effects of source radiation patterns on the phase s670p beneath the tonga subduction zone summary observations of short period s p converted phases from the 670 km discontinuity s670p provide unique constraints on the nature of the discontinuity and the seismic structure of deep subducted slabs because this is a subtle phase in the p codas from deep earthquakes great care is necessary in its identification in past work we identified s670p phases from a large number of deep events beneath tonga primarily by showing that the arrival times for s670p had the correct dependence upon hypocentral depth here we offer further confirmation of our interpretation of this s p converted phase by showing that the occurrence of the phase is a predictable consequence of the earthquake source mechanism we show this in two ways 1 independently determined moment tensor solutions for the earthquakes generally predict relatively high amplitude s p conversions where they are observed and 2 the observed amplitudes of s670p scp pp and p arrivals vary systematically in a way which confirms that s670p is generated with large amplitude when the s wave energy pattern is optimal comparison of scp observations with moment tensor solutions also suggests that much of the s wave energy results from the non double couple or for cmt solutions linear vector dipole lvd part of the source mechanism for the northern part of the tonga subduction zone where we observe the strongest s670p phases because these lvd components tend to be directed along fault planes inferred from deep seismicity patterns we suggest that they may be caused by rupture propagation directivity we conclude that the occurrence of s670p is primarily controlled by the source radiation pattern in a way that is consistent with our previous interpretation thus confirming that s670p is an s p conversion from an almost undeformed phase change boundary at about 670 km depth

shear coupled pl summary observed teleseismic shear coupled pl waves spl display a variety of waveforms depending on factors such as source depth source type and velocity structure using a wkbj spectral method for sv wave propagation synthetic seismograms of spl are produced to examine the factors important in sv and spl excitation results show that spl is preferentially excited by shallow sources compared to deep sources this is due to large source area reverberations which consequently leak as sv waves into the mantle interaction at the receiver area then sets up the classic prograde elliptical motion by which spl can be identified this is in accordance with the teleseismic observations and indicates that most previous models for the propagation of spl were not appropriate for shallow source since emphasis was placed on wave interactions occurring only near the receiver

analysis of asymmetric multipathing with a generalization of the double beam method abstract an array method integrating source and receiver array beamforming is presented the method is a generalization of previous work kruger et al 1993 to independent variation of slowness and azimuth in both the source and the receiver array compared to the analysis capabilities of a single array the resolution is enhanced and asymmetric scattering configurations and multipathing can be studied the generalized double beam method dbm is applied to a data set consisting of nuclear explosions detonated in eastern kazakhstan and recorded at the yellowknife array in northern canada the multipathing found for pcp and pdp is interpreted in terms of lateral inhomogeneity in the lowermost mantle below severnaya zemlya indications for rayleigh to p wave scattering observed for the shagan river and degelen subsites of the former main test site of the ussr in eastern kazakhstan can be related to the structure in the vicinity of the two subsites

on regional s the character of regional s wave trains is quite complex and consists of two major elements sn waves which principally travel in the upper mantle and lg waves guided in the earth s crust the effect of lateral heterogeneity is to modify the signals for these phases compared with the predictions for horizontally stratified models however complete synthetic seismogram calculations for simple stratified crustal models for a suite of distances out to 950 km including both sn and lg provide a range of insights into the nature of the s wave train in particular late picks of sn arrivals are likely to arise because the actual s onset has been missed in the p coda and a higher amplitude surface multiple phase with an extra crustal delay has been picked instead such a effect was first recognized by lapwood 1955 in a study of the iss readings for a sequence of japanese events recorded at regional distances but has usually not been taken into consideration in studies using regional networks lateral heterogeneity along the propagation path has the effect of inducing interconversion between sn and lg energy and thus alters the character of the s wave train the lower attenuation for s in the uppermost mantle compared with the crust frequently leaves sn as the most prominent phase at ranges greater than 1000 km long range propagation of high frequency lg requires a relatively undisturbed crust

shumagin seismic gap alaska peninsula history of great earthquakes tectonic setting and evidence for high seismic potential the shumagin seismic gap a segment of the plate boundary along the eastern aleutian arc has not ruptured during a great earthquake since at least 1899 1903 because at least 77 years have elapsed since the shumagin gap last ruptured in a great earthquake and repeat times for the 1938 rupture zone and part of the shumagin gap are estimated to be 50 to 90 years a high probability exists for a great earthquake to occur within the shumagin gap during the next one to two decades reconsideration of the rupture zones of the aleutian earthquakes of 1938 1946 and 1948 suggests that those events did not break the interplate boundary beneath the shumagin islands thus the shumagin seismic gap extends from the western end of the 1938 rupture zone to the eastern end of that of 1946 these boundaries also coincide with transverse structural features at least the eastern half of the shumagin gap broke in great earthquakes in 1788 and 1847 and possibly in 1898 1903 the shumagin gap is probably not the result of aseismic slip rather plate motion is accommodated there seismically and episodically and can be expected to produce large earthquakes in the future although there is no definitive evidence of long term precursors of a possible future earthquake several observations suggest that the shumagin gap is in an advanced stage of the earthquake cycle both teleseismic and local network data indicate a near absence of seismic activity m 2 above a depth of 30 km along the main thrust zone within the gap this is in strong contrast to adjacent portions of the arc where seismic activity is scattered across most of the main thrust zone two earthquakes with high stress drops 600 900 bars which occurred at the base of the main thrust zone may indicate the accumulation of a considerable amount of strain energy within the gap a possible seismic gap at the eastern end of the aftershock zone of the aleutian earthquake of 1957 has been identified near unalaska island an earthquake that nucleates in the shumagin gap could also rupture the possible unalaska gap to the west the 1938 aftershock zone to the east or both with resultant magnitude up to mw 9 0 alternatively the shumagin gap alone or in one of the above combinations could rupture in a series of very large earthquakes instead of a single great shock past alaska aleutian earthquakes including those of 1788 1938 1946 1957 1964 and 1965 have generated very large tsunamis future large earthquakes in the shumagin islands region could generate wave heights of several tens of meters along shorelines near the rupture areas the shumagin gap is one of two major gaps along the united states portion of the alaska aleutian plate boundary and is one of the few areas in the united states where processes leading to a great earthquake are likely to be observed within a reasonable span of time

the geometry of aleutian subduction three dimensional kinematic flow model we construct a steady state kinematic model of slab flow along the aleutian arc assuming the slab s geometry is known the slab geometry is constrained by local network data where the slab is seismically active and by p wave residual sphere analysis of 11 earthquakes and one nuclear explosion where it is aseismic the slab is approximated by a thin newtonian sheet subducting in a less viscous and incompressible mantle particles enter the trench at known rates of relative plate motion and are required to subduct along the predefined slab geometry for discrete but small 0 1 m y time steps we determine the speed and direction of each slab particle that minimizes dissipation power as the spherical shell oceanic lithosphere deforms to obtain the observed slab shape because the dip of the slab in the central aleutians is steep relative to the radius of curvature of the trench the slab must stretch along arc to obtain its present configuration our kinematic model represents one internally consistent flow that results in the observed geometry furthermore it provides a lower bound on the total amount of internal in plane deformation required by the shape and an estimate of stream lines and of the orientation and spatial distribution of the deformation rate the predicted in plane strain rate field is dominated by along arc extension reaching values as large as 10 15s 1 in the central aleutians and decaying by an order of magnitude to the east and west the slab accumulates an in plane strain of 10 by the time it reaches the depth of the seismicity cutoff along arc variations in both the magnitude and orientation of calculated strain rates are consistent with the spatial distribution of seismic moment release and source mechanism orientations of intermediate depth earthquakes suggesting that in order to understand intermediate focus earthquakes one must consider along arc deformation three dimensional analyses of subduction in a spherical geometry such as the present study are required to model along arc effects relative plate motions along the trench vary from nearly normal in the eastern aleutians to transform farther west coincident with this change the maximum depth of seismicity within the descending slab decreases from 250 to 50 km and active andesitic volcanism ceases the kinematic flow model predicts that the slab subducted obliquely along the central portion of the aleutian arc is transported laterally westward producing cold slab material to a depth of at least 300 km along the entire arc after only 15 m y of subduction the lack of volcanic activity in the western aleutians is apparently related to the lateral transport of the slab perhaps indicating that the slab is dehydrated and incapable of producing the volatiles necessary for arc magmatism the along arc variation in the maximum depth of seismicity can be explained thermally we construct temperature estimates within the slab using a quasi three dimensional finite difference scheme including entrained mantle flow stream lines from the kinematic model and the time history of the age of the subducting lithosphere as a function of position along the trench the theoretical temperature at the seismicity cutoff depth is remarkably close to a constant fraction of the depth dependent mantle solidus along the entire arc this observation is consistent with the hypothesis that seismicity terminates because of the initiation of high temperature steady state creep and provides an explanation for the lack of intermediate focus earthquakes in the western aleutians even though slab material exists there

