locating non volcanic tremor along the san andreas fault using a multiple array source imaging technique summary non volcanic tremor nvt has been observed at several subduction zones and at the san andreas fault saf tremor locations are commonly derived by cross correlating envelope transformed seismic traces in combination with source scanning techniques recently they have also been located by using relative relocations with master events that is low frequency earthquakes that are part of the tremor locations are derived by conventional traveltime based methods here we present a method to locate the sources of nvt using an imaging approach for multiple array data the performance of the method is checked with synthetic tests and the relocation of earthquakes we also applied the method to tremor occurring near cholame california a set of small aperture arrays i e an array consisting of arrays installed around cholame provided the data set for this study we observed several tremor episodes and located tremor sources in the vicinity of saf during individual tremor episodes we observed a systematic change of source location indicating rapid migration of the tremor source along saf

how are diagenesis and reservoir quality linked to depositional facies a deltaic succession edgeoya svalbard abstract middle to late triassic strata exposed at edgeoya eastern svalbard represents an uplifted part of the northwestern corner of the barents sea this interval is characterized by a predominantly mud dominated deltaic depositional system where rocks with petroleum reservoir potential are expected both in delta front and channelized sandstone deposits recent drilling campaigns into time equivalent rocks in the barents sea have however been disappointing with porosity and permeability below expectation this study improves the current understanding of reservoir potential within depositional systems of this kind by examining the link between different depositional facies diagenesis and their impact on reservoir quality five depositional facies were mapped and correlated channel floodplain shallow marine prodelta and offshore our study suggests that diagenetic signatures that control the quality of reservoir rocks vary systematically with these depositional facies mechanical compaction was the main cause of porosity destruction in the channel shallow marine and floodplain samples while early carbonate cementation occludes the intergranular volume in the prodelta samples moreover quartz cement reduced the porosity in the shallow marine depositional facies the estimated maximum burial temperature of 124 c indicates burial to several km depths depositional settings within a deltaic to shallow marine system do not exert a strong control on the abundance of chlorite but it does impact whether the chlorite is pore filling or grain coating and porosity preserving efficient chlorite coating have aided in preserving porosities up to 32 in the very well well sorted medium to fine grained sandstones deposited by fluvial dominated channels

a p wave velocity model of earth s core present earth core models derived from the retrieval of global earth structure are based on absolute travel times mostly from the international seismological centre isc and or free oscillation eigenfrequencies many core phase data are left out of these constructions e g pkp differential travel times amplitude ratios and waveforms this study is an attempt to utilize this additional information to construct a model of core p wave velocity which is consistent with the different types of core phase data available in conjunction with our waveform modeling we used 150 differential time measurements and 87 amplitude ratio measurements which were the highest quality observations chosen from a large population of global digital seismograph network gdsn records as a result of fitting these various data sets a one dimensional p wave velocity model of the core prem2 is proposed this model modified from the preliminary reference earth model prem dziewonski and anderson 1981 shows a better fit to the combined data set than any of the existing core models major features of the model include a sharp velocity discontinuity at the inner core boundary icb with a large jump 0 78 km s and a low velocity gradient at the base of the fluid core the velocity is nearly constant over the lower 100 km of the outer core the model features a depth dependent q structure in the inner core such that a constant t for the inner core fits the amplitude ratios and waveforms of short period waves moderately well this means the top of the inner core is more attenuating than the deeper part of the inner core in addition the p velocity in the lowermost mantle is reduced from that of prem as a baseline adjustment for the observed separations of the df and ab branches of pkp at large distances

late mesozoic magmatism in svalbard a review abstract late mesozoic mafic igneous rocks are widespread across the arctic region and are collectively referred to as the high arctic large igneous province halip in svalbard the halip is represented by the diabasodden suite an extensive system of predominantly basic intrusive doleritic rocks associated lava flows are exposed on the far east of the archipelago two main igneous centres have been proposed 1 central spitsbergen and 2 the eastern svalbard dolerite belt spanning hinlopenstretet and the islands of edgeoya and barentsoya offshore seismic and magnetic data suggest a link between the svalbard and franz joseph land igneous exposures intrusions occur predominantly as sills up to 100 m thick and laterally continuous for up to 30 km dykes are present locally and range in thickness from

opening of the fram strait gateway a review of plate tectonic constraints abstract we have revised the regional crustal structure oceanic age distribution and conjugate margin segmentation in and around the lena trough the oceanic part of the fram strait between the norwegian greenland sea and the eurasia basin arctic ocean the lena trough started to open after eurasia greenland relative plate motions changed from right lateral shear to oblique divergence at chron 13 times 33 3 ma earliest oligocene a new bouguer gravity map supported by existing seismic data and aeromagnetic profiles has been applied to interpret the continent ocean transition and the influence of eocene shear structures on the timing of breakup and initial seafloor spreading assuming that the onset of deep water exchange depended on the formation of a narrow oceanic corridor the gateway formed during early miocene times 20 15 ma however if the initial lena trough was blocked by terrigenous sediments or was insufficiently subsided to allow for deep water circulation the gateway probably formed with the first well developed magnetic seafloor spreading anomaly around chron 5 times 9 8 ma late miocene paleoceanographic changes at odp site 909 northern hovgard ridge are consistent with both hypotheses of gateway formation we cannot rule out that a minor gateway formed across stretched continental crust prior to the onset of seafloor spreading in the lena trough the gravity seismic and magnetic observations question the prevailing hypotheses on the yermak plateau and the morris jesup rise as eocene oceanic plateaus and the hovgard ridge as a microcontinent

