mantle upwelling in the big mantle wedge formed by the deep dehydration of the long stagnant pacific slab in the mantle transition zone under northeast china

p and s wave tomography and anisotropy in northwest pacific and east asia constraints on stagnant slab and intraplate volcanism we determined three dimensional p and s wave velocities and p wave azimuthal anisotropic tomography of the northwest pacific subduction zones by inverting 1 225 086 p wave and 335 117 s wave arrival times from 13 413 earthquakes our results show some differences between p and s wave images for the stagnant pacific slab in the mantle transition zone mtz beneath northeast china the stagnant slab looks thicker in the p wave image than that in the s wave image which may reflects the effects of both hydration and lower temperature in the mtz though differences in the resolution of p and s wave tomography may also have some effects the changbai intraplate volcanism is caused by hot and wet upwelling in the big mantle wedge above the stagnant pacific slab our p wave anisotropy tomography shows that the fast velocity direction fvd in the subducting philippine sea plate beneath the ryukyu arc is ne sw trench parallel which is consistent with the spreading direction of the west philippine basin during its initial opening stage suggesting that it may reflect the fossil anisotropy a striking variation of the fvd with depth is revealed in the subducting pacific slab beneath the northeast japan arc which may be caused by slab dehydration that changed elastic properties of the slab with depth the fvd in the mantle wedge beneath the northeast japan and ryukyu arcs is trench normal which reflects subduction induced convection beneath the kuril and izu bonin arcs where oblique subduction occurs the fvd in the mantle wedge is nearly normal to the moving direction of the downgoing pacific plate suggesting that the oblique subduction together with the complex slab morphology have disturbed the mantle flow

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

upper mantle discontinuity structure in the region of the tonga subduction zone we study the mantle structure below the southwest pacific in order to examine the geometry of the tonga slab at depth and its interaction with the 410 and 660 km discontinuities hereafter called the 410 and the 660 we utilize data from stations of both the lau basin ocean bottom seismogram experiment and island stations of the southwest pacific seismic experiment the tectonic complexity of this region containing both the tonga subduction zone and the associated lau back arc spreading center make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as common conversion point stacking of receiver functions we produce a high resolution image of the upper mantle near the tonga subduction zone to show the interaction between the discontinuities and the subducting slab our results show the 410 uplifted by 30 km near the tonga slab and the 660 depressed by 20 to 30 km as expected for thermally controlled olivine phase transitions

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

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

buoyancy bending and seismic visibility in deep slab stagnation abstract the petrological consequences of deep subhorizontal deflection stagnation of subducting slabs should affect both apparent seismic velocity structures and slab morphology we construct kinematic thermal models of stagnant slabs and perform thermodynamic modeling of the consequent perturbation of high pressure phase transitions in mantle minerals focusing upon japan as our study area we calculate associated thermo petrological buoyancy forces and bending moments which along with other factors such as viscosity variations and rollback dynamics may contribute to slab deformation we consider effects of variations in depth of stagnation post stagnation dip angle phase transition sharpness transition triplication due to multiple intersection of geotherms with phase boundaries and potential persistence of metastable phases due to kinetic hindrance we also estimate seismic velocity anomalies as might be imaged by seismic tomography and corresponding seismic velocity gradients as might be imaged by receiver function analysis we find that buoyant bending moment gradients of petrological origin at the base of the transition zone may contribute to slab stagnation such buoyancy forces vary with the depth at which stagnation occurs so that slabs may seek an equilibrium slab stagnation depth metastable phase bending moment gradients further enhance slab stagnation but they thermally decay after 4600 700 km of horizontal travel potentially allowing stagnant slabs to descend into the lower mantle stagnant slabs superimpose zones of negative velocity gradient onto a depressed 660 km seismic discontinuity affecting the seismological visibility of such features seismologically resolvable details should depend upon both stagnation depth and the nature of the imaging technique travel time tomography vs boundary interaction phases while seismic tomography appears to yield images of stagnant slabs discontinuity topography beneath japan resolved by migrated receiver functions appears to be consistent with slab penetration of the transition zone however model slabs which bottom around 4780 810 km and then bend upwards by a few degrees can match both the tomographic and receiver function images