mantle transition zone beneath eurasia mantle pds fig 1 converted phases are detected in the records of 11 seismograph stations in easternmost russia and china these data reveal neither a strong depression on the 660 km discontinuity nor a layer of partial melting atop the 410 km discontinuity that were found previously beneath this region in studies using long period underside sh reflections and multiple scs reverberations apparently a significant deepening of the 660 km discontinuity occurs only if the subducted plate penetrates the discontinuity which is not the case in the study region most estimates of thickness of the mantle transition zone mtz in eurasia based on the pds data are a few kilometres larger than the standard value 250 km and suggest that the average thickness of the mtz beneath eurasia is about 10 kilometres more than beneath the surrounding oceans

reflections from a distant subduction zone accurate models of the distribution of heterogeneities in the earth s mantle especially of subduction zones are crucial for our understanding of the composition and dynamics of the earth here we present broad band bb seismograms with p waves reflected at the subduction zone under southern japan at about 84 distance from german regional seismic network the earthquakes occurred in the mariana islands region at 100 to 105 distance array techniques allow the separation of this wave with an asymmetric path p1p2 from reflections and conversions under the source the receivers and the midpoint between the source and the receivers our observation demonstrates that subduction zones can act as strong seismic reflectors and can be mapped with a depth resolution of 20km with bb arrays at teleseismic distances the analysis of such data will help to derive a more detailed picture of the structure of the earth s mantle

rayleigh waves guided by sea trench topography a later phase of large amplitude is observed at a station in hokkaido from several events of the december 1991 off urup earthquake swarm in the kurile islands region the phase is characterized by a narrow frequency band with a dominant period of about 15 s and retrograde particle motion in a vertical plane with a strike very different from the great circle direction between the station and the epicenter the phase is observed from only the earthquakes that are located at shallow depths beneath the inner wall of the kurile trench where sea depth is greater than 4 km these characteristics indicate that the phase is a packet of rayleigh waves that arrive at the station from an azimuth considerably different from the great circle direction rayleigh waves with periods around 15 20 s are well excited by the shallow earthquakes beneath the inner wall of the trench this is supported by the dominance of a spectral peak at 15 20 s in the spectra of the direct rayleigh waves and the later phase rayleigh waves that propagate in an oceanic region are strongly influenced by the thickness of the water layer at periods shorter than about 25 s the group velocity decreases by 80 at 15 s with the increase of the sea depth from 4 km to 6 km the sea trench topography therefore forms a strong low velocity zone the observed later phase can be interpreted as a rayleigh wave packet guided by this velocity complexity

chemical remobilization of volcanic hosted sulphide deposits at rosebery and mt lyell tasmania abstract devonian hydrothermal activity has resulted in localised chemical remobilization of the cambrian massive sulphide deposits at rosebery and mt lyell in the mt read mineral province of western tasmania at rosebery part of the massive pyrite sphalerite galena chalcopyrite quartz phyllosilicate lens has been replaced volume for volume by a pyrite pyrrhotite chalcopyrite magnetite phlogopite tourmaline quartz assemblage with complete removal of cambrian sphalerite and galena the replacement is later than folding of probable devonian age the likely temperature the presence of tourmaline and the relatively reduced nature of the secondary assemblage suggest that the related fluids were derived from devonian granitoids which on independent evidence tourmaline veins pyrrhotite deposits metamorphic biotite and shallow granites nearby appear to extend beneath the rosebery mice lead and zinc dissolved from the cambrian sulphide lens may have been redeposited as veins and replacements in younger rocks the resulting lead is likely to yield anomalous isotopic ages and be associated with sulphur having the isotopic characteristics of cambrian massive sulphide deposits at mt lyell altered felsic volcanics host large disseminated pyrite chalcopyrite quartz phyllosilicate lenses and minor stratabound exhalative massive sulphide bodies all of probable cambrian age in the north lyell fault zone late cambrian early ordovician siliciclastic sediments have been replaced by cherty quartz bornite chalcopyrite chalcocite sericite pyrophyllite ores of likely devonian age this assemblage probably formed from acid pyrophyllite oxidized barite solutions that may have derived high a cu a fe by dissolution of cambrian chalcopyrite disseminated pyrite chalcopyrite ores of cambrian age have a well developed devonian cleavage in which hematite has crystallized thereby indicating circulation of oxidized fluids during or after cleavage development precipitation mechanisms for the bornite ores may have involved fluid sulphide and or fluid sediment reactions the fluids are unlikely to have been derived directly from devonian granitoids but they may have been modified groundwaters circulating either during cleavage growth or as a result of heating by post kinematic devonian granitoid plutons

s to p scattering at the 650 km discontinuity summary a search of seismograms recorded at the warramunga seismic array wra from events occurring below the izu bonin islands shows an arrival on some but not all of the records with an onset at 25 30 s after p which is not predicted by the standard travel time tables the slowness and azimuth of the phase show that it is generated almost in line with p and the variation of arrival time with the hypocentral depth of the earthquake indicates that its origin lies on the receiver side of the source it appears in fact to be an s to p conversion at a depth of 650 700 km which is seen only when the receiver is close to a node of the p radiation pattern and an antinode for s so that its amplitude compared with that of p is at a maximum finally the duration of the phase indicates that it is not simply a refracted wave but that it has a coda of scattered arrivals from lateral heterogeneity in the neighbourhood of 650 km below the izu bonin islands

are long period body wave coda caused by lateral heterogeneity summary data from two broad band arrays in western europe nars and grf are used to study the character of long period body wave coda the events studied are at epicentral distances of 40 to 60 in the hindu kush region and on the mid atlantic ridge sampling the upper mantle to a depth of about lo00 km the periods studied are 5 50 s the long period coda of p at grf interstation distance about 10 km are strongly coherent whereas the long period coda of pp ppp and s waves are incoherent this indicates that the latter coda consists of scattered waves to investigate the nature of the scattering process the data of grf are analysed for slowness and azimuth variations in the coda intervals a new beamforming algorithm is presented to deal with the low frequences and relatively short time intervals the method is based on backus gilbert inverse theory the results show that the incoherent long period coda intervals almost entirely consist of surface waves these waves are scattered from the preceding body waves some calculations with linearized theory for body wave to rayleigh wave conversion at topography at the surface or at the moho show that realistic scatterers can account for the observed constant coda level the beamforming results show that the phases in the p coda all arrive along the great circle as scattering calculations point out that body wave to body wave scattering is inefficient it is concluded that the long period p coda does not contain a significant amount of scattered energy synthetic seismograms obtained with the reflectivity method show that spherically symmetric upper mantle models can explain these coda waves for events in the hindu kush region an upper mantle with a thick lid overlying a pronounced low velocity zone lvz is necessary to explain the character of the p coda at grf such an upper mantle agrees with previous studies of similar great circle paths the strong coherence of the p coda at grf is lost on the scale of nars station separation about 200 km this suggests lateral variations in the upper mantle at a scale of about 200 km it appears from previous studies of the upper mantle under europe that these variations must be sought in the lvz it is shown that the long period p coda is sensitive to variations in the p velocity structure of the lvz this suggests the p coda i e pdp phases as a tool for monitoring lateral variations in the lvz and possibly to prove the existence or absence of a lvz in the p velocity

depositional ages of the stratiform base metal deposits at mount isa and mcarthur river australia based on u pb zircon dating of concordant tuff horizons potassium rich tuff beds occur interbedded with major stratiform pb zn mineralization in shales and dolomitic shales of the mount isa group northwest queensland and the mcarthur group northern territory these deposits are some 600 km apart yet they exhibit many similarities which together with regional correlations suggest that the deposits formed by similar processes and that the successions developed more or less contemporaneously zircon is sparsely distributed as tiny crystal fragments or phenocrysts in the tuffs and its morphology and u pb systematics affirm that it is primarily of igneous origin u pb zircon data from a tuff in the mineralized h y c pyritic shale member of the mcarthur group give an age of the tuff of 1 690 super 29 sub 25 m y and from a tuff in the mineralized urquhart shale of the mount isa group an age of 1 670 super 20 sub 17 m y these results are consistent with all other geologic controls and are regarded as the best estimates of the depositional ages of the groups the ages of the mount isa and mcarthur groups are thus the same to within the present experimental uncertainties essentially confirming geologic synchrony of these important sedimentary successions pooled whole rock data from several of the mount isa tuff marker beds give an updated rb sr age of about 1 480 m y interpreted as indicating that isotopic exchange and regional metamorphism had essentially ceased by this time