upper cretaceous exhumation of the western rhodope metamorphic province chalkidiki peninsula northern greece the vertiskos unit of northern greece is an elongated basement belt with a complex poly metamorphic history it extends from greece chalkidiki peninsula to the south up to serbia in the north and arguably represents the westernmost part of the rhodope metamorphic province northern greece to southern bulgaria the vertiskos unit experienced a medium pressure lower amphibolite facies metamorphic overprint during the alpine orogeny the available medium temperature geochronology implies that it remained at temperature of approximately 300 c or slightly higher during lower cretaceous in order to constrain its post lower cretaceous thermal history until near surface exposure we applied apatite fission track analysis the central ages obtained range from 68 5 3 8 to 46 6 3 6 ma uppermost cretaceous to middle eocene and mean track lengths between 13 and 13 5 m we applied two inverse thermal modeling approaches using either each sample independently high degree of freedomin the thermal history better data fit or all samples together interpreting them as a vertical profile simpler thermal history worse data fit irrespective of the modeling approach we conclude that the bulk thermal history of the vertiskos unit crosses the high temperature limit of the apatite partial annealing zone by the uppermost cretaceous and reaches near surface conditions as early as lower middle eocene these results contrast with the thermal history of the other domains of the rhodope metamorphic province further east namely the southern rhodope core complex and the northern rhodope complex and establish the vertiskos basement complex as the oldest exhumed coherent basement fragment of the rhodope metamorphic province and greece

structure of palaeogene sediments in east ellesmere island constraints on eurekan tectonic evolution and implications for the nares strait problem abstract the nares strait problem represents a debate about the existence and magnitude of left lateral movements along the proposed wegener fault within this seaway study of palaeogene eurekan tectonics at its shorelines could shed light on the kinematics of this fault palaeogene late paleocene to early eocene sediments are exposed at the northeastern coast of ellesmere island in the judge daly promontory they are preserved as elongate sw ne striking fault bounded basins cutting folded early paleozoic strata the structures of the palaeogene exposures are characterized by broad open synclines cut and displaced by steeply dipping strike slip faults their fold axes strike ne sw at an acute angle to the border faults indicating left lateral transpression weak deformation in the interior of the outliers contrasts with intense shearing and fracturing adjacent to border faults the degree of deformation of the palaeogene strata varies markedly between the northwestern and southeastern border faults with the first being more intense structural geometry orientation of subordinate folds and faults the kinematics of faults and fault slip data suggest a multiple stage structural evolution during the palaeogene eurekan deformation 1 the fault pattern on judge daly promontory is result of left lateral strike slip faulting starting in mid to late paleocene times the palaeogene judge daly basin formed in transtensional segments by pull apart mechanism transpression during progressive strike slip shearing gave rise to open folding of the palaeogene deposits 2 the faults were reactivated during se directed thrust tectonics in mid eocene times chron 21 a strike slip component during thrusting on the reactivated faults depends on the steepness of the fault segments and on their obliquity to the regional stress axes strike slip displacement was partitioned to a number of sub parallel faults on shore and off shore hence large scale lateral movements in the sum of 80 100 km or more could have been accommodated by a set of faults each with displacements in the order of 10 30 km the wegener fault as discrete plate boundary in nares strait is replaced by a bundle of faults located mainly onshore on the judge daly promontory

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

double beam analysis of anomalies in the core mantle boundary region seismic p waves from nuclear explosions at the kazakhstan test site in eastern kazakhstan kts observed at the yellowknife array yka canada are used to analyze the structure at the base of the mantle under severnaya zemlya arctic sea 18 events at the degelen subsite and 9 events at the shagan river subsite of kts are studied the simple waveforms and well controlled source parameters of nuclear explosions allow the use of the events as source arrays in addition to the usual receiver array configuration a new method double beamforming integrating both concepts is presented this method increases the resolution considerably for the degelen yka path one anomalous arrival 1 9 s before pcp slowness between p and pcp slowness is observed smaller anomalous arrivals are found 6 5 s degelen yka and 8 1 s before pcp shagan river yka these anomalous phases cannot be explained with standard earth models they are produced by reflections from an inhomogeneity in the lowermost mantle at 82 3n 107 5e under the area north of severnaya zemlya the strong variations between the wavefields for the two source receiver combinations indicate a heterogeneous structure in the lower mantle the anomaly found here is located close to an anomaly in the lowermost mantle beneath the nansen basin

development of the continental margins of the labrador sea a review abstract the labrador sea is a small oceanic basin that developed when the north american and greenland plates separated an initial period of stretching in early cretaceous time formed sedimentary basins now preserved under the continental shelves and around the margins of the oceanic crust the basins subsided thermally during late cretaceous time and a second episode of tectonism took place during latest cretaceous and early paleocene time before the onset of sea floor spreading in mid paleocene time around the northern labrador sea davis strait and in southern baffin bay voluminous picrites and basalts were erupted at and shortly after the commencement of sea floor spreading volcanism occurred again in early eocene time at the same time as sea floor spreading commenced in the northern north atlantic farther southeast along the labrador and southern west greenland margins oceanic crust is separated from continental crust by highly stretched but non magmatic transition zones which developed before sea floor spreading a complex transform zone which developed during sea floor spreading in late paleocene and early eocene time separates continental and oceanic crust along the baffin island margin the greenland and labrador ocean continent transitions are asymmetric across the only available conjugate cross sections however a cross section through the labrador margin farther north resembles the greenland cross section in the conjugate pair more than it does the labrador cross section of this pair consideration of the geological history of the area suggests that the non magmatic transition zones may have formed by slow extension of a few millimetres per year through a period of 53 ma during cretaceous and early paleocene time

