magma mixing in boninite sequence of chichijima bonin islands abstract boninite from chichijima can be divided into two major types a porphyritic type phenocryst mode exceeds 1 and b aphyric type phenocryst mode is less than 1 both types are further subdivided by means of microphenocryst assemblages into five subtypes i olivine ol clinoenstatite cen bronzite brz ii ol brz iii brz iv brz pigeonite pig augite aug v cen brz there is a systematic correlation between these rock types and the bulk composition of aphyric boninites the most magnesian boninites are of type i and the least magnesian ones are of types iii and v type ii is slightly higher in cao than type i at a given feo mgo ratio type iv is distinct from the others for its high cao and low incompatible element contents at a given feo mgo ratio type iv cannot be derived from the others by crystallization differentiation compositional data for the other types of boninite define rectilinear compositional variations which lie on a tie line connecting fe rich bronzite and primitive boninite microphenocrysts in equilibrium with boninite magma are more magnesian so it would appear that the fe rich bronzite was in equilibrium not with boninite magma but with some evolved descendant e g bronzite andesite magma the variations of aphyric boninites are then best explained by a combination of mixing of primitive boninite and evolved descendants together with equilibrium crystallization processes in order to account for this combination of processes a density stratified open system magma chamber of the type proposed for oceanic ridge systems seems appropriate the upper part of the magma chamber is composed of differentiated bronzite andesite magma and the lower part consists of hot dense and primitive boninite magma crystallization and sporadic overturn and mixing give rise to the spectrum of magma types erupted at the surface

propagation characteristics of lg across the tibetan plateau abstract we present the results of a study on the propagation of lg within the tibetan plateau we analyze over 1200 seismograms for lg propagation from 185 local and regional earthquakes recorded at distances from 150 to 2000 km 52 of which are located within the tibetan plateau although the propagation of lg has been observed across most of asia lg has not been observed for paths crossing the tibetan plateau we analyze paths that cross the plateau boundaries and paths contained entirely within the tibetan plateau lg is generated within the tibetan plateau and can propagate to epicentral distances of at least 600 km for events located outside of the tibetan plateau lg is either absent or severely diminished for paths that cross both the himalayan and kunlun ranges confirming that the margins of the plateau weaken lg transmission based on the observations from our qualitative analysis we invert lg amplitudes for event station paths that are confined to the tibetan plateau to estimate the quality factor q the inversion yields a frequency dependent q function q f 366 37 f 0 45 0 06 0 5 f 16 h z similar observations in other areas indicate that frequency dependent apparent q within the tibetan plateau is well below that expected for a typical continental interior instead it has a frequency dependence that is characteristic of a tectonically active region

strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

site amplification scattering and intrinsic attenuation in the mississippi embayment from coda waves we estimate site amplification intrinsic q q i and scattering q q s values from three component coda of broadband data recorded by cooperative new madrid sesimic network stations for center frequencies 0 2 0 5 1 0 2 0 and 5 0 hz near 0 5 and 1 0 hz site amplification factors for the transverse component recorded by stations in the mississippi embayment are between 1 7 and 18 compared with stations outside the embayment the amplification factor becomes smaller at frequencies lower than 0 5 hz and higher at those greater than 1 0 hz the radial component shows a similar amplification however the vertical component shows relatively less amplification q i and q s are estimated from coda using an energy flux model for two time windows after the direct s wave and at 0 5 and 1 0 hz center frequencies the two time windows include coda up to and after twice the s wave travel time at each station coda parameters change with the time window and are attributed to a mixture of incoherent coda with coherent s wave reverberations directly after the s wave q i and q s inferred from stations outside of the embayment have higher values than those from stations within the embayment a plane wave model qualitatively explains the lack of coda amplification at high frequencies compared with a low frequency without recourse to anelastic attenuation arguments however the plane wave model cannot explain the very large observed coda amplifications at 1 and 0 5 hz seen in the data suggesting that other wave scattering effects must be occurring in the embayment velocity structure this is consistent with the relatively low q s values inferred from the energy flux model for stations within the embayment in addition to coda amplification we performed a synoptic noise study that revealed that pre event ambient background noise behaves in a similar manner to the coda for stations within and outside of the embayment the presence of large lateral velocity heterogeneities in the embayment may cause q s values to be low there

the relation between site amplification factor and surficial geology in central california abstract site amplification factors at frequencies of 1 5 3 6 and 12 hz were determined for 132 stations of the usgs seismic network in central california from coda waves of 185 local earthquakes in this area using a recursive stochastic inversion method we found that the site amplification at a station is systematically related to the geology underlying that station the site amplification is high for young quaternary sediments and decreases with increasing geologic age at all frequencies between 1 5 and 12 hz the rate of decrease varies with frequency where site amplification at low frequencies shows a faster rate of decrease with age than at higher frequencies to quantify the relation between site amplification factor and site geology the surface geology of station sites was classified into five groups namely 1 quaternary sediments 2 pliocene sediments 3 miocene through cretaceous sediments 4 franciscan formation and mesozoic granitic rocks and 5 pre cretaceous metamorphic rocks the station site amplification factors for each group were logarithmically averaged and the mean value was assigned to the median geologic age of that group a smooth power law relation was found between the mean site amplification and the median geologic age this relation provides a simple way of estimating site effect at a specific site with known surficial geology our site amplification factors were compared with magnitude station residuals determined by eaton 1990 for the same network a remarkable linear correlation was found between the logarithmic coda amplification factor and the magnitude station residual value confirming that the coda method provides an effective means of estimating site amplification factor comparison of our site amplification factor with strong motion results obtained from the loma prieta earthquake boore et al 1989 maley et al 1989 shakal et al 1989 chin and aki 1991 suggests that weak and strong motion site amplification factors correlate well in the region outside the epicentral source region beyond epicentral distances of about 50 km within the epicentral source region however the weak motion amplification factors estimated from coda waves do not agree with the observed for strong ground motion suggesting a nonlinear behavior at sediment sites these results together with studies on the correlation between weak motion site amplification and earthquake intensity borcherdt 1970 borcherdt and gibbs 1976 chavez garcia et al 1990 king et al 1990 demonstrate the importance of in situ determination of weak motion site amplification for seismic zonation

seismic attenuation tomography in southwestern china insight into the evolution of crustal flow in the tibetan plateau abstract crustal flow in southwestern china is commonly attributed to the continental collision between the indian and the eurasian plates therefore studying crustal flow in this area can provide valuable insight into the collisional process the quality factor q is an indicator of seismic wave attenuation that provides constraints on lithology rheology and geodynamic processes which are vital for interpreting the nature of the crustal flow in a given region in this study we used local seismic waveforms to obtain regional attenuation models for p qp and s qs waves in the sichuan yunnan region syr the emeishan large igneous province elip is characterized by high qp and qs values in the middle yunnan block myb at depths greater than 5 km the qp and qs values in most of the fault zones surrounding the myb were lower than 300 indicating that the material within crustal faults causes high waveform attenuation our results suggest that the crustal flow was diverted into two channels in the northern myb due to resistance from the elip to illuminate the tectonic dynamics of the syr we propose a simple model that explains both the evolution of the crustal flow and the variation in the seismic wave attenuation

os pb nd isotope and highly siderophile and lithophile trace element systematics of komatiitic rocks from the volotsk suite se baltic shield abstract highly siderophile element hse ru pd re os ir and pt abundances and re os isotope systematics in combination with lithophile trace element and sm nd and pb pb isotopic data are reported for komatiites and basalts from the volotsk suite in the se baltic fennoscandian shield the new sm nd and pb pb whole rock isochron ages of 2850 84 and 2861 150 ma respectively define the emplacement age of the volcanic sequence this age is statistically indistinguishable from the emplacement age of the lower mafic ultramafic sequences of the adjacent sumozero kenozero greenstone belt the re os internal isochron derived from the analysis of a whole rock b2 4 cumulate and three chromite separates yields an age of 2878 81 ma which is consistent with the ages obtained from the lithophile element isotopic systems the komatiites of the volotsk suite were derived from a mantle source characterized by depletions in highly incompatible lithophile trace elements with the magnitude of the time integrated lree depletion initial nd 2 7 0 2 typical of this period in the earth s mantle history yet this source had an initial 187os 0 4 0 4 and thus evolved with a time integrated re os that was identical within uncertainty to that of the carbonaceous chondritic reference in addition the calculated relative abundances of os ir ru and pt in the komatiite source are similar to chondrites and may be explained by late post core segregation accretion to the earth of materials with high absolute and chondritic relative abundances of hse or by re entrainment of the core if the former interpretation is correct the late accreted materials were well homogenized within the upper mantle by 2 9 ga like many other archean komatiites the studied rocks have radiogenic nd isotopic composition indicative of their long term relative depletions in highly incompatible lithophile trace elements yet the long term re os in their source remained near chondritic modeling indicates that the komatiite source must have experienced only a small degree of partial melting 2 3 and extraction of basaltic melts ca 150 ma prior to emplacement of the komatiitic sequences larger degrees of melting would otherwise have led to loss of a substantial fractionation of re in the source which would then have evolved to a subchondritic 187os 188os by the time of komatiite formation