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

new lead isotope determination from five proterozoic sulfide deposits skellefte district sweden lead isotope analyses of 25 sulfide samples galenas iron sulfides and sulfosalts from five different mines of the skellefte district northern sweden demonstrate that the pb isotopic composition of galenas and other sulfides rich in lead varies between individual deposits within the district this contrasts with many other base metal districts where ore lead is isotopically homogeneous on a regional basis although all of the skellefte leads are depleted in 207pb relative to average global lead evolution models thus suggesting a large mantle derived component in their sources the nasliden deposit lying at the contact of the host volcanic rocks and the overlying metasediments contains a significant component of crustal lead it is concluded that while the pb isotope data are consistent with a volcanic exhalative origin of the ores of the skellefte district they also demonstrate that older crustal lead was incorporated into the sulfides during their emplacement and the subsequent period of magmatic and metamorphic activity which followed their deposition

base metal exploration of the mount read volcanics western tasmania pt ii lead isotope signatures and genetic implications lead isotope measurements of 13 deposits and prospects from western tasmania have been undertaken to establish a framework for the use of the lead isotope technique in exploration and to gain an understanding of the processes of ore formation lead isotope signatures for the cambrian volcanogenic deposits mount lyell rosebery hercules que river hellyer differ by more than 1 percent from those characterizing the post cambrian mineralization thought to be related to devonian carboniferous granitoids and or tabberabberan metamorphism e g renison bell queen hill spray mine farrell lodes the differences in lead isotope signatures allow discrimination of the two contrasting styles of mineralization in both surface material and drill core the isotopic signatures have been utilized in an exploration program at elliott bay which is described in companion papers the following important genetic conclusions can be drawn from the results of this exploration oriented investigation 1 a whole rock pb pb age of 580 or 170 m y 2 sigma was measured for polymictic volcaniclastic rocks from the que river area which although not tightly constrained because of the large error limits is consistent with paleontological evidence 2 the intersection of this whole rock line with the average que river ore lead isotope value is consistent with a common source for the lead 3 the lead isotope data are consistent with an origin of the post cambrian mineralization by remobilization from the host volcanic pile in response to devonian carboniferous metamorphism and or plutonism 4 all isotopic data for the volcanic rocks massive sulfides and post cambrian mineralization lie on linear arrays and on the uranogenic lead plot 207 pb 204 pb vs 206 pb 204 pb they intersect the lead evolution growth curve for other massive sulfide deposits at approximately 1 000 m y this is interpreted as evidence of a precambrian source for the metals and also of the cogenesis of the volcanics massive sulfide and later mineralization 5 the proterozoic source age preserved in the lead isotope data for the volcanics and mineralization is consistent with their formation in a rifting environment or a continental margin 6 the 206 pb 204 pb values of 18 27 to 18 35 are unusually high for cambrian massive sulfides and the calculated model ages of approximately 270 m y are about 50 percent too young and possibly result from an increase in the u pb ratio of the source in the last billion years a calculated 238 u 204 pb mu value for the stage from 1 000 or 1 500 to 540 m y ago is high at approximately 13 similar to that measured in the volcanic rocks today and may account for the unusual 206 pb 204 pb ratios 7 the th u value of 4 for volcanics and mineralization is normal for continental crustal rocks but is markedly different from the low th u of 1 to 2 in devonian granitoids 8 the unusual lead isotope signatures for the deposits cast doubt on the proposed conjunction of tasmania with the australian mainland in cambrian times the relationships with antarctica await future investigations

investigation of mantle discontinuities from a single deep earthquake converted phases have been identified in the p wave coda of teleseismic seismograms from a single large mw 6 8 deep 360 km earthquake south of honshu japan standard array techniques slantstacking applied to broadband data from the global broadband network the german regional seismic network grsn and the grafenberg grf array and synthetic seismogram modeling reveal that these unusual phases are s to p conversions from under the seismic source timing of the converted phase indicates that subduction has had little if any effect on the depth of the 660 km discontinuity this study shows that upper mantle seismic structure can be constrained from single events using the presently available broadband seismic data and applying array techniques to network data

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

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

rbsb age determinations on precambrian carbonate rocks of the carpentarian mcarthur basin northern territories australia abstract the applicability and the uncertainty range of rb sr age determinations on carbonate rocks are discussed briefly a study of clay minerals isolated from carbonate rocks of the mcminn formation provides an isochron age of 1429 31 ma and thus the age for cessation of the proterozoic mcarthur basin sedimentation the eukaryots recently described in this area are therefore the oldest occurrences found in earth history a second sequence of measurements on a homogeneous dolomite siltstone ca 23 km southeast of the mcarthur river h y c pb zn deposits yields an rb sr age of 1537 52 ma for the upper barney creek formation in broad agreement with recent age determinations on feldspar beds of this horizon these data are compared with previous isotopic results from data presently available it can be concluded that the sediments in the mcarthur basin the type locality of the carpentarian were deposited between 1800 1400 ma

small scale changes of core mantle boundary reflectivity studied using core reflected pcp abstract short period pcp phases from earthquakes in the western aleutians recorded at the canadian yellowknife array yka sample the core mantle boundary cmb beneath the alaskan kenai peninsula and the cook inlet the pcp to p amplitude ratios for these events show large variability with some amplitude ratios more than an order of magnitude larger than predicted by radial earth models such as iasp91 or prem the amplitude ratios vary laterally with pcp core mantle boundary reflection points showing a 1 region with exceptionally large amplitude ratios it is likely that the variability in relative pcp amplitudes is due in large part to cmb topography other mechanisms such as radiation pattern mantle attenuation focusing and defocusing and core mantle boundary velocity and density perturbations could cause variations on scale lengths of less than a degree as found in this study but are unable to produce variations as large as those we observe

analysis of near source contributions to early p wave coda for underground explosions iii inversion for isotropic scatterers a systematic array processing analysis is applied to 916 teleseismic shortperiod p wave signals from 25 underground explosions at pahute mesa in order to establish whether coherent arrivals scattered from near source velocity heterogeneity can be identified the signal coherence is measured using semblance which can be statistically evaluated using the noncentral f distribution if the noise properties are suitable the effects of unsuitable correlated noise due to common path and receiver contributions are suppressed by applying station weighting functions obtained by stacking all observations at a given receiver similar corrections for individual events are applied to reduce biases due to variations in surface reflection times and spall effects the characteristics of the spatial and phase velocity resolution of the semblance analysis are established by a series of synthetic calculations including cases with random arrivals in the p wave coda the procedure is applied to the actual data under the assumption that any scattering structures present radiate isotropically although this constraint could be relaxed given independent knowledge of a particular scatterer s nature the pahute mesa data show some evidence for enhanced scattering contributions to the first 15 sec of the p wave signals from the western edge of the silent canyon caldera the overall confidence level in this interpretation is fairly low because simulations with random scattering in the p coda successfully match many features observed in the stacked data