slab temperature and thickness from seismic tomography 2 izu bonin japan and kuril subduction zones delay times from teleseismic and local p wave arrivals are used to invert for a high resolution three dimensional velocity model beneath the northwest pacific the model shows high velocity slabs with average velocity anomalies of the order of 3 4 assuming the positive velocity deviations in the subducting lithosphere are to first order due to a temperature anomaly the results of a theoretical slab temperature profile based on the diffusion equation are converted to a synthetic slab velocity model temperature variations between the ambient mantle and the interior of the slab are converted to p wave velocity perturbations using dvp dt 4 8 10 4 km s 1 c 1 a nonlinear optimization scheme compares the tomograms obtained via tomography to the theoretically predicted models in order to determine the optimal values for slab thickness and mantle potential temperature using 1180 100 c as the potential temperature thickness estimates of 88 8 km 85 8 km and 84 8 km are obtained for the izu bonin japan and kuril slabs respectively a correlation exists between slab thickness and age which is strong if mantle temperature variations along the slab strike can be ruled out in the process of estimating slab thickness the predicted slab velocity model is used as a filter to enhance the initial minimum norm tomographic result the initial tomogram is modified to closely resemble the synthetic slab tomogram by using only null space components the use of the null space components guarantees that the enhanced solution will satisfy the original seismic delay times the enhanced slab images show very continuous and narrow slabs compared to the initial tomographic results

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

receiver function images of the mantle transition zone beneath ne china new constraints on intraplate volcanism deep subduction and their potential link abstract in order to better understand the deep subduction geometry of the pacific plate and genesis of intraplate volcanism in northeast china ne china we computed a total of 45 505 receiver functions from 788 teleseismic events recorded by 255 stations necessarray temporal and permanent stations in ne china we used a common conversion point stacking ccp method to generate a 3d reflectivity volume beneath the study area to position the p to s conversions to the correct depths we employed 3d crustal and mantle models as references to make time to depth conversion the 3d reflectivity volume was generated in an area between 115 135 e and 40 49 n in the depth range of 300 to 800 km we found significant topographic relief on the 660 km discontinuity across the study area in particular in a westward pacific plate subduction section between 40 n and 45 5 n the 660 km discontinuity is depressed by as much as 30 40 km along the western extension of the deep seismicity the depression is elongated along the strike of the deep seismicity and is confined to a 200 300 km region in the e w direction of subduction to the west of this depression the 660 km discontinuity is uplifted by 5 10 km in a rectangular area of 100 km by 200 km centered at about 125 e and 43 n in the north the 660 km discontinuity is moderately depressed 20 km in a broad area that extends further west the high and low regions in the 660 km topographic map correlate respectively with low and high velocity anomalies in the p and s wave tomographic velocity images at the same depth our results suggest that slab stagnation might not be occurring in the southern part of the ne china where the changbaishan volcanic complex is located thus the magmatism is unlikely caused by dehydration of the flat lying pacific slab in the transition zone the low velocity mantle upwelling arising from a gap of stagnant slabs is a likely source that feeds the volcanic complex in ne china

the crust and upper mantle structure beneath southeastern china we analyzed teleseismic waveforms recorded by 44 stations in the fujian and taiwan provinces of china and obtained 5344 high quality receiver functions the crustal thickness h and average crustal vp vs ratio k beneath every station were estimated using the h k stacking method crustal thicknesses near the fujian province range from 28 3 to 32 8 km with an average of 31 1 km and the corresponding vp vs ratios vary from 1 70 to 1 84 with a mean of 1 76 from inland to offshore of the fujian province the crustal thicknesses decrease and poisson s ratios increase these may indicate decreasing sio2 and increasing calcalkaline contents in the crust the discontinuity structures such as the moho subducting slab the 410 and 660 km discontinuities hereafter we call them the 410 and the 660 are also studied using common converted point ccp stacking of receiver functions along two nw se lines of central and northern taiwan the ccp stacking results show a western dipping structure at depths above 50 km suggesting that the philippine sea plate is probably subducting beneath the eurasian continent plate near the central and northern taiwan the ccp stacking results show sharp and flat 410 and 660 km discontinuities and the transition zone thickness tzt is the same as that of ambient mantle beneath fujian and taiwan strait but thickens in the east of taiwan these results suggest that 1 the subducting eurasian continent plate is confined to the depths above 410 km beneath fujian and taiwan strait and 2 the south china sea slab may reach the transition zone beneath the east of taiwan 2007 elsevier b v all rights reserved