evidence of a laterally variable lower mantle structure from p and s waves summary examples of grafenberg array data showing anomalous p waves which typically arrive 3 5s after the direct p wave and which have a slowness 0 7 0 8 sdeg smaller than direct p are presented this additional phase is most frequently observed for events located in the ne portion of the southern kurile island subduction zone 73 80 from grafenberg but systematically disappears for events in the sw portion of this zone because of the magnitude of the slowness difference these observations cannot be attributed to a complex source rupture process nor to multipathing through the descending slab likewise they may not be accounted for by near receiver structure because these phases are not seen for all kurile events if present they appear at all stations of the array but they follow direct p too closely to be a multiple from the moho therefore we conclude they are very likely caused by lower mantle velocity structure the most likely explanation is the presence of a p velocity jump of about 3 per cent approximately 290 km above the core mantle boundary since such a reflector in the lowermost mantle not only gives a good fit of traveltimes and slowness but is also able to model the waveform and the amplitudes of this additional p phase the distribution of bounce points on this reflector for the kurile events indicates a lateral extension of this velocity anomaly under northern siberia of about 150 km by at least 200 km the best fitting s wave model has a reflector in the same depth but the velocity contrast seems to be only about 2 per cent suggesting a different behaviour of the p and s velocity in d few events from other regions in this distance range are suitable for a definitive analysis of this kind from among this group some observations indicate a lower mantle anomaly under the lomonosow ridge and under northern greenland but since the lower mantle under western siberia northern novaya zemlya the azores islands region and the ussr afghanistan border region does not produce an additional phase in the grafenberg recordings it is very unlikely that such a velocity anomaly in the lowermost mantle is a global feature

crustal scattering at the ktb from a combined microearthquake and receiver analysis summary in december of 1994 a fluid injection experiment which triggered several hundreds of microearthquakes was conducted at the ktb main borehole oberpfalz germany these events were recorded with a temporal seismic network at the surface out of the complete data set a cluster of five events recorded at four mini arrays consisting of eight or nine stations was used to investigate the crustal scattering properties in the vicinity of the ktb for this purpose the double beam method dbm kruger et al 1993 1995 1996 and the double beam imaging method scherbaum kruger weber 1997 were extended to curved wave fronts to drop the restriction of plane wave propagation this technique is used for imaging the crustal scattering strength using earthquake clusters recorded at close by mini arrays the results of the array analysis show that the composition of the p coda is mainly avected by the site location of the arrays near surface and deeper crustal scattering contribute in a very complicated pattern furthermore with the present data set it was possible to identify reflections from the top of the erbendorf body this is a very pronounced arrival in most of the recorded traces in one of the arrays its amplitudes are even greater than the direct p phases five to eight coherent phases could be identified by the mini arrays using only these phases synthetic p coda traces were constructed which only contain the coherent part of the observed wavefield by subtracting the synthetic coherent wavefield from the original traces we achieve a variance reduction in the p coda of up to 37 per cent this leads to the conclusion that a large amount of the p coda at the ktb can be modelled by a simple deterministic single scattering model using a small number of individual scatterers

new insights into the tectono stratigraphic evolution of the southern stappen high and its transition to bjornoya basin sw barents sea abstract within the context of the southwestern barents sea the southern stappen high and its transition to the bjornoya basin are still underexplored improved quality seismic reflection data are utilised to describe new insights into the paleozoic to early cenozoic tectono stratigraphic evolution of the area as well as to discuss the structural inheritance and the rift development well defined syn rift wedges and better resolution images for both the deep carboniferous and permian successions are revealed in particular both the mid carboniferous and late permian earliest triassic extensional phases are characterized by widespread ne sw oriented normal faults that are mostly westward dipping although triassic is mostly considered as a tectonically stable period in the barents sea in the southern stappen high there is clear identification of a localised depocentre named herein intra stappen basin where syn tectonic geometries characterize the upper paleozoic and triassic deposits regional correlation to middle and upper triassic outcrops in southwestern svalbard reveals possible progradation from a west northwest northeast greenland provenance as a western sediment source area during the triassic in addition to the well known eastern sediment source area thin but distinct jurassic sequences are expected to be present on stappen high associated with prominent regional nw se extension throughout late jurassic that culminated during the earliest cretaceous furthermore structural and stratigraphic relations are observed within the study area that clearly indicate a distinct early aptian rift phase with increasing evidence for its occurrence in the southwestern barents sea upper cretaceous sequences bounded by major low angle west dipping detachment faults are observed in southwest stappen high during early cenozoic the study area was located at the proximity of the paleo coastline and paleo shelf edge for both paleocene and eocene gravity mass waste deposits these are most probably related to a progressively evolving steep bathymetric gradient between the developing margin mainly towards the west and to the south and the uplifted stappen high

an integrated model for the tectonic development of the frontal brooks range and colville basin 250 km west of the trans alaska crustal transect we present a kinematic model for the sequence of deformation and sedimentation in the frontal brooks range and adjacent colville basin in the etivluk river region 250 km west of the trans alaska crustal transect tact the model is based on a tectonic subsidence analysis of the foreland basin combined with structural stratigraphic and thermal studies of the northern edge of the brooks range thrust belt we interpret six discrete tectonic events that led to the present day configuration of the thrust belt in this area 1 emplacement of ophiolitic allochthons over the distal continental margin rocks in valanginian time hundreds of kilometers south of this study 2 hauterivian uplift of the barrow arch rift margin affecting the northern part of the colville basin 3 barremian contraction involving emplacement of distal continental margin and ophiolitic allochthons onto the endicott mountains allochthon and creation of a southward dipping flexural basin on the north slope autochthon 4 mid cretaceous exhumation of imbricated rocks in the brooks range during northward propagation of the thrust front into the foreland 5 minor thrusting in late cretaceous paleocene in the northern foreland to the northern limit of contractional structures and 6 regional exhumation of the orogen and the foreland in paleocene eocene time this sequence of deformation agrees well with a simple model of a forward propagating thrust system

imaging magma storage below teide volcano tenerife using scattered seismic wavefields summary tenerife canary islands spain is a volcanic island dominated by the teide pico viejo complex with a summit height of 3718 m after renewed signs of activity starting in 2004 an active seismic experiment was performed in 2007 to derive a tomographic model and identify seismic anomalies possibly associated with the magmatic system to complement the tomography a double beam forming analysis is applied on two orthogonal 2 d profiles crossing the island to look for evidence of the existence of a magma chamber numerical tests allow us to investigate the best measure of coherency between traces and show that the correlation and nth root semblance methods give better results than the classical semblance they also demonstrate that the technique is reliable for locating scattering structures at depth even when the velocity model is imperfect applying this technique to the tenerife data set two main anomalies can be identified one at approximately 7 9 km b s l depth in the northern part of the island and one shallower 1 4 km b s l beneath the main summit these structures could be linked to the magmatic system in good agreement with previous studies the shallowest one may be the phonolitic storage area feeding the teide pico viejo complex while the deepest structure may be related to the basaltic system