oxygenation of shallow marine environments and chemical sedimentation in palaeoproterozoic peritidal settings frere formation western australia the palaeoproterozoic frere formation ca 1 89 gyr old of the earaheedy basin western australia is a ca 600 m thick succession of iron formation and fine grained clastic sedimentary rocks that accumulated on an unrimmed continental margin with oceanic upwelling lithofacies stacking patterns suggest that deposition occurred during a marine transgression punctuated by higher frequency relative sea level fluctuations that produced five parasequences decametre scale parasequences are defined by flooding surfaces overlain by either laminated magnetite or magnetite bearing hummocky cross stratified sandstone that grades upward into interbedded hematite rich mudstone and trough cross stratified granular iron formation each aggradational cycle is interpreted to record progradation of intertidal and tidal channel sediments over shallow subtidal and storm generated deposits of the middle shelf the presence of aeolian deposits mud cracks and absence of coarse clastics indicate deposition along an arid coastline with significant wind blown sediment input iron formation in the frere formation in contrast to most other palaeoproterozoic examples was deposited almost exclusively in peritidal environments these other continental margin iron formations also reflect upwelling of anoxic fe rich sea water but accumulated in the full spectrum of shelf environments dilution by fine grained windblown terrigenous clastic sediment probably prevented the frere iron formation from forming in deeper settings lithofacies associations and interpreted paragenetic pathways of fe rich lithofacies further suggest precipitation in sea water with a prominent oxygen chemocline although essentially unmetamorphosed the complex diagenetic history of the frere formation demonstrates that understanding the alteration of iron formation is a prerequisite for any investigation seeking to interpret ocean atmosphere evolution unlike studies that focus exclusively on their chemistry an approach that also considers palaeoenvironment and oceanography as well the effects of post depositional fluid flow and alteration mitigates the potential for incorrectly interpreting geochemical data

analysis of broadband seismograms from the chile eru area broadband seismograms recorded at the graefenberg grf array in germany are analyzed these data from the chile peru area have often a very complicated signature we have attempted to identify pp using theoretical seismograms a third phase besides p and pp which may have higher frequencies was discovered in a number of broadband records interpretations that relate this phase to the source or the structure are discussed our preferred interpretation is an s to p conversion at the bottom of the descending plate

geobiology of the late paleoproterozoic duck creek formation western australia the ca 1 8 ga duck creek formation western australia preserves 1000 m of carbonates and minor iron formation that accumulated along a late paleoproterozoic ocean margin two upward deepening stratigraphic packages are preserved each characterized by peritidal precipitates at the base and iron formation and carbonate turbidites in its upper part consistent with recent studies of neoarchean basins carbon isotope ratios of duck creek carbonates show no evidence for a strong isotopic depth gradient but carbonate minerals in iron formations can be markedly depleted in 13 c in contrast oxygen isotopes covary strongly with depth 18 o values as positive as 2 vpdb in peritidal facies systematically decline to values of 6 to 16 in basinal rocks reflecting we posit the timing of diagenetic closure the duck creek formation contains microfossils similar to those of the gunflint formation canada they are restricted to early diagenetic cherts developed in basinal facies strengthening the hypothesis that such fossils capture communities driven by iron metabolism indeed x ray diffraction data indicate that the duck creek basin was ferruginous throughout its history the persistence of ferruginous waters and iron formation deposition in western australia for at least several tens of millions of years after the transition to sulfidic conditions in laurentia suggests that the late paleoproterozoic expansion of sulfidic subsurface waters was globally asynchronous

the nature of the 660 kilometer upper mantle seismic discontinuity from precursors to the pp phase global seismic network data were used to image upper mantle seismic discontinuities stacks of phases that precede the pp phase thought to be underside reflections from the upper mantle discontinuities at depths of 410 and 660 kilometers show that the reflection from 410 kilometers is present but the reflection from 660 kilometers is not observed a continuous lame s constant lambda and seismic parameter at the 660 kilometer discontinuity explain the missing underside p reflections and lead to a p wave velocity jump of only 2 percent whereas the s wave velocity and density remain unchanged with respect to previous global models the model deemphasizes the role of lame s constant lambda with regard to the shear modulus and constrains the mineralogical composition across the discontinuity

short period s p conversion in the mantle at a depth near 700 km summary data from 16 deep earthquakes originating in the tonga subduction zone and recorded at the warramunga seismic array wra in central australia show the presence of a distinct later arrival in the coda of p this phase which is not predicted by smooth velocity depth models of the earth has been called x in the paper slownesses and arrival azimuths of x are similar to those of the p wave it is unlikely that x was radiated as the result of multiple focal activity because a corresponding phase was not observed neither in the coda of scp nor on seismograms of seismic stations located at close epicentral distances relative to p the variation in arrival times with focal depth of the earthquakes indicates that x is produced by s p conversion below the source at a depth near 700 km most of the observed x phases have approximately the same signal duration as the p waves thus suggesting a transition zone near 700 km which is only a few kilometres thick synthetic seismograms calculated for a model with a first order seismic discontinuity at a depth of 700 km show reasonable agreement with the observations as demonstrated by the synthetic seismogram modelling the amplitude of x depends on the radiation pattern of the earthquake source variations in the radiation pattern offer a plausible explanation that first x is only intermittently detected at wra and secondly that x has most frequently been observed from the southern part of the tonga subduction zone

filamentous microfossils in the early proterozoic transvaal supergroup their morphology significance and paleoenvironmental setting abstract well preserved filamentous microfossils siphonophycus transvaalensis n sp are described here from the carbonate campbellrand subgroup to iron formation kuruman iron formation transition of the transvaal supergroup south africa estimated to be 2 5 2 3 ga years old the microfossils occur in petrographic thin sections of a core sample of carbonate chert they are preserved by permineralization in both chert and in sparry ferroan dolomite stratigraphically the fossiliferous core sample occurs as part of an upward transition from a stromatolitic dolomite and limestone sequence campbellrand to the overlying iron rich sediments of the kuruman griquatown iron formations the average 13 c value of the kerogen in the sample is about 36 9 the microfossils are filamentous unbranched tubular to somewhat flattened 15 25 m in diameter and a few to many hundred microns in length they exhibit a coriaceous finely granular external surface texture resulting from the presence of adhering randomly distributed fine mineralic carbonate needles in salient morphological characteristics they are comparable to the tubular originally polysaccharide encompassing sheaths of extant oscillatoriacean cyanobacteria in comparison with previously described precambrian microfossils these fossil filaments are unusual because of their preservation in sparry carbonate in addition to chert their relatively large diameter and their coating by adhering precipitated carbonate needles they appear to be among the oldest assured microfossils now known from proterozoic age sediments the microfossils are interwoven occurring in subparallel aggregates that form a stromatolitic mat like fabric they are considered to be of an endogenetic in situ benthic origin occurring at the proximal margin of a deep shelf environment at the front distal margin of the campbellrand carbonate platform the water depth for this environment at the break in slope between deep shelf and euxinic basin is estimated to have been of the order of 40 45 m paleomagnetic data support our interpretation that the micro organisms inhabited a warm water marine environment probably at low latitudes

the paleoecology and significance of the gunflint type microbial assemblages from the frere formation early proterozoic nabberu basin western australia abstract eight distinct microbial assemblages are preserved in the frere formation which was deposited in the nabberu basin western australia these assemblages contain taxa animikiea eoastrion gunflintia and huroniospora typical of gunflint type microbial assemblages which have been described from australia canada and south africa four assemblages are interpreted as primary composed of benthic microorganisms and four as secondary containing components that were transported before deposition these microbial assemblages have similar paleoecologies to other gunflint type microbial communities all gunflint type microbial assemblages including the frere assemblages were deposited in a marine subtidal environment the apparent worldwide distribution of these similar microbial assemblages supports the hypothesis that gunflint type microbiotas were characteristic of the biota living in subtidal settings during the early proterozoic however this hypothesis does not account for the lack of these assemblages in post early proterozoic rocks this may be explained by the absence of primary inorganic silica precipitation in open marine settings during the middle and late proterozoic primary silica precipitation may be necessary for the preservation of well preserved microbial assemblages in marine settings of normal salinity silica precipitation in early proterozoic open marine settings is commonly associated with banded iron formation deposition which is confined mainly to 2500 2000 ma ago