dynamic faulting studied by a finite difference method we have developed a finite difference method that is especially adapted to the study of dynamic shear cracks we studied a number of simple earthquake source models in two and three dimensions with special emphasis on the modeling of the stress field we compared our numerical results for semi infinite and self similar shear cracks with the few exact solutions that are available in the literature we then studied spontaneous rupture propagation with the help of a maximum stress criterion from dimensional arguments and a few simple examples we showed that the maximum stress criterion depended on the physical dimensions of the fault for a given maximum stress intensity the finer the numerical mesh the higher the maximum stress that had to be adopted a study of in plane cracks showed that at high rupture velocities the numerical results did not resolve the stress concentration due to the rupture front from the stress peak associated with the shear wave propagating in front of the crack we suggest that this is the reason why transonic rupture velocities are found in the numerical solutions of in plane faulting when the rupture resistance is rather low finally we studied the spontaneous propagation of an initially circular rupture two distinct modes of nucleation of the rupture were studied in the first a plane circular shear crack was formed instantaneously in a uniformly prestressed medium after a while once stress concentrations had developed around the crack edge the rupture started to grow in the second type of nucleation a preexisting circular crack became unstable at time t 0 and started to grow the latter model appeared to us as a more realistic simulation of earthquake triggering in this case the initial stress was nonuniform and was the static field of the preexisting fault

three dimensional finite difference modeling of the san andreas fault source parameterization and ground motion levels olsen et al 1995 recently simulated an mw 7 75 earthquake on the san andreas fault predicting long period t 2 5 sec ground velocities of 140 cm sec in the los angeles basin about 60 km from the fault these motions are much larger than estimates derived from empirical relations or other numerical simulations standard area magnitude relations predict that the 170 16 km fault used in the simulations would produce an mw 7 5 earthquake giving a moment of 2 0 1027 dyne cm which is 2 4 times smaller than the moment used by olsen et al 1995 further self similar scaling predicts a rise time of 3 sec for an mw 7 75 event and 2 2 sec for an mw 7 5 event the filtered impulse slip function used by olsen et al 1995 has an effective rise time of 1 6 sec yielding a response that is about 2 times larger than expected for periods less than 5 sec this combination of high seismic moment and short rise time along with the use of a uniform slip distribution leads to the extreme ground motion levels predicted by olsen et al 1995 to quantify the sensitivity of the long period ground motion response to source parameterization we have performed 3d finite difference fd simulations using various combinations of seismic moment source rise time and slip heterogeneity these calculations incorporate the same grid dimensions fault size and bandwidth employed by olsen et al 1995 with a moment of 2 0 1027 dyne cm a rise time of 2 sec and a smoothly heterogeneous slip distribution we simulate peak long period ground velocities of 155 cm sec in the near fault region and 40 cm sec in the los angeles basin these values are much closer to although still higher than empirical predictions a uniform slip distribution produces the largest peak motions both in the near fault region and in the los angeles basin whereas a rough asperity slip distribution noticeably reduces the maximum near fault ground velocities our results indicate that the accurate simulation of long period ground motions requires a realistic source parameterization including appropriate choices of seismic moment and rise time as well as the use of spatial and temporal variations in slip distribution

lg attenuation in northeast china using necessarray data nishath r ranasinghe 1 andrea c gallegos 1 andrea r trujillo 1 2 alexander r blanchette 1 eric a sandvol 3 james ni 1 thomas m hearn 1 youcai tang 4 5 stephen p grand 4 fenglin niu 5 6 yongshun j chen 7 jieyuan ning 7 hitoshi kawakatsu 8 satoru tanaka9 and masayuki obayashi9 1department of physics new mexico state university las cruces nm 88001 usa e mail nrana001 nmsu edu 2apache corporation 303 veterans airpark lane 600 midland tx 79705 usa 3department of geological sciences university of missouri 101 geology bldg columbia mo 65211 usa 4jackson school of geosciences the university of texas at austin austin tx 78712 usa 5state key laboratory of petroleum resource and prospecting and unconventional natural gas institute china university of petroleum beijing 102249 china 6department of earth science ms 126 rice university houston tx 77005 usa 7institute of theoretical and applied geophysics sess peking university beijing china 8earthquake research institute the university of tokyo tokyo japan 9institute for frontier research on earth and evolution japan agency for marine earth science and technology yokosuka japan

two crustal low velocity channels beneath se tibet revealed by joint inversion of rayleigh wave dispersion and receiver functions abstract competing geodynamic models such as rigid block extrusion continuous deformation and the mid lower crustal flow have been proposed to describe the growth and expansion of eastern tibet however the dynamic processes responsible for plateau evolution and deformation remain poorly understood partly due to resolution limitations of previous models of lithospheric structure on the basis of joint inversion of rayleigh wave dispersion and receiver functions using data from a newly deployed seismic array we have obtained a high resolution 3d image that reveals the distribution of low velocity zones lvzs with unprecedented clarity the prominent feature of our model is two low velocity channels that bound major strike slip faults in se tibet and wrap around the eastern himalaya syntaxis consistent with the clockwise movement of crustal material in this region most large earthquakes in this region occurred in the boundaries of the lvzs we propose that ductile flow within these channels in addition to shear motion along strike slip faults played a significant role in accommodating intensive lithospheric deformation during the eastward expansion of tibet in the cenozoic

3d simulation of near field strong ground motion based on dynamic modeling abstract we simulate the strong ground motion generated from the earthquake rupture process on a shallow strike slip fault using a 3d finite difference method the faulting process is modeled using a crack model with fixed rupture velocity the variability of peak ground velocity patterns correlated with fault location and source parameters such as stress drop or rupture velocity is investigated our findings suggest that these patterns are strongly affected by rupture directivity and the uppermost depth of the fault or that of the asperity when a fault breaks the ground surface the peak ground velocity and the peak ground acceleration show a narrow region of strong motion when a fault is buried under the ground the high peak ground velocity zone of the fault parallel component is apart from the fault trace by a distance comparable to the fault depth on the other hand the fault normal peak ground velocity is a maximum along the fault trace the fault length or asperity length is not so effective for peak ground velocities the effect of heterogeneity in stress drop and rupture velocity on strong ground motion is also investigated when stress drop is not uniform but increases linearly with depth from zero at the uppermost depth the peak ground velocity is reduced these results help better predict the strong ground motion generated from a potential fault

topographic slope as a proxy for seismic site conditions and amplification we describe a technique to derive first order site condition maps directly from topographic data for calibration we use global 30 arc sec topographic data and v s 30 measurements here v s 30 refers to the average shear velocity down to 30 m aggregated from several studies in the u s as well as in taiwan italy and australia v s 30 values are correlated against topographic slope to develop two sets of parameters for deriving v s 30 one for active tectonic regions where topographic relief is high and one for stable shields where topography is more subdued by taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear velocity observations we can recover to first order many of the spatially varying features of sitecondition maps developed for california our site condition map for the low relief mississippi embayment also predicts the bulk of the v s 30 observations in that region despite rather low slope ranges we find that maps derived from the slope of the topography is often well correlated with other independently derived regional scale site condition maps but the latter maps vary in quality and continuity and subsequently also in their ability to match observed v s 30 measurements contained therein alternatively the slope based method provides a simple approach to uniform site condition mapping