seismic evidence of partial melt within a possibly ubiquitous low velocity layer at the base of the mantle three source regions show evidence of a low velocity layer that is less than 15 kilometers thick on top of the core mantle boundary and require about a 3 1 ratio of shear to compressional velocity reduction which is consistent with partial melt layer thickness is correlated with travel time residuals of the seismic phases that are most sensitive to the lowermost mantle velocity these observations suggest that the layer is thinned beneath downwellings but is present everywhere along the core mantle boundary low viscosity accompanying partial melt can localize the upwelling of warmed mantle making the low velocity layer a plausible source of mantle plumes

probing the core mantle boundary beneath europe and western eurasia a detailed study using pcp summary we use pcp the core reflected p phase recordings of deep earthquakes and nuclear explosions from the grafenberg germany and norsar norway arrays to investigate the core mantle boundary region beneath europe and western eurasia we find evidence for a previously unknown ultra low velocity zone 600 km south east of moscow located at the edge of a middle size low shear velocity region imaged in seismic tomography that is located beneath the volga river region the observed amplitude variations of pcp can be modelled by velocity reductions of p and s waves of 5 and 15 respectively with a density increase of 15 travel time delays of pre and postcursors are indicating a thickness of about 13 km for this ultra low velocity region ulvz however our modelling also reveals highly ambiguous amplitude variations of pcp and a reflection off the top of the anomaly for various ulvzs and topography models accordingly large velocity contrasts of up to 10 in vp and 20 in vs cannot be excluded in general the whole volga river region shows a complex pattern of pcp amplitudes caused most likely by cmb undulations further pcp probes beneath paris kiev and northern italy indicate likely normal cmb conditions whereas the samples below finland and the hungary slovakia border yield strongly amplified pcp signals suggesting strong cmb topography effects we evaluate the amplitude behaviour of pcp as a function of distance and several ulvz models using the 1d reflectivity and the 2d gauss beam method the influence of the velocity and density perturbations is analysed as well as the anomaly thickness the dominant period of the source wavelet and interface topographies strong variation of the pcp amplitude are obtained as a function of distance and of the impedance contrast we also consider two types of topographies undulations atop the cmb in the presence of flat ulvzs and vice versa where a broad range of cmb topography dimensions lead to large pcp amplitude variations only large ulvz undulations generate significant amplitude scattering consequently this indicates that topography effects of anomalies may mask the true medium parameters as well as the ulvz thickness moreover there might be a possibility of misinterpreting the precursor as pcp in particular for thin ulvzs

burial and exhumation history controls on shale compaction and thermal maturity along the norwegian north sea basin margin areas abstract the north sea area has been subjected to significant erosion and subsequent deposition of sediments in the basin margin and deeper basin areas respectively during the late neogene a large amount of cretaceous early quaternary sediments have been removed below the angular unconformity along the west and southwest coast of norway and deposited in the huge north sea fan at the mouth of the norwegian channel at the same time a considerable thickness of early quaternary paleocene sediments was also eroded towards the east in the central north sea and subsequently deposited in the deeper basin areas to the west this study seeks to estimate exhumation from compaction and thermal maturity based techniques by using sonic velocities of shales carbonates and vitrinite reflectance data in a large number of boreholes in the central eastern and northern north sea the results indicate no or minor exhumation in the central graben and flanking high areas whereas more than 1 km sediments are eroded in the basin margin areas towards the norwegian coast more than 500 m sediments are eroded in the egersund basin and stord basin areas a similarity of exhumation estimates from the early cretaceous early miocene shales and late cretaceous early paleocene carbonates indicates maximum burial sometime after the early miocene in most of the central and northern north sea areas however the maximum burial throughout the north sea basin may be diachronous seismostratigraphic analysis indicates maximum burial sometime during the oligocene in the sorgenfrei tornquist zone area in the eastern north sea maximum burial in the norwegian danish basin varies from miocene pliocene in eastern parts to early pleistocene in western parts whereas sediments are currently at their maximum burial in the central graben and southern viking graben areas restoration of surface elevations to their original position before the onset of erosion indicated large subaerially exposed areas in the norwegian danish basin and along the southwest coast of norway this is also supported by predominantly coastal and or deltaic environments in the norwegian danish basin area during the late neogene these subaerially exposed areas may be linked to the regional tilting and erosion of the basin margin areas to the east and progressive basinward migration of deposition centres to the west since the oligocene the exhumation had significant effects on the petroleum system in the basin margin areas by cooling down the source rock however the deeper burial of sediments may also have changed the rheological properties of sediments from more ductile to brittle due to compaction and diagenetic processes which makes them more failure prone during exhumation leading to hydrocarbon leakage or seal failure in case of co2 injection

fine scale topography of the d discontinuity and its correlation to volumetric velocity fluctuations we constrained the correlation of the topography of the d discontinuity to volumetric velocity fluctuations we used data obtained from hi net tiltmeters deployed by the national research institute for earth science and disaster prevention japan and conducted stacking and migration to map the discontinuity beneath the southwestern pacific we compared the obtained map with the distribution of scs s travel time residuals an index of volumetric velocity fluctuations in the lowermost mantle we detected the fine scale of the order of a few hundred kilometers topography of the d discontinuity as well as fine scale volumetric velocity fluctuations regions with relatively higher velocities generally have a discontinuity with shallower depths and those with relatively lower velocities generally have a discontinuity with deeper depths the scattering plot between the observed depths of the discontinuity and the observed scs s residuals appears to exhibit a nonlinear trend these results may suggest the existence of both thermal and chemical anomalies such as those seen in thermochemical plumes