a study on the response of a soft basin for incident s p and rayleigh waves with special reference to the long duration observed in mexico city abstract the responses of two types of soft basins for incident sh sv p and rayleigh waves in a 2 d elastic half space are investigated with special reference to the long duration of strong motions observed in mexico city during the michoacan mexico earthquake of 1985 first the difficulty for simple 1 d models to reproduce the later part of the accelerogram observed in mexico city is shown then the effect of a 2 d deep basin structure of a relatively large scale 10 km wide and 1 km deep and a moderate impedance ratio 2 5 is studied the results show that surface waves generated at the edges of the basin are clearly propagating inside the basin back and forth however the increase in duration by these surface waves is 20 sec at most introducing another much softer layer inside the basin we found that the calculated durations and envelop shapes become very similar to the observed ones because of the slowness of surface wave propagation these theoretical studies suggest that the exceptionally long duration observed in mexico city might be caused by a strong constructive interaction of soft surface layers with a deep basin structure beneath the city

regional seismic phases across the ligurian sea lg blockage and oceanic propagation abstract the ligurian sea west mediterranean is an example of a narrow oceanic basin which stops lg wave propagation locating the exact point of lg extinction was one of the goals of the sisbalig ii seismological experiment which deployed both inland stations in provence and corsica and ocean bottom seismometers obs in the ligurian sea a precise analysis of all available regional seismological data using energy diagrams reveals the role of the provence margin in the lg phase extinction for seismic events occurring in the southwestern alps the extinction is not a progressive phenomenon along oceanic travel path but occurs in a narrow zone of about 20 km width in the vicinity of the provence margin the extinction is accompanied by the generation of two types of diffracted waves the mantle phase sn v s 3 5 km s 1 and very low velocity s waves appearing both near the provence margin and in the middle of the ligurian basin we performed numerical simulation of sh wave propagation in two dimensional 2d media to investigate the mechanisms of lg extinction the simulations confirm that simple crustal thinning at the continent ocean boundary alone cannot cause the observed extinction of lg waves although the change in moho depth does induce the diffraction of a part of the incident lg wavetrain into mantle waves giving rise to the sn phase on the other side of the basin the existence of soft oceanic sediments is a very important factor in explaining the extinction as a large part of the lg energy is transferred into slow s waves trapped in the sediments moreover comparison of synthetic seismograms and observed data shows that the observed two packets of low velocity waves are an indication of the asymmetry of the ligurian basin with the existence of two shallow basins with very low s velocity 1 0 km s 1 near the provence margin and in the middle of the ligurian sea however the attenuation of lg waves in the vicinity of the provence margin is more abrupt in the data than in the simulations this discrepancy could result from the complex geometry of the continent ocean transition zone and its possibly very diffractive nature two factors that could add to lg extinction at the margin and are impossible to take into account in models

effects of subduction zone structure on explosion generated rayleigh waves 3 d numerical simulations summary three dimensional linear elastic finite difference calculations were used to investigate effects of near source scattering on long period surface waves radiated by shallow explosions located in the arc of a subduction zone these simulations were motivated by the teleseismic rayleigh wave amplitude patterns of explosions detonated at the amchitka test site amplitude patterns of these explosion generated 20 40s surface waves show evidence of both tectonic release and source receiver path effects a 3 d velocity model was constructed based on the aleutian subduction zone structure of boyd craeger 1991 and crustal refraction results reported by lambert et al 1970 two 3 d finite difference calculations were performed to assess the sensitivity of the results to the source location and fine details of the velocity model each finite difference grid consisted of 2 million nodes and the calculations were designed to model surface waves in the 20 40s period range at distances up to 400 km from the source displacements on the free surface were analysed to show the effects of the velocity structure upon the 3 d propagation of rayleigh waves from an explosion source azimuthally dependent rayleigh wave amplitudes are clearly seen as well as rayleigh to love wave conversion along the strike of the subduction zone structures teleseismic rayleigh wave amplitude anomalies of up to a factor of two are clearly evident from the near source scattering love waves 1 3 to 1 4 the amplitude of the rayleigh waves are radiated in narrow azimuthal ranges a method for the continued propagation of surface waves is presented based on a 2 d fresnel kirchhoff integral for surface waves the fresnel kirchoff integral accounts for diffraction effects assuming uniform propagation outside the finite difference grid this hybrid procedure using both the finite difference simulation and the fresnel kirchoff integral predicts 50 per cent rayleigh wave amplitude anomalies at teleseismic distances due to structure within 400 km of the amchitka test site

inversion of p coda for isotropic scatterers at the yucca flat test site although many studies have addressed the statistical properties of p coda there have been few attempts to analyze coda in a deterministic framework in this analysis 1050 teleseismic p wave seismograms from 32 yucca flat explosions are inverted for the locations of any isotropic point scatterers in the source region that contribute coherent arrivals to the coda a rectangular grid of potential scattering locations is constructed around the source array and the semiblance a measure of waveform ensemble coherence for each point is computed by slant stacking the full set of seismograms at various move out velocities the potential resolution or sensitivity of the method for this data set is tested by constructing a corresponding suite of synthetic seismograms with direct arrivals plus arrivals from various point scatters and calculating the semiblance at each point on the grid the particular source station geometry produces better sensitivity near the array sensitivity is enhanced parallel to the trend of the elongated source array and diminished in the orthogonal direction stacking the data for a velocity of 2 5 km sec yields relatively high semiblance values in the central portion of the test site as well as about 10 km to the ene and moderately high semiblance valuesmore about 10 km to the wnw less

generation of the teleseismic p wave coda from aleutian earthquakes summary we investigate large amplitude phases arriving in the p wave coda of broad band seismograms from teleseisms recorded by the grafenberg array the german regional seismic network and the global seismic network the data set consists of all events mb 5 6 from the aleutian arc between 1977 and 1992 earthquakes with large amplitude coda waves correlate with the presence of oceanic crust in the source region the amplitudes sometimes approach those of the p wave much larger than predicted by theory modelling indicates that phases in the p wave coda cannot be p wave multiples beneath the source and receiver or underside reflections which precede pp from upper mantle discontinuities among the events seismograms are very similar where the arrival times of the unusual phases agree approximately with the predicted times of s to p conversions from the upper mantle discontinuities under the source because the large amplitude phases in the p wave coda have little if any dependence on event depth and have predominantly an sv wave radiation pattern towards the receiver we suggest that they originate as sv and or rayleigh waves and are enhanced by lateral heterogeneity and multipathing from the subducting aleutian slab

distribution and diagenesis of microfossils from the lower proterozoic duck creek dolomite western australia two distinct generations of microfossils occur in silicified carbonates from a previously undescribed locality of the lower proterozoic duck creek dolomite western australia the earlier generation occurs in discrete organic rich clasts and clots characterized by microquartz anhedra it contains a variety of filamentous and coccoidal fossils in varying states of preservation second generation microfossils consist almost exclusively of well preserved gunflintia minuta filaments that drape clasts or appear to float in clear chalcedony these filaments appear to represent an ecologically distinct assemblage that colonized a substrate containing the partially degraded remains of the first generation community the two assemblages differ significantly in taxonomic frequency distribution from previously described duck creek florules taken together duck creek microfossils exhibit a range of assemblage variability comparable to that found in other lower proterozoic iron formations and ferruginous carbonates with increasing severity of post mortem alteration duck creek microfossils appear to converge morphologically on assemblages of simple microstructures described from early archean cherts two new species are described oscillatoriopsis majuscula and o cuboides the former is among the largest septate filamentous fossils described from any proterozoic formation

observations on the genus huroniospora barghoorn implications for paleoecology of the gunflint microbiota abstract examination of exceptionally well preserved thin sections of gunflint chert reveals new morphological criteria which are utilized for a critical evaluation of the genus huroniospora the shape varies from spheroidal to ellipsoidal one third of the specimens examined have either simple circular apertures or breaks in the vessel wall wall structure and surface texture appear to vary with quality of preservation these and other features lead to the conclusion that huroniospora represents a spore or encysted cell rather than a vegetative cell the presence of huroniospora in the schreiber beach assemblage does not constitute direct evidence for the primary photosynthetic nature of the interlaminar community