velocity field in asia inferred from quaternary fault slip rates and global positioning system observations we perform a joint inversion of quaternary strain rates and 238 global positioning system gps velocities in asia for a self consistent velocity field the reference frames for all geodetic velocity observations are determined in our inversion procedure india in moves relative to eurasia eu about a pole of rotation at 29 78 n 7 51 e 0 353 myr 1 which yields a velocity along the himalaya within india that is 73 76 of the magnitude of the in eu nuvel 1a velocity and a vector azimuth that is 8 10 clockwise of nuvel 1a in eu vector azimuth relative to eurasia south china moves at 9 11 mm yr in the direction 110 120 with a pole position 64 84 n 156 74 e 0 12 myr 1 amurian block motion has a pole position in a similar location but at a slower rate 64 61 n 158 23 e 0 077 myr 1 and most of the amurian eurasia motion is accommodated by extension across lake baikal tarim basin moves relative to eurasia about a pole of rotation at 39 24 n 98 2 e 0 539 myr 1 and 16 18 mm yr of shortening is accommodated across the west central tien shan there is distributed e w extension throughout both southern and north central tibet within southern tibet between the longitudes of 77 e to 92 e the deformation field accommodates 16 19 mm yr of e w extension we compare predicted seismic moment rates with those observed in this century in asia total observed seismic moment rates within the entire area of central and east asia 2 2 107 km2 in this century are 2 26 0 7 1020 n m yr 1 as compared with a predicted total rate of 2 03 0 066 1020 n m yr 1 comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long term seismic moment rates from a catalog of limited duration 94 years an observation period of 10 000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates inferred from the joint inversion of gps and quaternary rates of strain we show that in general a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude mb 5 0 for the period january 1 1965 to january 1 1999

p wave tomography and dynamics of the crust and upper mantle beneath western tibet abstract the continental collision between the indian and asian plates plays a key role in the geologic and tectonic evolution of the tibetan plateau in this article we present high resolution tomographic images of the crust and upper mantle derived from a large number of high quality seismic data from the antilope project in western tibet both local and distant earthquakes were used in this study and 35 115 p wave arrival times were manually picked from the original seismograms geological and geochemical results suggested that the subducting indian plate has reached northward to the lhasa terrane whereas our new tomography shows that the indian plate is currently sub horizontal and underthrusting to the jinsha river suture at depths of 100 to 250 km suggesting that the subduction process has evolved over time the asian plate is also imaged clearly from the surface to a depth of 100 km by our tomography and it is located under the tarim basin north of the altyn tagh fault there is no obvious evidence to show that the asian plate has subducted beneath western tibet the indian and asian plates are separated by a prominent low velocity zone under northern tibet we attribute the low velocity zone to mantle upwelling which may account for the warm crust and upper mantle beneath that region and thus explain the different features of magmatism between southern and northern tibet but the upwelling may not penetrate through the whole crust we propose a revised geodynamic model and suggest that the high velocity zones under lhasa terrane may reflect a cold crust which has interrupted the crustal flow under the westernmost tibetan plateau

attenuation of multiphase surface waves in the basin and range province part i lg and lg coda summary broad band data recorded by the lawrence livermore laboratory network are used to investigate whether or not lg q and lg coda q have similar values in the basin and range province of north america in order to retrieve spectral estimates of the broad band lg and lg coda signals a hybrid method is used which retains high fidelity at lower frequencies while strongly smoothing the spectral estimates and minimizing the number of spectral points at high frequencies a stochastic approach is used to model the lg signal and ambient noise and to analyse results from two station determinations of lg q in the basin and range the effects of ambient noise frequency dependent station site response and focusing defocusing on lg q inversion are analysed it is shown that after averaging spectral ratios determined from repeated observations over four two station paths a modest site effect will not cause significant errors in the resulting lg q relation using this inversion scheme and spectral ratio estimates averaged over those paths at frequencies between 0 2 and 2 5 hz we obtain the relation qlg f 267 f 56 f0 37 0 06 for the basin and range province further constraints on lg q were obtained from lg coda which due to the natural averaging effect of scattering is insensitive to effects of focusing defocusing to investigate the validity of the single isotropic scattering model for lg coda in the basin and range province we calculated ischrons for scatterers corresponding to later lg coda signals as required by the single scattering model it is found that when significant portions of these isochrons are oceanic lg coda amplitude decay becomes irregular and abnormally slow suggesting a strong scattering reflection effect caused by the rapidly changing crustal structure along the coast applying the single isotropic scattering model to lg coda records with isochrons mostly confined to the continental crust results in q f 275 f 26 f 0 36 0 03 which is nearly identical to our qlg relation this indicates that even in a complicated geological province a joint interpretation of the dynamic properties of lg and lg coda can be successful and fruitful inconsistent lg q and coda q relations found in earlier studies in the basin and range province may be attributed to different settings of parameter trade offs different seismic phases or different dynamic properties being used

187os enriched domain in an archean mantle plume evidence from 2 8 ga komatiites of the kostomuksha greenstone belt nw baltic shield the re os data on archean komatiites from the kostomuksha greenstone belt in the baltic shield are presented this greenstone belt has been previously interpreted to represent a former oceanic plateau formed by the emplacement of an ancient plume head puchtel et al earth planet sci lett 155 1998 57 74 samples of flowtop breccia spinifextextured and cumulate komatiites and a chromite separate all collected from the core of a 300 m deep diamond drill hole yielded a re os isochron with an age of 2795 40 ma and an initial 187 os 188 os of 0 1117 0 0011 q 187 os 3 6 1 0 the high positive q 187 os t implies that the komatiites were derived from a mantle source with a time integrated suprachondritic re os ratio recycling of oceanic lithosphere to produce the enriched 187 os isotope signature is considered unlikely as 15 25 crustal component is required to be incorporated into the plume source as early as 3 5 4 3 ga such a substantial proportion of mafic material in the source would likely destroy the major and trace element characteristics of the komatiites our tentative interpretation is that the 187 os enrichment in the kostomuksha plume represents an outer core signature if confirmed by the ongoing pt os isotope studies the results would provide evidence for the existence of whole mantle convection in the late archean and might place constraints on the timing of core differentiation in the early earth fl 2001 elsevier science b v all rights reserved

re os results from odp site 801 evidence for extensive re uptake during alteration of oceanic crust abstract re and os concentrations and os isotopic ratios were determined for composite samples prepared from volcanoclastics vcl and basaltic flows flo from jurassic oceanic crust ocean drilling program leg 185 site 801 in the western pacific with the aim of determining the effect of seafloor weathering on the re os budget a supercomposite sample prepared from a proportionate mixture of the various composite powders served to represent the average composition of the altered oceanic crust kelley k a plank t ludden j and staudigel h 2003 composition of altered oceanic crust at odp sites 801 and 1149 geochem geophys geosyst 4 6 8910 doi 10 1029 2002gc000435 re contents vary from 0 2 to 1 3 ng g 1 and from 2 2 to 3 1 ng g 1 in the vcl and flo composites respectively os contents vary from 0 005 to 0 047 ng g 1 in the vcl and from 0 008 to 0 027 ng g 1 in the flo composites the flo composites have much higher re os ratios and thus have more radiogenic os compositions 187os 188os 1 38 to 8 48 than the vcl composites 187os 188os 0 32 to 4 40 the vcl composite from the upper section of the crust shows evidence for substantial re loss and os uptake consistent with oxidative weathering processes however re uptake during weathering processes under more reducing conditions evident in the flo samples from throughout the section and to a lesser extent in the lower vcl samples more than compensates for this re loss in the upper vcl os concentrations were essentially unchanged by these reductive processes model age calculations suggest that re uptake continued for tens of millions of years after crust formation abundant secondary pyrite is found throughout the altered hole 801c crust in zones of restricted seawater flow and this may have accommodated an important part of the input re the re content of the supercomposite 2 2 ng g 1 is about 1 ng g 1 higher than would be expected on the basis of its yb content if the results from hole 801c are typical they suggest that the re concentration of at least the upper part of the oceanic crust may be nearly doubled during seafloor alteration such large extents of re uptake would have a significant effect on the oceanic re budget furthermore assuming that they survive passage through the subduction zone these elevated re contents would greatly decrease the proportion of subducted oceanic crust required in the source region to explain the radiogenic os compositions of many ocean island basalts

breaking of a single asperity rupture process and seismic radiation the problem of spontaneous shear rupture of a single circular asperity on an infinite fault plane is studied initially the fault plane is broken everywhere except at a circular asperity an applied displacement at infinity results in a stress concentration along the bounding edge of the asperity the frictional stress on the broken part of the fault plane is taken to be a constant once a point on the asperity breaks the stress there drops to the same value as on the main fault surface the rupture is started by relaxing the shear stress at a point on the asperity edge and is then allowed to propagate spontaneously using a critical stress level fracture criterion the rupture process is calculated numerically it is found that for asperities of constant strength the rupture first propagates around the edge of the asperity and then inward a phenomenon best described by the well known term of classic military maneuver the double encircling pincer movement in the appendix the expressions for the far field seismic radiation due to the rupture of such an asperity are derived it is shown that the nth cartesian component of the far field displacement at x t for p sv and sh waves using the notation of aki and richards 1980 is given by un x t dni 4 c2r s0 i3 t r c ds thus the far field pulses can be directly found from the stress drops on the fault plane this formula is also true for crack or dislocation problems the directivity function dni for displacement for the asperity problem is found to be that for the double couple modified by some factor in particular the fault plane is a nodal plane for sv waves for the rupturing of asperities on a finite fault these directivity functions are applicable only to the initial part of observed pulses at a receiver provided the receiver is not located on the fault plane outside the broken part of the main crack edge in which case it is inapplicable for all time