seismology of the lower mantle and core mantle boundary the thermal and chemical processes operating near the core mantle boundary cmb play a major role in the planet s evolution influencing the magnetic field behavior chemical cycling in the mantle irregularities in the rotation and gravitation of the planet and the configuration of mantle convection the past decade has witnessed extensive investigation of the core mantle transition zone using seismological geodynamical geomagnetic and mineral physics experimental techniques the many multidisciplinary contributions on this region have been surveyed in several recent review articles e g loper and lay 1994 jeanloz and lay 1993 bloxham and jackson 1991 which provide a comprehensive survey of the recent literature i confine my attention here to recent seismological evidence for laterally heterogeneous structure in the d region the lowermost 200 km of the mantle while seismic models for d are still low resolution we have attained general constraints on the extent of heterogeneity which raise provocative questions as to the origins of the complex structures the idea that chemical and thermal heterogeneity in d is a manifestation of boundary layer processes is a long standing one bullen 1949 but we are far short of quantifying this notion in particular it is not known whether heterogeneities in d are relics of subducted slabs in situ accumulations of core mantle chemical reaction products primordial residuum from the core formation process or thermal heterogeneities within a hot unstable thermal boundary layer

a geophysical study of the northern svalbard continental margin summary in the summer of 1999 the first systematic seismic profiles were acquired across the northern svalbard continental margin east of 15 e approximately 1470 km of multi channel seismic reflection data as well as sonobuoy wide angle data were collected up to 82 n with few exceptions the signals imaged the whole sedimentary cover down to the acoustic basement the uppermost sedimentary deposits of the inner shelf yield p wave velocities of 2 km s 1 and higher indicating erosion and compaction due to a former ice load the inner shelf east of hinlopen strait has only a thin veneer of over consolidated sediments above the acoustic basement beneath the outer shelf up to 3 5 km of sedimentary deposits cover the down faulted acoustic basement the continental slope is heavily eroded due to bottom current activity and slumping at about 30 e the morphology of the continental slope has a smooth appearance shelf progradation only in the vicinity of glacial troughs crossing the shelf associated with submarine fans indicates main sediment transport by ice streams during former glacial periods the maximum sedimentary thickness in the sophia basin is more than 9 km and the nansen basin has a sediment thickness of 4 5 km close to the margin gravity modelling along the seismic profiles was performed to constrain the position of the continent ocean transition existing sedimentary thickness and structural maps were extended over the area investigated the new data provide no evidence for the presence of former extensive subaerial volcanic sequences seaward dipping reflectors which would have been emplaced during the break up along the margin thus we consider this part of the margin as non volcanic

investigating the base of the mantle using differential travel times abstract several techniques using differential seismic travel times to map lateral structure in the lowermost mantle are discussed results are shown for recent studies involving the established techniques of core reflected phases scs s and pcp p and diffracted phase profiles sdiff and new techniques involving the differential times of both core transmitted and core diffracted phases pkp pdiff and sdiff sks skks are described the recent databases of digital seismograms have allowed for a study of d velocities in the eastern hemisphere using scs s and sscs ss differential times from the many western pacific earthquakes the result is an image at a resolution of a few hundred kilometers of a slow velocity anomaly of 2500 km width beneath micronesia 2 relative to the preliminary reference earth model prem that is surrounded on three sides by fast d rock that is 3 faster than prem a study using the differential arrivals of core diffracted s waves sdiff from digital records is providing information about long wavelength variations in d shear velocities though the rigorous earthquake station geometry requirements limit the study to particular regions of the globe another study is using over 40 000 pcp p differential travel times as reported to the international seismological centre to map global p velocities at the base of the mantle and it shows that global coverage of the core mantle boundary cmb is very poor though there are some regions northern asia northern pacific central america with enough data sampling to allow a quantification of average d p velocities with a total robust range of 4 lateral variation they cover only a small portion of the total cmb as a means of increasing our understanding of the long wavelength variations of seismic velocities a description is given of two techniques that will take advantage of totally different sets of earthquake station geometries from the core reflected phase studies in the distance range from 120 to beyond 165 the differential times of pkp and pdiff can be used to map long wavelength d p velocities these two phases are very different in shape and frequency content so the differential times are found by a waveform cross correlation with reflectivity synthetic counterparts in the distance range from 105 to beyond 135 the differential times of sdiff sks and skks can be examined simultaneously to test models of velocity structure above and below the cmb also through comparison with synthetic counterparts

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

lateral variations in d thickness from long period shear wave data we explore global variations in d shear wave structure by examining 12 years of long period global digital seismograph network gdsn data to identify reflected phases from the d layer in the lowermost mantle we restrict our search to epicentral distances between 63 and 74 where a precritical d reflection will lie between the s and scs arrivals with gdsn long period records a d signal at these ranges is generally obscured by the stronger neighboring phases so to isolate the d reflections we develop a technique for stripping away the interfering s and scs waveforms we interpret travel time variations in the observed sds phases in terms of variations in d thickness and generate maps of inferred d thickness for areas of good seismic coverage the regions of observed d reflections are beneath australasia north central asia the arctic alaska and central america we find considerable variation in d structure in these regions a result consistent with previous studies which have found evidence for lateral heterogeneity in d on average the estimated d thickness is 260 km but there is a fairly uniform distribution of thicknesses between 150 km and 350 km a possible correlation is observed between d thickness and regions of predicted strong horizontal mantle flow