palaeomagnetism of iron formation from the late palaeoproterozoic frere formation earaheedy basin western australia palaeogeographic and tectonic implications abstract oriented cores of unweathered frere formation 1 9 1 8 ga including shaly and peloidal iron formation were obtained from two holes drilled to 116 m depth on opposite limbs of an anticline in the southern earaheedy basin at the northeastern margin of the yilgarn craton anisotropy of magnetic susceptibility measurements indicated that the frere formation in this area was unaffected by regional metamorphism the predominant magnetic mineral in the iron formation is hematite but a small amount of magnetite also is present in some samples stepwise thermal demagnetisation was conducted for 263 specimens many specimens acquired spurious components of magnetisation after treatment to 300 500 c ascribable to the oxidation of siderite to magnetite on heating however other specimens displayed a stable high temperature 680 c dual polarity component a carried by hematite that gave a bedding dip corrected direction of d 314 3 i 24 8 k 38 5 95 2 6 n 78 a fold test was positive at 99 level of confidence indicating acquisition of remanence prior to stanley folding in the earaheedy basin that occurred at 1 78 ga near the end of capricorn orogenesis the presence of polarity reversals in the stratigraphic section from one drill hole and within some specimens argues for the identification of a dipole field axis and the sufficient averaging of secular variation to define a palaeopole for the frere formation located at 45 2 s 220 0 e dp 1 3 dm 2 4 component a is ascribed to a chemical remanent magnetisation crm acquired soon after deposition and indicates a palaeolatitude of 13 0 1 5 for the frere formation the frere pole plots adjacent to but is still significantly different from late to post capricorn orogeny overprint poles from the hamersley province of the southern pilbara however vertical axis relative anticlockwise rotation of 10 15 for the frere pole brings it close to these pilbara overprint poles the data imply that the earaheedy basin and possibly the yilgarn craton underwent clockwise rotation of 10 15 relative to the pilbara craton during stanley folding this finding accords with evidence for tectonic settings marked by sinistral strike slip deformation adjacent to the yilgarn pilbara cratonic boundary during deposition and subsequent folding in the earaheedy basin the data also imply that the west australian and north australian cratons were in close proximity by 1 8 ga

organic remains in late palaeoproterozoic granular iron formations and implications for the origin of granules toward the end of the palaeoproterozoic era over 109 billion tonnes of banded bif and granular gif iron formations were deposited on continental platforms granules in iron formations are typically sub spherical structures 0 2 to 10 mm in size whereas concretions are 10 mm both types of spheroids are preserved throughout the sedimentological record their formation has typically been interpreted to originate from reworked fe rich sediments in high energy wave agitated shallow marine environments new evidence from six different late palaeoproterozoic granular iron formations gif however suggests that some granules are the result of diagenetic reactions in addition to other features driven by microbial processes and mechanical movements characteristic coarse grain interiors and septarian type cracks inside granules akin to those features in decimetre to meter size concretions are interpreted as desiccation features from hydrated diagenetic environments where sulphate and or ferric iron were reduced while organic matter om was oxidised inside granules those granules derived from sulphate reduction preserve diagenetic pyrite rims whereas those formed via ferric iron reduction preserve diagenetic magnetite along their rims other diagenetic minerals including apatite mixed with om and various carbonate phases are commonly preserved within granules combined with systematically 13c depleted carbonate these diagenetic mineral assemblages point to the oxidative decay of om as a major process involved in the formation of granules spheroidal equidistant haematite laminations surround some granules and contain apatite associated with carbonate om and ferric ferrous silicates and oxides that further suggest these structures were not shaped by wave action along sediment water interfaces but rather by chemical wave fronts and biomineralisation our results demonstrate that the formation mechanisms of gif also involve microbial activity and chemically oscillating reactions as such granules have excellent potential to be considered as promising biosignatures for studying precambrian biogeochemistry as well as astrobiology

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

propagation of pl and implications for the structure of tibet pl is a long period 20 s or more wave train beginning just after the p arrival in seismograms and continuing until the s arrival or the rayleigh wave the wave train can be observed at epicentral distances of about 5 20 with instruments sensitive to these low frequencies pl propagates as a partially trapped p sv wave in the crust s wave energy is lost to the mantle during propagation making pl a leaky mode we study pl propagation for a variety of earth models using the synthetic seismogram algorithm wave number integration and find that the vertical travel time in the crust is the most important parameter controlling pl s oscillation period this period can vary by more than a factor of two between oceanic and continental paths p sv leaky mode propagation includes many different modes the low frequency motion termed pl is only the first or fundamental mode in this family a second higher frequency mode roughly equal in amplitude to the fundamental appears for models without a net positive velocity gradient in the crust we use these results to match an observed pl wave train whose propagation path consisted almost entirely of the tibetan plateau by considering first and second pl mode behavior we find that the tibetan crust is about 85 km thick with an uncertainty of about 20 km and possesses a significant 0 01 s 1 positive velocity gradient the presence of a large low velocity zone in the lower tibetan crust thus seems unlikely much higher frequency pl modes also appear in the green s functions for layer over half space models and appear to be responsible for the high frequency pg phase making pl and pg different members of the same type of leaky mode propagation

geology and tectonic evolution of palaeoproterozoic basins of the eastern capricorn orogen western australia abstract the palaeoproterozoic yerrida bryah padbury and earaheedy basins extend for about 700 km along the northern margin of the yilgarn craton and are part of the capricorn orogen the basins developed between ca 2 2 and 1 8 ga recording periods of sedimentation volcanism rifting accretion and passive margin tectonism the oldest is the 2 17 ga yerrida basin which began as an intracontinental sag within which siliciclastics and evaporites accumulated at about 2 0 ga convergence of the yilgarn craton and the glenburgh terrane closed an oceanic arm resulting in the accretion of oceanic crust and plateaux onto the yilgarn margin and the western side of the yerrida sag basin this accretion produced the narracoota formation which together with the clastic and chemical banded iron formation sedimentation that followed forms the bryah basin continuing collision resulted in the establishment around 1 96 ga of the padbury foreland basin developed on top of the bryah basin rocks the ca 1 83 ga convergence and collision of the pilbara and yilgarn cratons capricorn orogeny resulted in deformation of the bryah padbury basins and orogenic uplift with coarse clastic sedimentation accompanied by the eruption of flood basalts into the yerrida basin to the east of the collision zone and possibly postdating collision a passive margin was present sedimentation along it consisted of clastic and chemical sediments granular iron formation which form the earaheedy basin during the ca 1 79 ga yapungku orogeny due to the convergence of the north australian and western australian cratons the northern margin of the earaheedy basin was deformed stanley fold belt this last deformation event concludes the palaeoproterozoic history of the eastern capricorn orogen basins

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

crustal reflections and the nature of regional p coda abstract f k spectra for regional p coda are presented for events recorded at the scandinavian noress finesa and arcess arrays and the new england nynex array the spectra are dominated by on azimuth energy with apparent velocities between pn or faster and lg following this analysis reflection coefficients calculated with an efficient boundary integral scheme are used to study the role irregular interfaces play in the creation of regional p coda we find that observed crustal scattering in these regions is strikingly consistent with p p and p sv scattering from the two dimensional 2d irregular moho and even more consistent with scattering from a 2d irregular near surface interface

subduction zone calibration and teleseismic relocation of thrust zone events in the central aleutian islands abstract teleseismically determined epicenters for thrust zone earthquakes in the central aleutian islands are systematically mislocated by about 40 km to the north relative to local network solutions we attribute the mislocation to travel time anomalies produced by the higher p wave velocity in the subducting slab these travel time anomalies can be estimated by seismic ray tracing through a thermally modeled velocity structure for the slab observed teleseismic station residuals and slab travel time anomalies computed from local network hypocenters plotted as a function of event distance to the center of curvature of the island arc show good agreement this distance versus residual relationship can be used to validate the local network hypocenters and to constrain the length of subducting lithosphere computed slab source corrections when incorporated in a standard location program reduce teleseismic mislocations to 20 km the addition of empirical station corrections reduces the mislocation to about 10 to 15 km depth and origin time estimates can be further improved through use of better station corrections a local station within 2 and depth phases