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

moho undulations beneath tibet from grace integrated gravity data summary knowledge of the variation of crustal thickness is essential in many applications such as forward dynamic modelling numerical heat flow calculations seismologic applications and geohistory reconstructions we present a 3 d model of the moho undulations over the entire tibetan plateau derived from gravity inversion the gravity field has been obtained by using the gravity recovery and climate experiment grace potential field development which has been integrated with terrestrial data and is presently the best available in the studied area for the effective use of the global geopotential model that has no height information of observation stations upward continuation is applied the moho model is characterized by a sequence of troughs and ridges with a semi regular pattern which could reflect the continent continent collision between the indian and eurasian plates the three deep moho belts troughs and shallow moho belts ridges between them are clearly found to have an e w directional trend parallel to the border of the plateau and tectonic lines while variation of the directionality is observed in central to southeast tibet to describe the distinctive shape of the moho troughs beneath tibet we introduce the term moho ranges the most interesting aspects of the moho ranges are 1 that they run in parallel with the border and tectonic sutures of the plateau 2 that the distances between ranges are found at regular distances of about 330 km except in northeast tibet and 3 that the splitting of the ranges into two branches is found as the distance between them is increasing from our study we conclude that the distinctive undulations of the tibetan moho have been formed by buckling in a compressional environment superimposed on the regional increase in crustal thickness according to our analysis the grace satelliteonly data turns out to have good enough resolution for being used to determine the very deep moho beneath tibet our moho model is the first one that covers the entire plateau

petrogenesis of volcanic rocks from saipan and rota mariana islands and implications for the evolution of nascent island arcs an 40 ar 39 ar age of 45 1 ma determined for lavas from northern saipan confirms that these high silica rhyolites erupted during the proto arc stage of volcanism in the izu bonin mariana system which is characterized elsewhere by eruption of boninitic lavas incompatible trace element concentrations and sr hf nd and pb isotope ratios for these rhyolites are transitional between those of c 48 ma boninitic lavas and post 38 ma first arc andesites and dacites from saipan and rota that have typical subduction related compositions these transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust a second stage of rayleigh fractionation in the upper crust is required by sio 2 concentrations that exceed 77 wt and near zero compatible element concentrations first arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first arc magmas themselves the mantle sources of the proto arc and first arc lavas from saipan and rota are similar to those of philippine back arc basin basalts based on nd and hf isotopic compositions the pb isotope compositions of these lavas are between those of pacific sea floor basalts and jurassic and younger cherty and clay rich sediments this contrasts with the boninitic proto arc volcanic rocks from guam and deep sea drilling project sites 458 and 459 that have pb isotope compositions similar to pacific basin basalts and volcaniclastic sediments the preferred explanation for the difference in the nature of proto arc volcanism between saipan and other fore arc locations is that the crust ceased extending 3 4 myr earlier beneath saipan this was caused by a change from mantle upwelling fore arc extension and shallow melting to an environment dominated by more normal mantle wedge convection stable crust and deeper melting

on the nature of regional seismic phases i phase representations for pn pg sn lg summary an operator development of the seismic wavefield is used to generate descriptions of the propagation processes contributing to the main regional seismic phases pn pg sn lg these operator forms are valid for laterally heterogeneous crust and mantle models and include the major processes of interconversion between wavetypes these representations of the regional phases are used to examine the theoretical basis for discriminants between earthquakes and underground explosions based on the relative amplitudes of p and s phases the ratio of sn to pn amplitude looks promising as a high frequency discriminant however the ratio of lg to pn amplitudes is not as useful because of the complex nature of the propagation characteristics of lg

a novel technique for measuring lg attenuation results from eastern canada between 1 to 10 hz spectral amplitudes of regionally recorded lg waves are studied in detail between 0 6 and 10 hz using vertical component velocity seismograms of the eastern canada telemetered network stations and a supplementary seismic research observatory type station at glen almond gac quebec we find that the site responses vary among these stations by more than a factor of 3 within the frequency range of interest furthermore they are found to be strongly frequency dependent consequently it is essential that they be taken into consideration in studies of lg wave attenuation and lg source spectra of regionally recorded seismic events we present a new method of measuring interstation surface wave attenuation which is closely related to the conventional two station method while retaining all the desirable features of the conventional two station method the new technique which we will call the reversed two station method allows simple direct one parameter determination of the lg wave attenuation from sparse spectral data in a manner unaffected by station site effects and associated instrument error the reversed two station method is successfully tested over weakly attenuating short 53 to 210 km interstation paths in eastern canada a difficult experimental condition by normal standards the lg attenuation coefficient 0 6 to 10 hz in eastern canada is found to be frequency dependent and of the form f 0 0008 f 0 81 km 1 at higher frequencies the lg attenuation appears to be essentially frequency independent this latter finding is preliminarily interpreted as evidence that regionally recorded lg waves in the canadian shield are as in the case of lg waves propagating through the structurally complex appalachian province contaminated by the high frequency coda of sn waves the lg contamination over the shield paths becomes severe starting at 14 hz twice the frequency above which the lg signal propagating over the appalachian province becomes completely dominated by non lg arrivals

strong lg wave attenuation in the northern and eastern tibetan plateau measured by a two station two event stacking method 1 the regional seismic phase lg which involves shear wave reverberations in the crust is strongly attenuated along propagation paths in northern and eastern tibet robust estimates of the attenuation quality factor qlg in these regions are obtained using a two station two event stacking method for broadband vertical component lg data in the passband 0 2 1 0 hz we find average 1 hz qlg values qo of 79 4 in northern central tibet and 119 17 in eastern tibet for the 0 2 0 5 hz passband localized regions of eastern tibet have qo estimates ranging from 66 to 121 these results confirm and augment the spatial coverage of recent estimates of low qo values in north central and south central tibet and support the notion of widespread partial melting of the crust throughout the plateau

estimation of site effects and q factor using a reference event a method to determine site effects and the q factor using a reference event is described spectral ratios between pairs of earthquakes and a reference one were written as logarithmic summations in order to obtain individual solutions for the numerators and denominators given the seismic moment and corner frequency for one of the events following the 2 model a source model was assumed to constrain the amplitude of the solution the validity and limitations of this technique were tested on synthetically generated data we analyzed the variances of the results depending on the different number of stations used the amount of noise present in the records and the change in the seismic moment corner frequency and source model of the reference event aftershocks of the 2000 tottori japan earthquake were used to estimate the site effects and q factor in that region two strong motion networks k net and kik net recorded the aftershocks the latter network contained both surface and borehole receivers at every station data were inverted first using only surface records and then using the borehole records from kik net stations in order to study amplitude differences our results suggest that the study area presents a low q value and that there is also amplification at borehole sites manuscript received 18 january 2002

specific distribution of ground motion during the 1995 kobe earthquake and its generation mechanism the 1995 kobe hyogo ken nanbu earthquake mw 6 9 is the most damaging in the recent japanese history and its notable feature is that most of the damage and over 6000 casualties occurred in a narrow belt through kobe and neighboring cities the kobe hanshin area usually a fault rupture propagation generates strong ground motions along the direction of propagation and this directivity effect should be significant above the source faults and beyond its leading end however the damage zone of the kobe earthquake is migrated noticeably from the fault trace into the center of kobe city three dimensional 3 d simulations of ground motion in kobe show that this migration is due to the strong amplification and ray bending in the sedimentary basin below the city in cooperation with the multipathing effects at a basin bedrock boundary

azimuthal anisotropy of lg attenuation in eastern tibetan plateau 1 the q of seismic phase lg is measured to approach the crustal q which can be used to infer crustal rheology a reverse two station event method is used to measure the interstationqlg which theoretically eliminates effects from the source and site response strong azimuthal anisotropy of 1 qlg in northern tibet is observed which approximately correlates with the seismic velocity anisotropy observed using both surface waves and teleseismic s waves both the intrinsic and scattering attenuation contribute the isotropic q whereas the anisotropic q may be associated with the fractures and faults in the crust strong 1 qlgis observed in the northwestern songpan ganzi fold belt where the high qdirections correspond with the orientations of major strike slip faults intrinsic and scatteringq values are estimated for different tectonic terranes in eastern tibetan plateau the intrinsic qvalues are approximately 500 in the northern tibetan plateau which suggests a hot 700 c crust consistent with low seismic velocity measurements our result suggests that the crust of northwestern songpan ganzi fold belt may be hotter than that of the qiangtang terrane which may be related to the strain heating along the major strike slip fault zones that dominate the northernmost tibet