synthetic seismic illumination of small scale growth faults paralic deposits and low angle clinoforms a case study of the triassic successions on edgeoya nw barents shelf abstract fault activity and sandstone body geometries and spread all bear significant weight to understanding the potential hydrocarbon systems on the nw barents shelf synthetic seismic modelling of onshore sedimentary successions provides insight into the seismic resolution and expression of various sedimentological features on edgeoya i sandy growth basins in pro delta mudstones ii paralic sedimentary deposits and iii low angle tidally influenced progradational successions synthetic modelling suggests that the lithological contrast associated with sand infilled growth basins in pro deltaic shales even with offsets as small as 50 75 m will create distinct geometries in seismic data modelling suggests that while optimal 90 illumination low frequency bands and more typical sedimentary velocities around 2000 m s will generate very clear discernible growth faults the angular lithological contrast should generate discernible features even with the high velocities and typical 20 30 hz frequency band of seismic sections on the nw barents shelf comparing to actual seismic data in which multiples noise and in places overburden are influential it is possible to identify growth fault geometries more confidently and to link them to larger planar fault activity modelling other features identified in the paralic sedimentary system it is apparent that many of the massive channel or lenticular shaped sandstone bodies should be identifiable in the actual seismic although their expression is less distinct and more easily misinterpreted than that of growth faults it is apparent that features such as igneous intrusions unless imaged in a perfect survey can be difficult to properly identify particularly near vertical connections the velocity contrast creates strong impedance along horizontal sections but heavily fractured igneous intrusions with lower velocities could easily be assumed to be isolated sandstone bodies while the modelling appreciates the overall wedging nature of the successions the simplified lithological observations onshore cannot predict the probable erosion condensation contrast associated with low angle clinoforms which are visible in the seismic data and hence not reproduce them easily in the models

neural network based model for assessing failure potential of highway slopes in the alishan taiwan area pre and post earthquake investigation abstract this paper is aimed at creating an empirical model for assessing failure potential of highway slopes with a special attention to the failure characteristics of the highway slopes in the alishan taiwan area prior to and post the 1999 chi chi taiwan earthquake the basis of the study is a large database of 955 slope records from four highways in the alishan area artificial neural network ann is utilized to learn from this database the developed ann model is then used to study the effect of the chi chi earthquake on the slope failure characteristics in the alishan area significant changes in the degrees of influence of several factors variables are found and possible reasons for such changes are discussed the novelty of this paper lies in the fact that the developed ann models are used as a tool to investigate the slope failure characteristics before and after the chi chi earthquake

post caledonian tectonics along the billefjorden fault zone svalbard and implications for the arctic region the north south billefjorden fault zone of northern spitsbergen in the svalbard archipelago has been widely regarded as one of the classic areas for transpressional tectonics the fault zone coincides with a late caledonian shear zone and presently constitutes the eastern boundary of the main devonian basin of northern svalbard fold axial traces and thrust faults that developed in the devonian to permo carboniferous rocks parallel the strike of the fault zone there is no evidence for the rotation of fold hinges and nonaxial planar transecting cleavages are lacking folds and thrusts with similar geometries affect the whole 60 km wide devonian basin fill these observations together with analyses of fault and fold patterns indicate a pre devonian caledonian left lateral ductile shearing b late silurian early devonian extension c late devonian early carboniferous inversion accompanied by folding of devonian sedimentary rocks d early carboniferous extension followed by platform wide subsidence into late mesozoic time e late cretaceous paleocene east west contraction thrusting and folding and e post paleocene east west extension the late devonian deformation along the billefjorden fault zone is compared with the late devonian to early carboniferous ellesmerian orogeny of the canadian arctic the late devonian megashear model for the tectonic evolution of the north atlantic region is not supported by the evidence from svalbard

comments about the age of the canada basin abstract the canada basin appears to have formed over a 60 ma interval during the cretaceous period the opening process can be divided into two parts continental breakup widespread in hauterivian through aptian time about 113 131 ma ago along what was about to become the polar margin of north america and seafloor formation active during the extended cretaceous interval of normal geomagnetic polarity to marine magnetic anomaly 33 time about 118 79 ma ago presently available age controls indicate that all parts of the north american polar margin could have formed at about the same time two styles of margin formation are evident from brock to axel heiberg island there appears to be an extended 70 ma stratigraphic hiatus and northeastward from ellef ringnes island extensive tholeiitic igneous activity associated with continental breakup from banks island to northeastern alaska the breakup interval is abbreviated 20 30 ma and few igneous rocks are observed a progressive difference may be indicated in the amount and or rate of crustal stretching during margin formation with relatively less slower stretching having occurred along the segment southwest of brock island and relatively more faster stretching having taken place along the segment to the northeast

upper mantle structure beneath central eurasia using a source array of nuclear explosions and waveforms at regional distances we have observed very consistent broadband regional three component p waveforms from a set of 11 explosions that occurred at the former soviet test site in kazakhstan during 1988 and 1989 and have modeled most of the prominent features in these waveforms to determine upper mantle structure beneath central eurasia using a subset of events as a source array we have identified four consistent arrivals during the first 15 s of p waves recorded at the soviet incorporated research institutions for seismology iris stations arti aru d 1500 km and garm gar d 1380 km these arrivals show consistent variations in frequency content relative timing and amplitudes from event to event despite a range in source magnitude mb between 4 9 and 6 1 we have also identified consistent features in p waveforms of these events recorded at obninsk obn d 2880 km we argue that most of the prominent features in these waveforms can be explained by reflections at or refractions near discontinuities or large velocity gradients in the upper mantle we show that a model with discontinuities of approximately 3 0 and 6 5 near 200 km and 400 km respectively produces a better fit to the broadband data at aru and gar than previous models for this region this model also produces a good match to waveforms recorded at obn and noress however this model is not unique for example we show that it is also possible to produce similar waveforms by replacing the 200 km discontinuity with a large notch shaped low velocity zone between approximately 100 and 140 km based on reflectivity synthetics we suggest that additional data especially from distances less than 1000 km should indicate which of these models is more accurate although our models are not unique our observations do constrain the following features 1 there must be reflectors between about 140 and 200 km and near 400 km depth in the upper mantle to explain the large secondary arrivals at both aru and gar 2 based on the relative size of some of the secondary arrivals at aru and gar the discontinuity near 400 km must be large however it cannot be much greater than about 6 5 because there is no evidence in our data of a reflection from 400 km at obn 3 the relatively small amplitude of the first arrivals at both aru and gar suggests that either the gradient in the uppermost mantle is very small or that a shallow low velocity zone is present comparison of our results with other upper mantle studies shows that the central eurasian upper mantle is similar to the upper mantle of the central and eastern united states