effects of near source seafloor topography on long period teleseismic p waveforms in the study of the shallow earthquakes in the subduction zones reverberations inside the water layer near the source are known to affect very significantly the far field body waveforms the usual correction is to take a horizontally layered model as the near source structure but it is not realistic for the trench geometry therefore finite difference simulation combined with far field ray theory is employed in order to examine the effects of the realistic non flat seafloor topography as a realistic trench model the japan trench is selected the response of the structure near the source is calculated by two dimensional finite difference simulation then teleseismic p waveforms are calculated by using reciprocal theorem our results suggest that 1 around the trench bottom effects of non planar structure are remarkable and the realistic seafloor cannot be approximated by any model of planar structure however 2 effects of the continental slope are well approximated by a model of planar dipping seafloor of appropriate dip therefore the near source structure must include the realistic seafloor topography or the waveform analyses may lead to spurious subevents in a later stage of rupture process and mislocation of events our results also show that present knowledge about the seafloor topography is enough to incorporate the complicated geometry

a global study of s to p and p to s conversions from the upper mantle transition zone summary long period data of the global digital seismograph network gdsn recorded over the three year period from 1984 to 1986 were studied for the occurrence of s p and p s conversions from the upper mantle transition zone that appear as precursors to teleseismic s arrivals conversions of this type were identified on a large number of single station records simple stacking of many records enhanced the appearance of converted phases and demonstrated that no major lateral variations in the nature of the transition zone exist between various tectonic regions s p and p s conversions from the 400 km discontinuity were best observed at distances between 70 and 85 while conversions from the 670 km discontinuity showed up best at distances beyond 87 the analysis of published source mechanisms and comparison with synthetic seismograms suggests that the appearance of converted phases is primarily governed by the earthquake radiation pattern phases that have undergone s p conversions beneath the receiver are best observed from dip slip events that radiate strong sv and weak p waves towards the station p s conversions beneath the source area on the other hand are frequently observed from events that radiate strong p and little sv energy towards the station and also from some strike slip events comparison of observed with synthetic seismograms suggests that the prem model of dziewonski anderson 1981 explains most of the observations observed s p and p s conversions from the 670 km discontinuity however often have larger amplitudes than in the synthetics constructive interference of converted waves with the p wave coda source radiation effects and a velocity contrast across the 670 km discontinuity which is higher than in prem may all contribute to the discrepancy

waveform inversion for rupture process using a non flat seafloor model application to 1986 andreanof islands and 1985 chile earthquakes abstract yoshida s 1992 waveform inversion for rupture process using a non flat seafloor model application to 1986 andreanof islands and 1985 chile earthquakes in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 45 59 we inverted teleseismic p waveforms to infer rupture processes of subduction zone earthquakes using green s functions computed for realistic seafloor topography because reverberations in the water layer near the source affect the teleseismic waveforms we modeled the seafloor by a series of several plane surfaces with appropriate dip and performed three dimensional ray tracing to compute green s functions from the gdsn seismograms spatial and temporal slip distributions were estimated for the 1986 andreanof islands earthquake m s 7 7 and the 1985 chile earthquake m s 7 8 to suppress unstable parts of the least squares solution we introduced some stabilizing constraints the 1986 andreanof islands earthquake was analyzed using two kinds of source models the first one has a prescribed rupture velocity and the second one allows any point to slip at any time the solutions by the two models gave similar patterns of slip distribution a relatively small subevent initially occurred and then a dominant subevent with a moment of about five times larger occurred with a center about 80 km west of the hypocenter for the 1985 chile earthquake a small initial subevent was also found and a major rupture with a ten times larger moment extended approximately from 20 to 70 km south of the hypocenter for both the earthquakes it was found that most of the one day aftershocks occurred in areas where a small moment was released during the main shock

bathymetric effects on body waveforms from shallow subduction zone earthquakes and application to seismic processes in the kurile trench resolution of earthquake source characteristics is limited by structural complexities in the source region this study explores the effects of dipping near source bathymetry and structure on long period teleseismic body waveforms from shallow subduction zone earthquakes ray theory synthetics calculated for shallow double couple sources and velocity structures incorporating planar dipping interfaces suggest that p waveforms are most sensitive to water depth and sea bottom orientation for sources beneath a substantial water layer the seafloor dip of the inner trench slope can give rise to prominent later arrivals and waveform complexity not found on synthetics calculated for horizontal bathymetry sediment thickness and the dip of the sediment crust and crust mantle interfaces generally have lesser effects the high amplitude arrivals result largely from changes in the source takeoff angles of the reverberations these arrivals present difficulties for source depth and time function determination from p waveforms and may cause spurious complexity in the derived pattern of earthquake moment release sh waveforms show little effect of sea bottom orientation or local structure for shallowly dipping thrust faulting events suggesting that sh waveform analysis can stabilize source parameter studies with respect to unknown or incompletely parameterized bathymetry and structure effects of nonhorizontal structure are observed in the p waveforms from shallow kurile trench events ms 5 8 7 2 which show prominent later arrivals not observed on sh waveforms synthetics calculated for a dipping water crust interface provide a better fit to these arrivals than those calculated for a horizontal interface suggesting that the p waveforms show systematic effects of seafloor dip the synthetics are highly sensitive to the assumed orientation of the seafloor the data are fit best for seafloor orientations consistent with the dip of the inner trench slope the distorting effect of the water layer gives rise to systematic differences in p wave complexity between shallow events near the seismic front which occur beneath a thick water layer and show numerous later arrivals and deeper arcward events which occur beneath a thin water layer and show simple waveforms large reverberations in the p waveforms of the october 20 1963 and june 10 1975 kurile tsunami earthquakes have previously been used as evidence for a long source duration and a change in focal mechanism during rupture however sh waveforms suggest simple source time functions with durations of 25 and 43 s inversion of p and sh waveforms and amplitudes yields shallowly dipping thrust mechanisms with no conclusive evidence for a previously proposed steepening of the fault dip during rupture the large p wave reverberations cannot be fit using a point source and a horizontal seafloor detailed modeling of the 1963 event as an upward propagating rupture extending through low velocity sediments to the ocean bottom combined with the effects of the dipping seafloor allow the reverberation amplitudes to be matched the anomalous tsunami excitation appears to be related to the location of the sources at shallow depths within low velocity sediments

finite difference modelling of lg blockage summary a p sv finite difference method is used to calculate seismic wave propagation in the crust and upper mantle the anelastic effect is excluded from the calculation and therefore any wavefield changes can be attributed to either geometrical or structural effects snapshots of displacement distrihutions within and synthetic seismograms at the free surface of different models revealed that the inclusion of a water column over an appropriate section of the crust is necessary to explain the severe lg attenuation widely observed in many parts of the world in particular for an oceanic propagation path they clearly show that the effect of a water column over the crust is an important factor in blocking lg piupagation a propagation path of more than 100 km over an oceanic crust can completely block the transmission of lg waves originating in a neighbouring continental crust the energy of the lg wavetrain is redistributed at continental margins most of the incoming energy is either converted to waves which propagate into the mantle or is backscattered to form waves which propagatc in the continental crust in the opposite direction very little energy is presented at the expected arrival times at the other side of the oceanic crust which is consistent with the lg blocking phenomena simplified 2 d models of the north sea central graben are used to simulate lg propagation in a laterally varying continental path the inclusion of a water column over the crust enhances the attenuation of lg the deeper the water column the greater the attenuation the undulation of sea bottom can further enhance the attenuation of lg