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

pn anisotropic tomography of northeast china and its implications to mantle dynamics abstract in this study we manually pick a total of 9197 high quality pn arrival time data from seismograms of 457 events recorded at 271 provincial seismic stations and 69 necessarray portable seismic stations in northeast ne china using these pn arrivals having much better crosscrissing coverage of ray paths in the uppermost mantle we determine a new high resolution anisotropic tomographic model our new model shows distinct lateral heterogeneities in both velocity and anisotropy which are closely related to surface geology obvious high pn velocities are imaged under the songliao hailar and erlian stable basins prominent low pn velocities are observed under the changbaishan jingpohu longgang wudalianchi and datong volcanic areas as well as the halaha and abaga volcanic groups furthermore some low pn velocities are well connected or they are separated by high pn velocities but connected by the fast directions of pn propagation these results suggest the existence of some correlations among these volcanoes in the present day or the traces preserved in the past tectonic evolution history we propose a possible dynamic model that illustrates the material and energy exchange among the volcanoes which could be caused by a complicated rather than simple e w horizontal flow of mantle convection in the big mantle wedge due to the stagnancy and dehydration of the western pacific slab in the mantle transition zone these results provide new insights into the mechanism of genesis of active volcanoes and complex mantle dynamics of ne china

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

detailed structure of mantle transition zone beneath southeastern china and its implications for thinning of the continental lithosphere abstract this paper presents a new study on 3 d structure temperature and water content estimation of the mantle transition zone mtz by p wave receiver functions common conversion point ccp method on broadband array with 267 temporary stations and 203 permanent stations in southeastern china the results show different structural characteristics of the mtz to the north and south of the 29 n line in the northern part the 660 km discontinuity is generally deeper than globe models and there are two independent local depressions with characteristics of high velocity low temperature and water shortage we interpret them as two previous subducting plates stagnated in mtz possibly related to two separate subduction events at different times while in the southern part the 410 km discontinuity is locally depressed resulting in thinning of the mtz particularly in the cathaysian block south of the 29 n line based on the mtz structure and the estimation of temperature and water content we conside dehydration of the mtz as one of the important mechanisms of lithospheric thinning through interaction with the asthenosphere we then propose a dynamic model that combines paleo pacific flat slab subduction and upper mantle convection to explain the lithospheric extension and thinning magmatic rock distribution weak seismic activity and basin and range geomorphology this model differs from that north of the 29 n line in the north china block

multiple seismic discontinuities near the base of the transition zone in the earth s mantle the seismologically defined boundary between the transition zone in the earth s mantle 410 660 km depth and the underlying lower mantle is generally interpreted to result from the breakdown of the gamma spinel phase of olivine to magnesium perovskite and magnesiowustite laboratory measurements of these transformations of olivine have determined that the phase boundary has a negative clapeyron slope and does indeed occur near pressures corresponding to the base of the transition zone but a computational study has indicated that because of the presence of garnet minerals multiple seismic discontinuities might exist near a depth of 660 km ref 4 which would alter the simple negative correlation of changes in temperature with changes in the depth of the phase boundary in particular garnet minerals undergo exothermic transformations near this depth acting to complicate the phase relations and possibly effecting mantle convection processes in some regions here we present seismic evidence that supports the existence of such multiple transitions near a depth of 660 km beneath southern california the observations are consistent with having been generated by garnet transformations coupling with the dissociation of the gamma spinel phase of olivine temperature anomalies calculated from the imaged discontinuity depths using clapeyron slopes determined for the various transformations generally match those predicted from an independent p wave velocity model of the region