the effect of long term low temperature exposure on apatite fission track stability a natural annealing experiment in the deep ocean abstract since studies on deep sea cores were carried out in the early 1990s it has been known that ambient temperature may have a marked affect on apatite fission track annealing due to sluggish annealing kinetics this effect cannot be quantified by laboratory annealing experiments the unknown amount of low temperature annealing remains one of the main uncertainties for extracting thermal histories from fission track data particularly for samples which experienced slow cooling in shallow crustal levels to further elucidate these uncertainties we studied volcanogenic sediments from five deep sea drill cores that were exposed to maximum temperatures between 10 and 70 c over geological time scales of 15 120 ma mean track lengths mtl and etch pit diameters dpar of all samples were measured and the chemical composition of each grain analysed for age and track length measurements was determined by electron microprobe analysis thermal histories of the sampled sites were independently reconstructed based on vitrinite reflectance measurements and or 1d numerical modelling these reconstructions were used to test the most widely used annealing models for their ability to predict low temperature annealing our results show that long term exposure to temperatures below the temperature range of the nominal apatite fission track partial annealing zone results in track shortening ranging between 4 and 11 both chlorine content and dpar values explain the downhole annealing patterns equally well low chlorine apatite from one drill core revealed a systematic relation between si content and dpar value the question whether si substitution in apatite has direct and systematic effects on annealing properties however cannot be addressed by our data for samples which remained at temperatures 30 c the predictions of the laslett et al 1987 model agree with the measured mtl data within error levels with few exceptions predictions by the ketcham et al ketcham r donelick r and carlson w 1999 variability of apatite fission track annealing kinetics iii extrapolation to geological time scales am mineral 84 9 1235 1255 annealing model are consistent with the measured data for samples which remained at temperatures below 30 c for samples which experienced maximum temperatures between 30 and 70 c and which are rich in chlorine the ketcham et al 1999 model overestimates track annealing

slowness power spectrum analysis of the coda composition of two microearthquake clusters in northern switzerland abstract the s wave coda composition of two microearthquake clusters in northern switzerland has been analysed using the slowness power spectrum analysis proposed by spudich and bostwick the purpose of this paper is to estimate the directions in which individual coda wavelets leave the source region for lapse times more than six times the s wave travel time the data set consists of 12 and 16 events in the magnitude range of 1 9 2 6 and 1 4 3 4 respectively the depths range from 7 to 8 km and from 6 to 7 km digital data from five vertical component stations enb rbf gef gif and tsb and two three component stations acb and che were available the hypocentral distances to the recording sites vary between 29 and 63 km for the first cluster gunsberg and 10 and 24 km for the second cluster laufelfingen respectively all events have been relocated using a master event technique with inter event arrival times estimated by a cross correlation method the slowness power spectra for the laufelfingen cluster at all stations show essentially two different patterns in agreement with the results of spudich and bostwick we found that the early coda following the direct s wave immediately was composed of wavelets leaving the source region with the same slowness vector as the direct wave we interpret this coda part as being very probably produced close to the recording site however with increasing lapse times we observe a different coda type which receives its energy from wavelets leaving the source region in a variety of directions the transition between the two coda types takes place often at 1 5 2 times and always before three times the s wave arrival time

thermochronological constraints to late cenozoic exhumation of the barents sea shelf abstract the cenozoic evolution of the barents sea has been widely debated for its implications on hydrocarbon exploration in this paper we provide the first for the area apatite u th he thermochronology data on early middle jurassic and early cretaceous sandstone reservoir samples obtained from three wells located on the western part of the barents shelf a large range of grain ages have been detected ranging from 1 58 to 50 65 ma these thermochronological data initially integrated with vitrinite reflectance measurements to properly constrain the burial history evolution have been modelled on the basis of estimated maximum temperature in order to evaluate the cooling path to present day temperatures outputs from modelling indicate that 1 the amount of net uplift and denudation is in the order of 1000 m and 2 the last important phase of exhumation occurred during late miocene early pliocene time therefore challenging the prevailing idea of substantial uplift linked to the observed shelf progradation along the margin as a result of the plio pleistocene glaciations the timing of the distinct exhumation event documented here may be instead attributed to different mechanisms these may include basin inversion widespread on the nw european margin that is probably related to local changes in the north atlantic spreading vector while other mechanisms may include thermal and lithospheric scale anomalies observed at the northwestern corner of the barents svalbard shelf

crustal structure in eastern kazakh u s s r from teleseismic receiver functions broadband receiver functions determined from teleseismic p waveforms recorded at two seismic stations in eastern kazakh u s s r are inverted for the vertical velocity structure beneath the stations the detailed broadband receiver functions are obtained by stacking source equalized radial components of teleseismic p waveforms a time domain inversion of the radial receiver function is used to determine the structure assuming a crustal model parameterized by flat lying homogeneous layers the general features of the inversion results are a complex shallow crust velocities less than 6 km s in the upper crust a high velocity 6 9 7 5 km s lower crust and a moho that varies between 47 and 57 km depth these results compare favorably with a composite velocity model from deep seismic sounding data but show lower velocities in the upper crust and higher velocities in the lower crust the results indicate that the crust in this region is relatively uniform however the nature and depth of the moho changes significantly across the region the receiver function inversion structure for the kazakh sites is similar to a published receiver function structure for a site on the canadian shield

catalog of locations of nuclear explosions at balapan kazakhstan 1965 to 1985 abstract geographic locations for 81 explosions at the balapan test site in kazakhstan are derived with a precision of about 200 m from time sequence landsat mss and spot satellite images combined with teleseismic epicenter estimates the landsat images were registered to ground control points from a spot image to provide high relative accuracy for feature comparison initial seismic location estimates for the explosions were derived using joint epicenter determination for most events there is little or no ambiguity in the association between seismic events and image features owing to the frequency of the landsat images and the spatial separation between explosions