distribution and origin of igneous rocks from the landward slopes of the mariana trench implications for its structure and evolution the landward slope of the mariana trench is composed largely of igneous rocks serpentinites and serpentinized ultramafic rocks occur at nearly all structural levels on the slope from depths of 8000 to 1200 m seamountlike features on the trench slope break are the surface expression of serpentinite diapirs cumulate and massive gabbros are found several varieties of volcanic rocks are common including boninites altered and metamorphosed basalts andesites and dacites the chemical characteristics of the volcanic rocks indicate that nearly all are products of island arc volcanism together with the gabbros these volcanic rocks represent what is probably a late eocene arc complex these rocks were probably the first volcanic products to result from the subduction of the pacific plate beneath the phillippine sea plate their exposure on the trench slope today implies a significant amount of tectonic erosion of the landward slope since eocene time most of this removal of material appears to have occurred during the early stages of subduction there are isolated occurrences on the landward slope of rock assemblages including alkalic basalts chert hyaloclastites upper cretaceous siliceous sediments and shallow water limestones these assemblages are very similar to rocks dredged from seamounts on the offshore flankmore of the trench and their presence on the landward slope suggests that since the cessation of vigorous tectonic erosion there has been episodic accretion of seamount fragments to the landward slope less

rayleigh wave phase velocity maps of tibet and the surrounding regions from ambient seismic noise tomography ambient noise tomography is applied to the significant data resources now available across tibet and surrounding regions to produce rayleigh wave phase speed maps at periods between 6 and 50 s data resources include the permanent federation of digital seismographic networks five temporary u s program for array seismic studies of the continental lithosphere passcal experiments in and around tibet and chinese provincial networks surrounding tibet from 2003 to 2009 totaling 600 stations and 150 000 interstation paths with such a heterogeneous data set data quality control is of utmost importance we apply conservative data quality control criteria to accept between 5000 and 45 000 measurements as a function of period which produce a lateral resolution between 100 and 200 km across most of the tibetan plateau and adjacent regions to the east misfits to the accepted measurements among passcal stations and among chinese stations are similar with a standard deviation of 1 7 s which indicates that the final dispersion measurements from chinese and passcal stations are of similar quality phase velocities across the tibetan plateau are lower on average than those in the surrounding nonbasin regions phase velocities in northern tibet are lower than those in southern tibet perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained at short periods 20 s very high velocities are imaged in the tarim basin the ordos block and the sichuan basin these phase velocity dispersion maps provide information needed to construct a 3 d shear velocity model of the crust across the tibetan plateau and surrounding regions

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

lateral variation of pn and lg attenuation at the cdsn station lsa abstract the propagation efficiencies of the regional phases lg and pn are indicative of how active and recent tectonics influence crust and uppermost mantle properties respectively in this study regional scale lateral heterogeneity of lg and pn attenuation for the region in and around the southern tibetan plateau is investigated the data set is comprised of seismograms recorded at the chinese digital seismogram network cdsn station lsa from regional events with epicentral distances within 11 attenuation of lg and pn is calculated using spectral methods and assuming constant q lg and q pn models for the frequency bands 0 3 to 3 0 hz and 0 5 to 4 0 hz respectively lateral variation in attenuation is estimated by analyzing data on an event by event basis significant lateral variation is observed with q lg decreasing from about 520 for events south of lsa to about 340 for events north of lsa and q pn 670 for southern backazimuths while q pn 240 for northern events for pn this north south variation is consistent with other observations indicating partially melted upper mantle beneath north central tibet for lg the azimuthal variation in attenuation indicates that lg propagation as observed at lsa is efficient for paths crossing the southern boundary of the tibetan plateau relative to paths within the plateau itself

crustal structure of western tibet revealed by lg attenuation tomography abstract the lateral lg wave attenuation in western tibet and surrounding regions is studied a total of 706 two station method tsm and 533 reverse two station method rtsm pairs are generated from 282 regional earthquakes the earthquakes are recorded at 48 seismic stations operated across western kunlun and western tibet regions the lg phases are extracted from the vertical component of seismograms with magnitude greater than 4 6 mb and focal depth less than 50 km to measure the tsm and rtsm lg q at 1 hz q0 the lgq0 tomography map is generated with grid size 0 5 0 5 using the least squares orthogonal factor decomposition lsqr algorithm lateral variation of lg attenuation is found to exist across the region the results provide new insights into the attenuation properties of the crust and its linkage with regional geodynamics the low lgq0 200 is revealed in the central part of lhasa terrane in western tibet from both tsm and rtsm images the high lgq0 values complemented with the high pn velocities beneath the central part of lhasa terrane reveal the trace of underthrusting indian lithosphere beneath the western tibet

pg attenuation tomography within the northern middle east abstract a seismic attenuation map of regional phase pg is constructed for the northern middle east using two station frequency dependent q measurements and seismic attenuation tomography pg is widely used in source discrimination but its attenuation behavior is not well understood we investigate the functional form of the geometrical spreading of pg for both uniform and more complex seismic velocity structures we find that there is significant trade off between attenuation and geometrical spreading of pg and we quantify this trade off to interpret the observed pg attenuation in the middle east the northern middle east attenuation tomographic images show significant lateral q pg variations which may be caused by lateral variations of crustal rheology if intrinsic attenuation dominates the seismic attenuation mechanism of this region we observe very large variations in the q pg model such as low q pg within the anatolian plate and high q pg within much of the stable arabian plate the low q pg anomalies generally coincide with active fault systems or quaternary volcanisms in order to perform a systematic comparison with lg propagation we have also updated the q lg map of zor et al 2007 these two new maps provide estimates for crustal q pg and q lg for the northern middle east

convergence of the indian and eurasian plates under eastern tibet revealed by seismic tomography 1 a three dimensional p wave velocity model of the crust and upper mantle down to 400 km depth beneath eastern tibet is obtained using many temporary seismic stations of the ascent project and the namche barwa broadband seismic network we collected 16 508 arrival times of p pn and pg phases from 573 local and regional earthquakes and 7 450 p wave arrivals from seismograms of 435 teleseismic events our high quality data set enables us to reconstruct the 3 d velocity structure under eastern tibet in more detail than the previous studies in the shallow depth our results show that the low velocity zones are not interconnected well no wide spread low velocity zones which may reflect the complex pattern of material flowing in the study region below the moho we find that the indian lithospheric mantle underthrusts sub horizontally under eastern tibet and the extent of the northward advancing indian lithosphere decreases from west to east in the north the asian lithospheric mantle is detected under the vicinity of the qaidam basin between the indian and asian lithospheric mantles there is an obvious low velocity anomaly which may reflect an upwelling mantle diapir

estimation of the high frequency radiation of the 2000 tottori japan earthquake based on a dynamic model of fault rupture application to the strong ground motion simulation abstract in the present study we investigate the high frequency hf radiation mechanism of the 2000 tottori earthquake in japan based on a 3d spontaneous fault rupture dynamic model we generalize the model of hf radiation of a suddenly stopping circular crack madariaga 1977 boatwright 1982 to the radiation from a general 3d rupture in a planar fault where hf is radiated during gradual changes of rupture velocity at the rupture front local rupture velocity changes are expressed as the divergence of local rupture velocity vectors that are derived from gradients of rupture times from the dynamic model our numerical model of the tottori earthquake indicates that rupture velocity changes are largely induced by barriers locally stronger fault sections across the fault plane and that hf radiation mainly originates within asperities large stress drop regions in areas where the product of dynamic stress drop and rupture velocity changes is maximum we develop a strong ground motion simulation methodology that incorporates hf radiation inferred from a dynamic fault rupture model using this methodology we investigate the hf radiation of the tottori earthquake by inverting observed near source acceleration envelopes of the earthquake our inversion results corroborate that hf radiation originates within asperities and show that significant hf radiation represents no more than a 20 of the total asperity area our results show that the incorporation of a directivity factor on the basis of a well defined physical rupture model to the radiation pattern leads to a significant improvement in fitting of observed ground motions our simulated near source strong ground motions of the tottori earthquake are also able to reproduce the 2 radiation theoretically predicted in 2d dynamic fault rupture models