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

impact craters on venus initial analysis from magellan magellan radar images of 15 percent of the planet show 135 craters of probable impact origin craters more than 15 km across tend to contain central peaks multiple central peaks and peak rings many craters smaller than 15 km exhibit multiple floors or appear in clusters these phenomena are attributed to atmospheric breakup of incoming meteoroids additionally the atmosphere appears to have prevented the formation of primary impact craters smaller than about 3 km and produced a deficiency in the number of craters smaller than about 25 km across ejecta is found at greater distances than that predicted by simple ballistic emplacement and the distal ends of some ejecta deposits are lobate these characteristics may represent surface flows of material initially entrained in the atmosphere many craters are surrounded by zones of low radar albedo whose origin may have been deformation of the surface by the shock or pressure wave associated with the incoming meteoroid craters are absent from several large areas such as a 5 million square kilometer region around sappho patera where the most likely explanation for the dearth of craters is volcanic resurfacing there is apparently a spectrum of surface ages on venus ranging approximately from 0 to 800 million years and therefore venus must be a geologically active planet

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

crustal structure and extensional deformation of thinned lithosphere in northern china abstract we herein present an interpretation of a 320 km long wide angle seismic profile between anxin and kuancheng which was obtained in 2002 the profile runs from the north china plain ncp where the lithosphere is just 70 km thick to the yanshan mountain folded belt ymfb where the lithosphere is180 km thick our model shows a crustal thickness that varies from 31 km under the ncp to 36 km under the ymfb the observed thinning of the crust in the ncp is about 14 which compares with an average extension of 24 41 at basin scale and 25 at lithosphere scale this finding suggests that the extensional deformation of the lithosphere in the north china block depends on depth the thin high velocity crust mantle transition zone has most likely originated after a delamination of the bottom of the crust and a concomitant intrusion of materials from the mantle the lower velocity of the lower crust may be attributed to the destruction of the lithosphere which permitted the lateral flow of melting materials above the moho from the ncp to the ymfb the differences found between the crust and the lithospheric mantle help to dispel any of the remaining uncertainty in the extensional factors and they may be attributed to detachment of middle crust and an intrusion of magma that originated in either lithosphere or asthenosphere we infer that the detachment of the middle crust lower crustal flow and magma intrusion probably lead to the underestimation of the crustal scale extensional factor and may represent the crustal response to the thinning of the lithosphere

crustal structure beneath northeast china from ambient noise tomography abstract to investigate the deep origin of cenozoic intraplate volcanism in northeast ne china especially the wudalianchi volcanism a new 3 d s wave velocity model of the crust and uppermost mantle is constructed through ambient noise tomography rayleigh wave phase velocity dispersion curves of 5 40 s periods are collected from seismograms recorded at our newly deployed wavesarray portable stations around the wudalianchi volcano and necessarray portable stations and cea provincial seismic stations in ne china compared to images resolved in previous studies the addition of wavesarray data provides much higher resolution in ne china under the songliao erlian and hailar basins obvious low velocity low v anomalies are imaged above 5 km depth which are well correlated with the sediments and have good correlations with the outlines of the basins under the changbaishan volcano and abaga and halaha volcanic groups low v anomalies extend continuously down to the uppermost mantle suggesting their continuous mantle magma provision under the wudalianchi volcano low v anomalies above 20 km depth and in the upper mantle are revealed but a high velocity anomaly zone appears in the lower crust suggesting that the wudalianchi volcano is a type of mantle origin but parts of magma have been solidified in the lower crust likely due to intermittent mantle magma provision all these results indicate that the intraplate volcanism in ne china could be related to complicated mantle dynamics of the big mantle wedge that has been formed due to the stagnancy and dehydration of the subducted pacific slab in the mantle transition zone