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

segmentation of the aleutian plate boundary derived from stress direction estimates based on fault plane solutions we propose a new method to investigate stress homogeneity along plate boundaries based on the cumulative misfit of individual fault plane solutions calculated using assumed stress tensors using this method some segments of faults can be defined without the time consuming inversions for stress directions from earthquake fault plane solutions we assume that the misfits are relatively constant within segments of uniform stress orientation but that they change abruptly at boundaries of segments this assumption is supported by the pattern of misfits observed for about 400 earthquakes located along the aleutian subduction zone from 170 e to 145 w which occurred between 1964 and 1993 the efficacy of the approach is confirmed by stress inversion analysis of moving windows the cumulative misfit as a function of strike along the aleutian arc shows there are five first order boundaries where the stress directions change significantly the first one is near 177 5 e which corresponds to the northward projection of the rat fracture zone the second one falls on the adak fracture zone the third is near the locus where the amlia fracture zone intersects the trench the fourth lies on the easternmost end of the 1957 aftershock zone the last is located where the two major asperities of the 1964 rupture are separated the segmentation boundaries found in our analysis may be controlled by the fracture zones in the subducted plate this observation may be interpreted as due to decoupling within the underthrusting plate along the zones of weakness the fracture zones across which stress may not be transmitted fully the boundaries defined by our method may also be related to the asperity and aftershock distributions of great earthquakes the focal mechanism data in the segments defined as stress homogeneous by our method are then used to invert for the principal stress orientations by employing the focal mechanism stress inversion fmsi computer programs of gephart we find that the stress directions within these segments are different from each other where they can be defined well and the average misfits in these volumes are very small 2 8 and 5 5 the method is not robust enough to detect all the stress segmentation boundaries based on only one reference stress tensor analyses with different reference stress models help to define most or all the stress segmentation boundaries

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

variability of p660s phases as a consequence of topography of the 660 km discontinuity abstract records from the broad band nars nl array deployed in the netherlands western germany and belgium have been analyzed for p to s converted phases from the 660 km discontinuity p660s no individual seismogram from this array shows a clear p660s phase stacks of up to 113 crosscorrelograms show p660s energy that is weak in comparison with similar stacks elsewhere very high amplitude observations of p660s in single seismograms from the same region have been made in a previous study paulssen 1985 geophys res lett 12 709 712 paulssen 1988 j geophys res 93 10489 10500 such variations may be due to relief on the 660 km discontinuity in this paper we quantify the effects of different topographies on the time delays amplitudes and waveforms of the direct p wave as well as on the p660s converted wave synthetic p and p660s waveforms are calculated by applying a kirchhoff helmholtz integration over the wave fields at the 660 km discontinuity interface for topography on scales of a few hundreds of kilometers the effects on the p wave are very small compared with those on the p660s phases time delays focused and defocused amplitudes and waveform distortion of p660s phases influenced by such topography diminish the coherence of p660s in synthetic stacks our results show that topography of the 660 km discontinuity with dominant scale lengths of a few hundreds of kilometers and variations in depth of 15 km to 25 km is consistent with the pattern of p660s observations in single seismograms and stacks

early proterozoic microfossils and penecontemporaneous quartz cementation in the sokoman iron formation canada early proterozoic microfossils from the sokoman iron formation northeastern canada are indistinguishable from those of the gunflint formation in both morphology and inferred community structure the contemporaneity of the sokoman assemblage with the bitter springs like cyanobacteria of the belcher supergroup indicates that differences between the two major types of early proterozoic microbiotas are primarily ecological and not temporal evolutionary in nature in arenaceous iron formations microfossils are restricted to peloids and are absent from pore filling silica interpreted as cement cemented arenaceous intraclasts indicate that some of the silica was penecontemporaneous and the abundance of minus cement porosity in arenaceous iron formations demonstrates that early pre compaction cementation was common

pb and nd isotope and trace element constraints on the origin of basic rocks in an early proterozoic igneous complex minnesota abstract a suite of early proterozoic basic to granitic rocks exposed near st cloud minnesota are cut by northeast trending basaltic dikes pb isotope data for all of these rocks overlap and plot about an 1800 ma pb pb correlation line the basic rocks which are light ree enriched have nd values between 0 4 and 4 8 petrogenetic considerations suggest that the basic rocks were derived from a light ree enriched source which had an fe mg ratio greater than that for pyrolite the enrichment in fe mg is probably a result of the addition of basic melts to the source the light ree enrichment may have a mantle origin by the addition of mantle derived light ree enriched basic melts or fluids or a sedimentary origin by the addition of light ree enriched fluids or melts derived from subducted sediments with an early proterozoic provenance in either case the nd isotopes suggest that the light ree enriched component existed since c 2300 ma the igneous complex may have formed at a convergent margin an anorthositic gabbro near mora has an nd of 5 indicating that it was derived from a mantle source with a history of light ree depletion this gabbro may have been part of an early proterozoic ocean crust

shelf facies microbiotas from the odjick and rocknest formations epworth group 1 89 ga northwestern canada moderately well preserved microfossils are reported from two formations of the epworth group in the wopmay orogen an assemblage in a stromatolitic unit in the uppermost part of the odjick formation contains the microfossils huroniospora spp gunflintia sp and frutexites sp this assemblage which is remarkable for its preservation in a carbonate matrix is similar to one found in a stromatolitic chert facies in the gunflint iron formation of ontario taxa in four members of the overlying rocknest formation are preserved in cherty dolomites of inner shelf shallowing upward cycles capped by small digitate stromatolites these assemblages comprise the spheroidal taxa huroniospora spp melasmatosphaera magna sphaerophycus sp and palaeoanacystis sp the filamentous taxa archaeotrichion sp eomycetopsis sp siphonophycus sp brevitrichoides sp biocatenoides incrustata gunflintia sp and archaeonema sp the acritarch eomicrhystridium sp and rare specimens of two unidentified and problematic taxa

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

comparison of broadband and short period seismic waveform stacks implications for upper mantle discontinuity structure stacks of short period and broadband seismic waveforms from four deep h 550 km earthquakes in south america recorded in california exhibit small amplitude signals between the direct p and surface reflected pp phases the anomalous and variable slownesses of these signals in short period stacks as well as differences between short period and broadband stacks suggest that the weak arrivals originate by reflection from laterally varying structure in the mantle near the subduction zone the only structure consistently producing near source and near receiver reflections is the 410 km discontinuity which varies in depth by 10 km and has an impedance contrast comparable with that predicted by reference earth models

the geometry of aleutian subduction three dimensional seismic imaging we constrain the subduction geometry of the aleutian slab by the distribution of seismicity recorded at local seismic networks and by a teleseismic p wave residual sphere analysis of 11 earthquakes and one nuclear explosion the source depth and mechanism of each earthquake are determined by modeling teleseismically recorded long period p and sh waveforms we construct theoretical residual spheres by tracing rays through three dimensional thermal models of the subducting slab assuming a pressure independent thermal coefficient of velocity vp t rays are initiated at the estimated source depths and at epicenters whose relative locations are consistent with those obtained from the observed travel times after correction for slab effects for each event we determine the values of slab strike and dip that provide the best match to the observations average values of 67 confidence intervals determined by an f test are 5 in dip and 10 in strike strike varies along the arc by 60 for all but one of the modeled events the estimated orientation of the slab s aseismic extension agrees to within two standard deviations of that derived from seismicity distributions assuming a constant dip a slab penetration depth of 600 km fits the central aleutian events best this is at least 300 km below the deepest earthquakes slabs terminating above 400 km or below 1000 km provide significantly worse fits though there is a trade off between vp t and penetration depth the best fitting value of vp t using a 600 km deep slab is 0 5 0 1 ms 1 k 1 this is consistent with both ultrasonic laboratory measurements and values obtained for deeper earthquakes in several other subduction zones vp t estimated from shallow thrust events is 20 50 larger than that for intermediate focus events suggesting that a simple thermal model with constant vp t under predicts the velocity anomalies shallower than 100 km depth the fact that a 600 km deep slab provides better fits than a 1000 km deep slab implies that the downdip p rays exit the slab near a depth 600 km a slab bend to either a shallower or steeper dip would satisfy the p times hand picked pcp travel times are most consistent with the slab bending to a 20 steeper dip before plunging into the lower mantle in the western aleutians the relative plate motion becomes transform seismicity ceases at less than 100 km depth and volcanism is no longer active we image a slab like high seismic velocity zone apparently associated with the along arc lateral transport of cold subducted lithosphere from the east an earthquake whose location is known from local observations is analyzed in detail the velocity structure inferred using this location is nearly identical to that obtained using our scheme of annihilating location information from both theoretical and observed times a severe bias would result however from the commonly used procedure of analyzing travel times computed with respect to an incorrect event location such as that of the isc