an improved method to extract very broadband empirical green s functions from ambient seismic noise the extraction of green s functions by cross correlation of continuous seismic records at station pairs is best achieved in a diffuse wave field where energies radiated by random sources have equal power to partially satisfy the diffuse wave field condition original seismic records must be normalized as they are highly nonstationary and may have amplitude variations of orders of magnitude within the cross correlation time window we adapt a frequency time normalization method to obtain seismograms with an even spectrum at all times within the data processing unit compared with the commonly used one bit normalization the new method improves the signal to noise ratio of empirical green s functions by a factor of 2 and increases the effective data recording duration by a factor of 4 assuming random local and instrument noise it yields useful rayleigh waves at periods up to 300 s for program for array seismic studies of the continental lithosphere passcal type deployments and 600 s for permanent stations with very broadband sensors thus the new method makes it possible to extend surface wave signals to frequencies beyond those in traditional earthquake based surface wave tomography at both the high and low frequency ends

re os systematics of the ca 2 7 ga komatiites from alexo ontario canada abstract the re os isotope systematics of a suite of komatiitic flows from alexo township in the abitibi greenstone belt ontario were examined in order to characterize the partitioning behaviors of re and os during the generation and subsequent differentiation of komatiitic magma and also to determine the initial os isotopic composition of the late archean mantle source of these rocks this suite of komatiites was suitable for these objectives because of their high os concentrations 1 ppb and low 187 re 188 os ratios ranging between 0 4 and 2 4 the re os isotope results for whole rock komatiites and chromite separates yield a model 3 isochron age of 2762 76 ma this age is in general agreement with previously determined u pb ages of zircons from associated felsic units and also with sm nd and pb pb isochron ages of these flows this suite of alexo komatiites thus demonstrates relatively limited mobility of re and os at the whole rock scale even after large scale alteration and ambient lower greenschist facies metamorphism the initial os isotopic composition 187 os 188 os i 0 1080 0 0012 obtained from the regression is essentially chondritic os 0 1 1 0 our results therefore suggest an absence of any large scale deviation in initial os isotopic composition of the mantle source of these komatiites relative to the putative chondritic composition of the contemporaneous convective upper mantle for these komatiites os was moderately compatible with the mantle residue d os mantle melt 2 1 whereas re was incompatible d re 0 20 these d s are consistent with those previously estimated for other archean komatiites the apparent d os olivine liquid and d re olivine liquid in alexo komatiites are 1 7 and 0 66 respectively our results therefore suggest that os is slightly compatible whereas re is moderately incompatible in olivine or a coprecipitating phase during magmatic differentiation of komatiitic liquids the concentrations of re and os in alexo komatiites are also comparable to those reported for komatiites from other greenstone belts the apparent magmatic behaviors of both re and os in the whole rock komatiites indicate that the re os elemental systematics are primary features of these rocks the os isotopic results obtained for the alexo komatiites combined with those from the spatially associated munro township komatiites suggest a source for komatiites in the southern volcanic zone of the abitibi greenstone belt that was dominated by os with a chondritic isotopic composition also the os isotopic results combined with previous nd and pb isotopic results are consistent with derivation of the komatiites from either the contemporaneous convective upper mantle or a deep mantle source with isotopically similar characteristics these results imply that if the chondritic os isotopic composition of the mantle sources for the komatiites were set by a chondritic late veneer most late accretion and subsequent homogenization within the mantle must have occurred prior to their eruption at 2 7 ga

lithospheric and upper mantle structure of the northeastern tibetan plateau 1 we use receiver functions calculated for data collected by the indepth iv seismic array to image the three dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern tibet our results indicate an average crustal thickness of 65 to 70 km in northern tibet in addition we observe a 20 km moho offset beneath the northern margin of the kunlun mountains a 10 km moho offset across the jinsha river suture and gently northward dipping moho beneath the qaidam basin a region in the central qiangtang terrane with higher than normal crustal vp vs ratio of 1 83 can be the result of the eocene magmatic event in the qiangtang terrane we observe a significant lithospheric mantle discontinuity beneath the bangong nujiang suture at 80 km depth which dips 10 to the north reaching 120 km depth we interpret this feature as either a piece of lhasa terrane or remnant oceanic slab underthrust below northern tibet we detect a 20 km depression of the 660 km discontinuity in the mantle transition zone beneath the northern lhasa terrane in central tibet which suggests this phase transition has been influenced by a dense and or cold oceanic slab a modest 10 km depression of the 410 km discontinuity located beneath the northern qiangtang terrane may be the result of localized warm upwelling associated with small scale convection induced by the penetration of the sinking indian continental lithosphere into the transition zone beneath the central tibetan plateau

os isotope systematics of komatiitic basalts from the vetreny belt baltic shield evidence for a chondritic source of the 2 45 ga plume the vetreny belt in the southeast baltic shield is a large volcano sedimentary basin containing a 4 to 8 km thick sequence of basaltic to komatiitic lavas which were erupted 2 45 ga ago in a continental rift setting during the interaction of a mantle plume and the archean continental crust of the karelian granite greenstone terrane re os isotope data for olivine cumulate samples and chromite separates from victoria s lava lake and golets flow 3 define isochrons with ages of 2387 57 and 2432 34 ma respectively these ages are in good agreement with the previously reported and new nd pb and u pb zircon ages these data coupled with the evidence for immobile behavior of the re and os indicate that the re os system remained closed since the lava eruption the weighted average initial 187os values range between 0 43 0 10 for golets flow 3 and 0 07 0 13 for the lava lake a single chromite separate from golets flow 1 has a 187os t of 0 06 0 15 nd pb isotope and lithophile trace element data for the vetreny belt komatiitic basalts were used to monitor the effect of crustal contamination on the os isotope system and to estimate the initial os isotope composition of the vetreny plume source to be approximately chondritic with a 187os t of 0 9 this implies that the mantle source evolved with a long term nearly chondritic re os ratio the data provide further evidence that by the end of the late archean the earth s upper mantle was well homogenized with respect to the highly siderophile elements added during the accretion of a late veneer on a time scale of 1 ga

1 hz lg q tomography in central asia we have applied tomographic techniques to a data set of over 1700 regional distance lg amplitudes from 12 stations for paths inside a 30 by 40 region of central asia our purpose is to create high resolution path correction maps for use in regional distance source discrimination and magnitude determination as well as to study lg propagation characteristics in one of the most heterogeneous portions of the earth for 1 0 hz data tomography reduced variance 33 relative to a best fit uniform q model lgq varied with geologic region with low values in tibet the tien shan and the pamir range q 200 to 400 intermediate values in basins such as the tarim q 500 and high values for platforms and older crust q 800 the relatively efficient propagation across the tarim differs from the poor propagation that is generally observed across sedimentary basins

distinct lateral contrast of the crustal and upper mantle structure beneath northeast tibetan plateau from receiver function analysis abstract we investigate the crustal thickness poisson s ratio and the thickness variation of the mantle transition zone using the p receiver function technique on data recorded by the 68 seismic station of the ascent experiment beneath the northeast tibetan plateau our results reveal a distinct crustal and upper mantle structure contrast between the western and eastern part of the northeast tibetan plateau along two migrated profiles in the eastern region the moho is relatively smooth and continuous and varies in depth between 80 and 51 km whereas the depth of the moho beneath the western region varies between 77 and 52 km and is offset 20 km beneath the boundary between the kunlun fault and qaidam basin the delay times for the ps phases from both the 410 km and 660 km discontinuities in the western region are 1 2 s later than that in the eastern region indicating that the average velocities of the upper mantle in the western region are lower than that in the eastern region crustal rocks with low to moderate poisson s ratios are interpreted to be dominated by felsic to intermediate compositions implying that large scale middle lower crustal flow does not occur easily the nearly constant thickness of the mantle transition zone with an average value of 255 km 24 4 s from south to north implies that no lithospheric fragments correlating to the india asia collision have entered into the mantle transition zone

high resolution regional seismic attenuation tomography in eastern tibetan plateau and adjacent regions 1 the q of regional seismic phases lg and pg within the crust is assumed as a proxy for crustal q and q which is used as a constraint of crustal rheology we measure regional phase q of the eastern tibetan plateau and adjacent areas this method eliminates contributions from source and site responses and is an improvement on the two station method tsm we have generated tomographic images of crustal attenuation anomalies with resolution as high as 1 in general we observe low q in the northernmost portions of the tibetan plateau and high q in the more tectonically stable regions such as the interior of the qaidam basin the calculated site responses appear to correlate with topography or sediment thickness furthermore the relationship between earthquake magnitudes and calculated source terms suggest that the rtm method effectively removes the source response and may be used as an alternative to source magnitude