atmospheric effects on cratering on venus a paraboloidal bow shock model is developed in order to estimate the surface distribution of gas shock induced modifications surrounding venusian impact craters we apply two dimensional oblique shock dynamics to describe a three dimensional paraboloidal shaped bow shock impinging upon an assumed incompressible venusian surface the effects of the hypersonic atmospheric shock acting on the venusian surface are considered in terms of induced maximum gas pressure density particle velocity and temperature for varying angles and velocities of impact the maximum boulder size that can be saltated by the shock wave induced gas flow and the degree of mutual collision of the surface materials are also considered the present calculations quantitatively predict the areal extent of the gas shock perturbed surface for normal and oblique impact as a function of impact angle and velocity and radii of impactors for a 1 km radius stony meteorite impacting normally at 20 km s the radius of the disturbed area extends 10 17 times the 3 5 km crater radius the perturbed surface affects the surface radar properties and the present results can provide an explanation of the wide dark bright halos surrounding some of the venusian impact craters observed via magellan imagery for example a 50 km radius bright halo surrounding a 20 km dark halo is observed around the 3 1 km radius crater located at 16 5 north latitude and 334 4 longitude the average value of the radar backscatter cross section of the 20 km radius dark halo indicates that 50 cm thick layer of porous lithologic material is superimposed upon an assumed undisturbed basement rock surface the bright halo indicates that the surface roughness in this region is 30 greater than that of the surrounding original surface these features can be induced by atmospheric shock waves the present model can relate the observed crater halo radii to the impact parameters such as projectile radius and density and the impact velocity and angle

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

the upper mantle discontinuity structure beneath eastern china 1 we apply a common conversion point stacking technique of receiver functions from the north china interior structure project ncisp to image the upper mantle discontinuity structure beneath eastern china the stacking results of dense receiver functions reveal a prominent discontinuity structure the appearance of the 410 km discontinuity is sharp and consistent with little influence by western pacific subduction slabs the 520 km discontinuity is relatively weak and consists of splitting phases at depths ranging from 500 km to 550 km as for the 660 km discontinuity double discontinuities and a narrow depression zone are detected near a depth of 660 km indicating a more complex structure there phase transformations in non olivine components such as from garnet to perovskite seem to be a more plausible interpretation for a complex 660 km discontinuity in this region

mapping the subducting pacific slab beneath southwest japan with hi net receiver functions abstract we have used 4th root receiver function stacks and pre stack receiver function depth migrations to study the transition zone discontinuity structure beneath southwestern japan receiver functions were calculated from the quiet short period seismograms recorded by a recently deployed borehole network hi net we found that a relatively broad frequency band can be retrieved from a short period seismogram by a deconvolution of the instrument response the quality of the receiver functions formed from large earthquake recordings is comparable to those from broadband instruments we applied common conversion point gathering to the receiver function data to image the p to s conversion events beneath the network by stacking with a 4th root technique to improve lateral coherence we found that the topographic anomalies of the 410 and 660 km discontinuities beneath southwest japan have very different length scales the former is characterized by a narrow 150 200 km wide topographic high while the latter exhibits a broad 400 km wide moderate topographic low together with a small scale larger amplitude depression a 2 5d pre stack depth migration of the receiver functions shows the transition zone features clearly as well as images of a change of slope in the subducting slab at the 410 discontinuity and flattening of the slab onto the 660 km these observations show that the subducted pacific slab is deflected when it encounters the upper and lower boundaries of the transition zone and is flat lying either above or across the 660 km discontinuity the flat lying slab is however restricted to the bottom of the transition zone and probably experiences much less thickening than is suggested by some global tomographic images in which subhorizontal high velocity anomalies are seen throughout the transition zone between the two discontinuities

the crust and upper mantle discontinuity structure beneath alaska inferred from receiver functions in this study three receiver function stacking methods are used to study the detailed crust and upper mantle structure beneath south central alaska we used teleseismic waveform data recorded by 36 stations in the broadband experiment across the alaska range beaar and 4 permanent stations in alaska h stacking method using p to s converted wave and its multiply reflected waves between the earth s surface and the moho discontinuity is adopted to estimate the crustal thickness h and average crustal vp vs ratio in this region the receiver function results for 24 stations show that the crustal thickness under alaska ranges from 26 0 to 42 6 km with an average value of 33 8 km and the vp vs ratio varies from 1 66 to 1 94 with an average value of 1 81 which corresponds to an average poisson s ratio of 0 277 with a range from 0 216 to 0 320 high poisson s ratios under some stations are possibly caused by partial melting in the crust and the uppermost mantle common converted point ccp stacking results of receiver functions along three lines show clear moho and slab images under this subduction zone the depths of the slab from our ccp stacking images are consistent with those estimated from the wadati benioff zone wbz in the area between two stations dh2 147 8 w 63 3 n and dh3 147 1 w 63 0 n a moho depth offset of about 10 km is found by both the h and ccp stacking techniques common depth point cdp stacking of receiver functions shows not only the 410 520 and 660 km discontinuities but also significant variations 30 to 15 km in the transition zone thickness under the southwest and southeast parts of the study region the transition zone becomes thinner by 20 30 km indicating that the temperature there is 150 200 k higher than that of the normal mantle 2004 elsevier b v all rights reserved