seismic evidence for significant lateral variations in lithospheric thickness beneath the central and western north china craton abstract we have migrated teleseismic s receiver functions to construct detailed lithospheric structure images that cover the three constituent parts of the north china craton ncc our images show that in contrast to the eastern ncc where significantly thinned lithosphere 60 100 km is widespread the central and western ncc are characterized by the coexistence of both preserved thick and dramatically thinned lithosphere the thick lithosphere 200 km is present beneath the stable ordos plateau and the thinned lithosphere up to 80 km is found in the late cenozoic yinchuan hetao and shaanxi shanxi rift areas with sharp changes occurring over a lateral distance of

pgq model for nepal himalaya abstract we investigate the spatial variations of pg attenuation structure in nepal himalaya we implement the two station method on 2325 waveforms obtained from 435 events recorded at 151 stations deployed across nepal himalaya to measure the pg wave quality factor q these station station measurements are inverted further for investigating the spatial variations of pg attenuation using a back projection algorithm the model contains variations across the region with much of the region being dominated by high attenuation it is observed that the areas around the existing faults and lineaments exhibit very low q values a few exceptional high q pockets are observed to be limited predominantly in the north western side of our study area we hypothesize that intrinsic attenuation plays a major role in causing the high apparent pg attenuation in the crust of nepal himalaya this may be mainly caused by highly pressurized fluids trapped within a thin low velocity layer lvl at shallow depths moreover it is interesting to observe that all the past earthquakes of m w 7 since 1934 fall into the regions of low q values but not the lowest it may be anticipated that the very low q areas are too weak to sustain the accumulation of high energy release during a large magnitude earthquake therefore the observed low q structures across the study area may imply a weak crust and subsequently a favorable zone for future rupture

ancient depleted mantle as a source of boninites in the izu bonin mariana arc evidence from os isotopes in cr spinel and magnetite abstract boninite is a volcanic rock derived from shallow melting of highly depleted hydrous mantle fluxed with water from subducted slabs the eruption of boninite early in the history of the izu bonin mariana ibm arc 48 45 ma suggests generation by melting of upper mantle material that was relatively unmodified by subducted components thus the boninite composition should largely reflect that of the sub arc mantle for better understanding of the mantle sources of nascent arc settings and the contributions of different components to arc melts we analyzed os isotope ratios 187os 188os of bulk rocks and mineral separates euhedral cr spinel from boninites and euhedral cr spinel magnetite mixtures from tholeiites younger than 45 ma that erupted after boninites from the bonin islands and guam the age corrected initial os isotope ratios of the whole rock samples 0 1179 0 2050 were more radiogenic and variable than those of the mineral separates possibly because of contamination with crustal materials during magma ascent or alteration after emplacement the age corrected os isotope ratios of euhedral cr spinel in boninite from the bonin islands 0 1187 0 1254 and from guam 0 1220 0 1269 are unradiogenic relative to primitive mantle and those of the cr spinel magnetite mixtures from the tholeiites from the bonin islands are similar to or slightly more radiogenic 0 1224 0 1382 the most depleted os isotope ratio of the cr spinel from boninite yielded a model re depletion trd age of 1 4 ga suggesting that the mantle source of the boninite experienced melt extraction prior to 1 4 ga the source of the boninites is interpreted to be mostly highly depleted mantle with a small contribution of slab flux arising from altered oceanic crust that has radiogenic os components with or without contributions from components with relatively unradiogenic os such as volcaniclastics of oceanic island basalt affiliation or very young mid ocean ridge basalt

heterogeneous distribution of the dynamic source parameters of the 1999 chi chi taiwan earthquake 1 the spatial and temporal distribution of the stress on the fault plane of the 1999 chi chi taiwan earthquake is calculated from kinematic inversion results using a three dimensional finite difference method for solving the elastodynamic equations we analyze the relations between stress and slip for all grid positions on the fault and use these relations to infer the friction law for the rupture the dynamic source parameters were also determined our results show that for most of the points on the fault the relation between stress and slip was consistent with the slip weakening law during the rupture process especially for those points with large slip however consistency with the velocity weakening law is not clear from the observed relation between stress and slip velocity the distributions of the dynamic parameters on the fault are very heterogeneous the peak value of the static stress drop is 35 mpa in general high stress drop occurred in the areas with large slip the estimated strength excesses are generally small suggesting that the tectonic shear stress had reached the level of the fault strength before the main shock the slip weakening distance dc and the fracture energy gc are proportional to the final slip the aftershock activity correlates with the spatial distribution of dynamic source parameters usually the aftershocks near the fault plane are concentrated in regions with small or negative static stress drop and there were few aftershocks in regions which had large values of critical slip weakening distance dc

new insights into the origin of o hf os isotope signatures in arc lavas from tonga kermadec abstract o hf and os isotope data are presented for lavas from the highly depleted tonga kermadec arc o isotope values overlap with those of morb limiting the amount of interaction with the arc crust 18 o does not increase northwards as would be expected from the 4 fold increase in subduction rate if slab derived fluids had high 18 o 16 o ratios thus the overall northward decrease in hfse concentrations likely reflects depletion due to prior melt extraction not increasing extents of melting hf isotopes are strongly negatively correlated with be isotopes consistent with mixing of subducted pelagic sediment into the mantle wedge and do not require hf to be fluid mobile with the exception of a boninite from the north tongan trench the northern tonga lavas do not overlap the hf isotope composition of either the samoan plume or the subducting louisville volcaniclastic sediments thus the pb isotope signatures in these lavas must have been added by fluids and sediment melts derived from the louisville volcaniclastics with minimal mobilisation of hf this suggests conservative behaviour for this element due to the formation of residual zircon during partial melting of the subducted sediments 187 os 188 os ranges from 0 1275 to 0 4731 and the higher os isotope ratios reflect the sensitivity of this system to even minor interaction with altered arc crust conversely the lowest os ratios are subchondritic and indicate that transfer of radiogenic os from the slab is not all pervasive and provide an important constraint on the composition of the mantle wedge remarkably the least radiogenic sample is a dacite demonstrating that evolved magmas can develop by fractionation from mantle derived magmas with minimal interaction with the arc crust

computer simulation of strong ground motion near a fault using dynamic fault rupture modeling spatial distribution of the peak ground velocity vectors computer simulation was used to study the nature of the strong ground motion near a strike slip fault the faulting process was modeled by stress release with fixed rupture velocity in a uniform elastic half space or layered half space the fourth order 3 d finite difference method with staggered grids was employed to compute both ground motions and slip histories on the fault the fault rupture was assumed to start from a point and propagate circularly with 0 8 times shear wave velocity in the present paper we focused on the spatial pattern of ground velocity vectors i e the direction of strong motions in the case of bilateral rupture propagation the strong fault parallel ground motion appeared near the center of the fault the fault normal motions of ground velocity appeared near the edges of the fault in the case of unilateral rupture the fault parallel motion appeared near the starting point however the amplitude was lower than that for the bilateral rupture case the fault normal motion was predominant near the terminal point of the rupture the results were applied to the earthquake damage data especially the directions that simple bodies overturned and wooden houses collapsed caused by the 1927 tango the 1930 kita izu and the 1948 fukui earthquakes the spatial distributions of the direction data were found to reflect the strong ground motions generated from the earthquake source process

lg attenuation tomographic models of himalaya and southern tibet abstract we investigate the lg attenuation structure of the crust beneath the tectonically complex himalaya and southern tibet regions adopting a tomographic regionalization method a total of 1671 earthquake waveforms registered at 38 seismic stations operated in the region are selected for the initial lgq measurements using the standard two station method q0 1 hz lgq values of 76 high quality interstation paths are finally considered as input for the tomographic inversion the estimates of q0 exhibit distinct variations in the crustal attenuation from north to south across the whole region the zones of lowest q0 values

strong lg wave attenuation in the middle east continental collision orogenic belt abstract using lg wave q tomography we construct a broadband crustal attenuation model for the middle east the q lg images reveal a relationship between attenuation and geological structures strong attenuation is found in the continental collision orogenic belt that extends from the turkish and iranian plateau to the pamir plateau we investigate the frequency dependence of q lg in different geologic formations the results illustrate that q lg values generally increase with increasing frequency but exhibit complex relationships both with frequency and between regions an average q lg value between 0 2 and 2 0 hz q lg 0 2 2 0 hz may be a critical index for crustal attenuation and is used to infer the regional geology low q lg anomalies are present in the eastern turkish plateau and correlate well with low pn velocities and cenozoic volcanic activity thus indicating possible partial melting within the crust in this region very strong attenuation is also observed in central iran the afghanistan block and the southern caspian sea this in line with the previously observed high crustal temperature high conductivity layers and thick marine sediments in these areas suggests the high lg attenuation is caused by abnormally high tectonic and thermal activities