seismological detection of low velocity anomalies surrounding the mantle transition zone in japan subduction zone in the japan subduction zone a locally depressed 660 discontinuity has been observed beneath northeast asia suggesting downwelling of materials from the mantle transition zone mtz vertical transport of water rich mtz materials across the major mineral phase changes could lead to water release and to partial melting in surrounding mantle regions causing seismic low velocity anomalies melt layers implied by low velocity zones lvzs above the 410 discontinuity have been detected in many regions but seismic evidence for partial melting below the 660 discontinuity has been limited high frequency migrated ps receiver functions indicate lvzs below the depressed 660 discontinuity and above the 410 discontinuity in the deep japan subduction zone suggesting dehydration melting induced by water transport out of the mtz our results provide insights into water circulation associated with dynamic interactions between the subducted slab and surrounding mantle

crustal and lithospheric structure of northeast china from s wave receiver functions abstract lithospheric thickness is closely associated with lithospheric response to tectonic forces or thermal processes analysis of s wave receiver functions srfs from a dense seismic array has revealed lithospheric thickness variations along a 1200 km long profile that spans all major geological terranes of northeast china the srf images identify the shallowest moho and lithosphere asthenosphere boundary lab depths beneath the rifted songliao basin thus spatially correlating well with the thick sediment accumulation these features can be explained by a pure shear regime operating in the lower crust and upper mantle suggesting the predominance of roughly symmetric lithosphere stretching from continental rifting in contrast thicker lithosphere in western and eastern terranes of ne china indicates the lithosphere in these regions was not strongly affected by the extensional processes during the late mesozoic flat lab structure beneath the erguna and xing an terranes along with the overlying relatively deeper moho beneath the great xing an range suggests the north south gravity lineament may not be a trans lithospheric structure

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

distinct velocity variations around the base of the upper mantle beneath northeast asia abstract both the global and regional p wave tomographic studies have revealed significant deep structural heterogeneities in subduction zone regions in particular low velocity anomalies have been observed beneath the descending high velocity slabs in a number of subduction zones the limited resolution at large depths and possible trade off between the high and low velocities however make it difficult to substantiate this feature and evaluate the vertical extent of the low velocity structure from broadband waveform modeling of triplicated phases near the 660 km discontinuity for three deep events we constrained both the p and sh wave velocity structures around the base of the upper mantle in northeast asia for the two events beneath the southern kurile the rays traveled through the lowermost transition zone and uppermost lower mantle under the descending pacific slab our preferred models consistently suggest normal to lower p and significantly low sh velocities above and below the 660 km discontinuity extending to about 760 km depth compared with the global iasp91 model corroborating previous observations for a slow structure underneath the slab in contrast both high p and sh velocity anomalies are shown in our preferred models for the japan subduction zone region likely reflecting the structural feature of a slab stagnant above the 660 km discontinuity the velocity jumps across the 660 km discontinuity were found to be on average 4 5 and 7 for p and s waves under the south kurile and 3 and 6 under the japan subduction zone the respective velocity contrasts in the two regions are consistent with mineralogical models for colder slab interior and hotter under slab areas based on mineral physics data the depth averaged 1 5 p and 2 5 sh velocity differences in the depth range of 560 760 km between the two regions could be primarily explained by a 350 450 k temperature variation although the presence of about 0 5 1 wt water might also contribute to the subtle velocity variations near the base of the transition zone in the southern kurile from our modeling results we speculate that the slow structure in the southern kurile may be correlated to the low velocity zone observed previously around the 410 km discontinuity under northern honshu if this is the case both may be associated with a thermal anomaly rooted in the lower mantle beneath the subduction zone in northeast asia

origin and significance of deep earthquake clusters surrounding a pronounced seismic gap in northeast china abstract from analysis of earthquakes in the global centroid moment tensor gcmt catalog we identify an intermediate depth 250 350 km seismic gap having dimensions of 300 km 500 km within the subducting northwestern pacific plate npp and surrounded by several groups of isolated earthquakes near northeast china normally groups of unusual deep earthquakes may represent a detached slab segment as in tonga vanuatu and a deep seismic gap maybe either be the aseismic continuation of a subducting slab or a possible gap in the subducting plate we evaluate the characteristics of deep earthquakes in the npp and focus on three isolated clusters particularly one directly beneath the seismic gap and northeast china this sub gap cluster has a peak in the number of quakes near depth 580 km unlike seismicity in the npp as a whole its b value 0 60 is significantly smaller than b values in two other nearby clusters 0 81 and 0 86 in the npp focal mechanisms in this sub gap cluster also show a variation with depth and projections of their t p and b axes differ from what is observed for the other isolated clusters we speculate why these sub gap earthquakes are isolated and suggest two possible models explaining the presence of the gap and the characteristics of the underlying cluster beneath northeast china in the first model the seismic gap is attributable to aseismic deformation caused by abnormally high temperatures in the subducting lithosphere and the characteristics of the sub gap clusters and its related slab structures are attributed to slab buckling caused by resistance to subduction in the second model the seismic gap represents absent subducting lithosphere and the missing material has migrated to the bottom of subducting slab although we favor the second model both models are consistent with our knowledge of deep earthquakes and the results of recent studies near northeast china

pervasive seismic low velocity zones within stagnant plates in the mantle transition zone thermal or compositional origin abstract we exploit conversions between p and s waves for large scale high resolution imaging of the mantle transition zone beneath northwest pacific and the margin of eastern asia we find pervasive reflectivity concentrated in two bands with apparent wave speed reduction of 2 to 4 about 50 km thick at the top of the transition zone and 100 km thick at the bottom this negative reflectivity associated with the scattered waves at depth is interpreted jointly with larger scale mantle tomographic images and is shown to delineate the stagnant portions of the subducted pacific plate in the transition zone with largely positive shear wave velocity contrasts the upper reflectivity zone connects to broad low velocity regions below major intra plate volcanoes whereas the lower zone coincides locally with the occurrence of deep focus earthquakes along the east asia margin similar reflectivity is found in pacific northwest of the usa we demonstrate that the thermal signature of plates alone is not sufficient to explain such features alternative explanations for these reflective zones include kinetic effects on olivine phase transitions meta stability compositional heterogeneities within and above stagnant plates complex wave propagation effects in the heterogeneous slab structure or a combination of such factors we speculate that part of the negative reflectivity is the signature of compositional heterogeneities as revealed by numerous other studies of seismic scattering throughout the mantle and that such features could be widespread across the globe

structure of the mantle transition zone beneath the north china craton abstract detailed information on deep structural features can improve current understanding of lithospheric thinning and destruction of the north china craton in this study we analyzed receiver functions rfs from teleseismic p waveforms recorded by 246 seismic stations beneath the north china craton ncc to determine the topography of the mantle transition zone mtz the results reveal specific features in the 410 and 660 km discontinuities and outlined mtz thickness beneath the study area the mtz structure shows two anomalously thickened regions beneath the ncc up to 30 40 km thicker than the global average which are mainly concentrated in the eastern and western sections of the eastern ncc encc the thickened regions are discontinuous and separated by a distance of about 400 km double 660 km discontinuities also appear in the southeastern encc combining these results with previous findings we propose a model in which mtz thickening is caused by discrete remnants of the subducted western pacific slab discontinuities resulting from the stagnant slabs may retain evidence of changes in the rate of pacific plate subduction as well as the phase transition of the subducted slabs within the mtz