new osmium isotope evidence for intracrustal recycling of crustal domains with discrete ages new 187os 188os ratios of quaternary mount adams volcanic rocks from the cascade arc in southern washington vary by 300 187 os 188 os 5 0 165 0 564 and fall into high 0 319 and low 0 166 to 0 281 groups of 187 os 188 os ratios that are sub stantially more radiogenic than mantle values these os isotope compositions and groupings are interpreted to reflect recycling of discrete intracrustal domains with high 187os 188os ratios but dif fering ages thus recording the process of crustal hybridization and homogenization os isotope compositions provide new constraints on amounts of intracrustal recycling in young subduction zone en vironments that reflect the magmatic history of the arc sr nd hf and pb isotope variations in this young mafic arc complex are too small to allow such constraints

lg attenuation in the central and eastern united states as revealed by the earthscope transportable array abstract lg waveforms recorded by earthscope s transportable array ta are used to estimate lg q 0 in the central and eastern united states ceus attenuation is calculated based on lg spectral amplitudes filtered at a narrow band from 0 5 to 1 5 hz two station and reverse two station techniques are used to calculate q 0 values 39 events occurring from 2010 to 2012 and ranging from magnitude 3 to magnitude 6 are used in this study 39 258 and 4 627 interstation q 0 values are determined using the two station and reverse two station techniques respectively the results show northeast trending high q 0 regions low attenuation ranging from eastern new mexico to wisconsin regions of low q 0 high attenuation are seen along the minnesota wisconsin border the gulf coastal plain gcp along the south dakota nebraska border and along the oklahoma texas border a positive correlation between heat flow sediment thickness recent tectonic activity fluids and low q 0 was observed areas with low heat flow thin sediment cover no recent tectonic activity and a lack of fluids were observed to have consistently high q 0 these new models use a greater amount of data attain more coverage than previous studies and better constrain attenuation this increase in detail can improve high frequency ground motion predictions of future large earthquakes for more accurate far field hazard assessment and improve the overall understanding of the structure and assemblage of the ceus

three dimensional simulation of the near fault ground motion for the 1995 hyogo ken nanbu kobe japan earthquake abstract the 17 january 1995 hyogo ken nanbu earthquake is a typical example showing that the ground motions along basin edge faults can be very destructive in this study we simulate the near fault ground motion from this earthquake based on a kinematic fault model and a simplified 3d velocity structure of the kobe area the kinematic earthquake rupture and the wave propagation are modeled using a 3d finite difference method fdm our simulation identifies the basin edge effect as an important factor that influenced the ground motion amplification pattern in the kobe area we found that the coupling of the source directivity and basin edge effects causes impulsive ground motions with extremely high amplitude at periods greater than 1 sec and in a narrow zone offset less than 1 km from the basin edge the combination of these effects acted to create a fairly continuous band of amplification that extends about 30 km in an elongated zone parallel to the basin edge boundary in some areas localized site effects might have been as important as the abovementioned effects but they cannot explain the continuity of the extended east west zone of damage

geology and geochemistry of early arc volcanic rocks from guam the island of guam is located at the southern end of the mariana fore arc and was the site of early arc volcanism between 43 and 32 m y ago two volcanic units were erupted during the early evolution of guam the late middle eocene facpi formation and the late eocene to early oligocene alutom formation the facpi formation is composed largely of interbedded boninite series pillow lavas pillow breccias and dikes although arc tholeiite series rocks cap the formation in some areas the alutom formation is composed of interbedded volcanic breccias tuffaceous sandstones lava flows and sills calc alkaline arc tholeiite and boninite series compositions are all found in the alutom formation primitive boninite series rocks are relatively high in sio 2 mgo and ni and low in a1 2 o 3 and tio 2 contents compared to basaltic rocks from the arc tholeiite series additionally they have low ti zr ratios and high k zr rb zr and ba zr ratios although no calc alkaline basalts are found in the early arc formations of guam basalts with chemical characteristics transitional between boninite and tholeiite series rocks may be genetically linked to the calc alkaline series among the analyzed samples here are a few with anomalously high y and rare earth element ree contents chemical variation in the boninite series can be explained by early fractionation of olivine and clinopyroxene followed by plagioclase clinopyroxene and orthopyroxene the arc tholeiite series chemical trends can be explained by early olivine fractionation followed by plagioclase clinopyroxene orthopyroxene and magnetite the composition of the least silicic calc alkaline series andesite can be modeled by fractional crystallization of olivine clinopyroxene plagioclase and magnetite from a transitional basalt derivation of more silicic calc alkaline series rocks requires magma mixing or crustal assimilation in addition to crystal fractionation involving hornblende all basalts from the early volcanic series of guam were erupted in an arc setting the boninite series magmas were produced by hydrous partial melting of depleted mantle at relatively shallow levels whereas the arc tholeiite and calc alkaline series magmas were generated by partial melting of less depleted mantle at deeper levels we suggest that because the calc alkaline series parent basalts most likely were richer in incompatible high field strength hfs elements and sio 2 than were the tholeiitic parents they were generated by lower degrees of melting

three dimensional finite difference simulation of fault dynamics rectangular faults with fixed rupture velocity we analyze three dimensional finite difference solutions for a simple shear crack model of faulting to determine the effects of fault length and width on the earthquake slip function the fault model is dynamic with only rupture velocity fault dimensions and dynamic stress drop prescribed the numerical solutions are accurate for frequencies up to 5 hz and are combined with asymptotic results for shear cracks in order to characterize the slip function at higher frequencies near the hypocenter the slip velocity exhibits a square root singularity whose intensity increases with hypocentral distance at distances greater than the fault width w growth of the velocity intensity ceases and the slip function becomes nearly invariant with distance along the fault length closed form expressions are developed for the dependence of static slip s slip rise time tr and slip velocity intensity v on fault geometry along the center line of a long narrow fault at hypocentral distances exceeding w these expressions reduce to s w tr 0 5 w vr and v w 2 vr where is the dynamic stress drop the shear modulus and vr the rupture velocity the numerical results imply that uniform dislocation kinematic earthquake models in which slip is represented by a ramp time function will underpredict high frequency ground motion relative to low frequency ground motion a further implication of the numerical solutions is that the nature of inelastic processes at the advancing edge of a long fault will depend on fault width but will be independent of rupture length

crustal flow pattern beneath the tibetan plateau constrained by regional lg wave q tomography abstract as a prominent geophysical anomaly unusually high seismic wave attenuation is observed in the crust and upper mantle of the tibetan plateau particularly along its northern area theoretical and laboratory investigations show that the strong seismic attenuation can indicate high temperatures and partial melting which may decrease the viscosity of the material and cause it to flow thus seismic attenuation distribution may provide useful constraints to the crust flows if they exist using lg wave q tomography we construct a 0 05 10 0 hz broadband high resolution crust attenuation model for the tibetan plateau and its surrounding regions the maximum spatial resolution is approximately 1 0 1 0 in well covered areas and for frequencies between 0 05 and 1 5 hz this attenuation model reveals an apparent low qlg belt stretching along the northern and eastern tibetan plateau combining the lg wave q model with other geophysical data two possible crust flow channels are found in the tibetan plateau the main flow channel is from north to east and then turns to southeastern tibet along the western edge of the rigid sichuan basin while a second channel starts from southern tibet and crosses the eastern himalayan syntaxis

platinum group element fractionation in a komatiitic basalt lava lake precise pge os ir ru pt pd abundance data for bulk rock samples and mineral separates from a deep differentiated komatiitic basalt lava lake in the vetreny belt baltic shield were obtained using an icp ms id technique the composition of the erupted liquid mgo 5 15 is characterized by a highly fractionated pge pattern pd os n 5 48 with subchondritic os ir n 5 0 43 analysis of separated olivine and chromite revealed that ru os and ir were slightly compatible to moderately incompatible in olivine d 5 1 7 0 8 and were compatible with chromite d 5 100 150 platinum and pd were highly incompatible with olivine d 5 0 08 0 03 and moderately compatible with chromite d 5 1 6 3 3 bulk solid liquid partition coefficients for pges ni cr and cu estimated from the whole rock regressions were os 7 9 ir 6 6 ru 4 5 pt 0 53 pd 0 09 ni 6 2 cr 4 4 and cu 0 01 the highly incompatible behavior of cu and pd indicates that there was no separation of fenicu immiscible sulfide liquid during the differentiation of the lava lake comparison of the bulk partition coefficients with those calculated from the olivine and chromite compositions implies that the fractionating mineral assemblage included 10 2561 mass fraction of pyrrhotite the data obtained provide new constraints on the effects of fractional crystallization on the pge compositions of mafic ultramafic magmas copyright 2001 elsevier science ltd