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

shock wave equation of state of serpentine to 150 gpa implications for the occurrence of water in the earth s lower mantle the shock equation of state of serpentine has been determined to 150 gpa four distinct regions occur along the hugoniot a low pressure phase a mixed phase region a high pressure phase and a very high pressure phase the low pressure phase lpp exists under shock pressures to about 40 gpa this material exhibits shock properties that are partially consistent with those of low pressure serpentine but steep release paths and a low value of k 2 77 suggest transformation to another possibly amorphous assemblage thermodynamic calculations indicate that under equilibrium conditions serpentine would decompose to oxides plus water at conditions below 10 gpa along the hugoniot a mixed phase region begins at 40 gpa with complete transition to a high pressure phase occurring by about 55 gpa the high pressure phase hpp occurs at shock pressures between 55 gpa and 125 gpa model hugoniots based on perovskite plus periclase plus water and brucite plus periclase plus stishovite reproduce the serpentine hpp hugoniot within experimental error so definitive identification of the hpp as a distinct hydrous mineral phase or as a free water containing mixture is not possible above 125 gpa a transition to a very compressible phase possibly a hydrous partial melt occurs the serpentine hpp hugoniot is about 15 20 less dense than the earth s lower mantle models of the lower mantle based on shock equations of state for olivine pyroxene and serpentine indicate that for an atomic mg mg fe ratio of 0 80 the presence of 2 wt h 2o is consistent with seismically determined lower mantle density estimates greater amounts of h 2o can be accommodated if accompanied by an increase in fe content calculated hugoniot sound speeds of the serpentine hpp although poorly constrained are broadly consistent with lower mantle sound speeds thus the high pressure density and sound speed of an h 2o rich magnesium silicate determined from shock equation of state experiments indicate that the observed seismic properties of the lower mantle allow the existence of several weight percent of water in the lower mantle

interstation surface wave attenuation by autoregressive deconvolution a new technique for calculating interstation green s functions and attenuation coefficients for seismic surface waves is presented

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

a detailed receiver function image of the sedimentary structure in the bohai bay basin abstract if the site at which receiver functions are constructed is filled by sediments then the waveforms from these receiver functions are dominantly controlled by the sedimentary structures within the first few seconds after the direct p arrival based on this observation waveform data collected at 44 temporary seismic stations have been used to image the sedimentary structure of the bohai bay basin a major continental petroliferous basin in eastern china an adapted hybrid global waveform inversion method was applied to the receiver functions to extract structural information beneath each of the stations the derived s velocity structure provides for the first time a basin scale seismic image of detailed sedimentary stratification the sedimentary cover of the basin is about 2 12 km thick consisting of cenozoic mesozoic and paleozoic strata from top to bottom the structural features presented in the s velocity image coincide quite well with the depression uplift type of tectonic system in the bohai bay basin the reconstructed morphology of the sedimentary layers provides seismological evidence for the two stage evolution of the intracontinental basin that were caused by an intensive tectonic regime transition in late mesozoic immediately following the lithospheric reforming of the eastern china continent

mantle dynamics of western pacific and east asia insight from seismic tomography and mineral physics abstract recent results of high resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of western pacific and east asia the most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation corner flow processes in the mantle wedge the combination of the two processes may have caused the back arc spreading in the lau basin affected the morphology of the subducting philippine sea slab and its seismicity under southwest japan and contributed to the formation of the continental rift system and intraplate volcanism in northeast asia which are clearly visible in our tomographic images slow anomalies are also found in the mantle under the subducting pacific slab which may represent a small mantle plumes b upwellings associated with the slab collapsing down to the lower mantle or c sub slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite water we proposed a petrologic model for arc volcanism arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge a large amount of fluids can be released from hydrated morb at depths shallower than 55 km which move upwards to hydrate the wedge corner under the fore arc and never drag down to the deeper mantle along the slab surface slab dehydration reactions at 120 km depth are the antigorite related 5 reactions which supply water rich fluids for forming the volcanic front phase a and mg surssasite breakdown reactions at 200 and 300 km depths below 700 c cause the second and third arcs respectively moreover the dehydration reactions of super hydrous phase b phases d and e at 500 660 km depths cause the fluid transportation to the mantle boundary layer mbl 410 660 km depth the stagnant slabs extend from japan to beijing china for over 1000 km long indicating that the arc trench system covers the entire region from the japan trench to east asia we propose a big mantle wedge bmw model herein where hydrous plumes originating from 410 km depth cause a series of intra continental hot regions fluids derived from mbl accumulated by the double sided subduction zones rather than the india asia collision and the subsequent indentation into asia are the major cause for the active tectonics and mantle dynamics in this broad region