localized double array stacking analysis of pcp d and ulvz structure beneath the cocos plate mexico central pacific and north pacific abstract a large high quality p wave data set comprising short period and broadband signals sampling four separate regions in the lowermost mantle beneath the cocos plate mexico the central pacific and the north pacific is analyzed using regional one dimensional double array stacking and modelling with reflectivity synthetics a data screening criterion retains only events with stable pcp energy in the final data stacks used for modelling and interpretation this significantly improves the signal stacks relative to including unscreened observations allows confident alignment on the pcp arrival and allows tight bounds to be placed on p wave velocity structure above the core mantle boundary cmb the pcp reflections under the cocos plate are well modelled without any ultra low velocity zone from 5 to 20 n at latitudes from 15 to 20 n we find evidence for two p wave velocity discontinuities in the d region the first is 182 km above the cmb with a ln vp of 1 5 near the same depth as a weaker discontinuity

studies of the seismic coda using an earthquake cluster as a deeply buried seismograph array loosely speaking the principle of green s function reciprocity means that the source and receiver positions in a seismic experiment can be exchanged without affecting the observed seismograms consequently the seismograms observed at a single observation location o and caused by a cluster of microearthquakes at locations ei are identical to the time series that would be measured by an array of stress meters emplaced at positions ei recording waves generated by a source acting at o by applying array analysis techniques like slant stacking and frequency wave number analysis to these seismograms we can determine the directions and velocities of the component waves as they travel in the earthquake focal region rather than at the surface we have developed a computationally rapid plane wave decomposition which we have applied to single station recordings of aftershocks of the 1984 morgan hill california earthquake the analysis is applied to data from three seismic stations having considerably different site geologies one is a relatively hard rock station situated on franciscan metamorphics one is within the calaveras fault zone and one is on semiconsolidated sand and gravels we define the early coda to be the part of the coda initiating immediately after the direct s wave and ending at twice the s wave lapse time the character of the s wave and early coda varies from being impulsive at the first station to highly reverberative at the last we examine waves in sequential time windows starting at the s wave and continuing through the early part of the coda at all seismic stations the early coda is dominated by a persistent signal that must be caused by multiple scattering probably within 2 km of each seismic station despite clear station to station differences in the character of the early coda coda q values measured in the late coda greater than twice the s lapse time agree well among stations implying that the mechanisms causing the varying behavior of the early coda do not control the coda decay rate at the stations we have considered coda q values measured on horizontal components of motion agree within a factor of 2 with those measured on vertical components we have not been able to determine the composition of the late coda because of a low signal to noise ratio our analysis technique however is quite appropriate for such a task

detection of p wave reflector in d beneath the south western pacific using double array stacking the double array event and receiver array stacking is applied to the p wave data from 6 fiji is events recorded by a local network of short period seismographs in japan to investigate vp structure in the lowermost mantle a signal which may be interpreted as the reflection from a discontinuity at 170 km above the cmb core mantle boundary is observed between p and pcp in the stacked record as compared with the receiver array stacking with a small number of stations applied to a single event the double array stacking is effective to detect the weak signal of pdp

the theropod that wasn t an ornithopod tracksite from the helvetiafjellet formation lower cretaceous of boltodden svalbard

new evidence on the structure of the labrador sea greenland continental margin a 275 km section of a regional seismic reflection profile from the greenland side of the labrador sea has been reprocessed and interpreted three zones of different structural style have been identified the innermost of which is block faulted magnetic modelling based on the seismic line shows that sea floor spreading anomalies are confined to the outermost zone and the oldest identified anomaly is 27n all attempts to model the area landward of anomaly 27n as a series of remanent magnetizations of alternating polarity failed however a model in which the innermost zone is interpreted as extended and block faulted continental crust and the intermediate zone as continental crust intruded and overlain by reversely magnetized igneous material fits the magnetic and seismic data it is concluded that at least in the northern labrador sea sea floor spreading started in palaeocene times chron 27 and large areas formerly thought to be underlain by oceanic crust should now be considered to be continental one plate kinematic consequence may be a reduced need to postulate large strike slip movements along nares strait

mesozoic extension versus tertiary compression along the billefjorden fault zone south of isfjorden central spitsbergen the billefjorden fault zone displays a complex tectonic history going back into precambrian times observed thickness variations within mesozoic sequences across the fault zone have previously been related to mesozoic extensional movements along the fault zone recent structural stratigraphical and sedimentological investigations suggest that the thicknessvariation observed across the fault zone is due to tertiary compressional tectonics and that mesozoic extensional movements are minor or nonexistent tertiary deformation is characterized by a combined thin skinned thick skinned structural style thin skinned thrusting is due to the development of decollement zones in the jurassic cretaceous janusfjellet subgroup and in the triassic sassendalen group thick skinned faulting is related to reverse faulting along a steep east dipping fault rooted in the pre mesozoic basement interacting thin skinned thrusting and thick skinned faulting has resulted in out of sequence thrusting and is responsible for the thinning of mesozoic sequences across the fault zone

late mesozoic and cenozoic tectono thermal history and geodynamic implications of the great xing an range ne china abstract the mesozoic cenozoic tectonics of northeastern china involves complex multistage interaction of different tectonic domains and has long been controversial the tectono thermal evolutionary history recorded by the widespread outcropped granitoids in the great xing an range gxr may provide significant constraints on the tectonics illuminating the deep geodynamic process here we present new low temperature thermochronology data from the gxr and thus reconstruct the detailed thermal history zircon fission track dating yielded a group of central ages ranging from 80 4 to 185 10 ma while apatite fission track ages range from 55 3 to 75 5 ma the modeled time temperature paths of all the samples reveal two relatively rapid cooling events at 100 60 ma and 50 0 ma and one intervening reheating episode at 60 50 ma the initial uplift exhumation and induced cooling events which represent strong orogenic activity are related mainly to closure of the mongol okhotsk ocean in the north and were subsequently influenced by the paleo pacific plate subduction in the east the evolution of the basin mountain system consisting of the songliao basin and gxr as well as the accretion metamorphism and magmatism at the northeast asian continental margin are all dominated or indirectly influenced by the paleo pacific subduction this indicates that the paleo pacific subduction had affected the evolution of the gxr to the west since the jurassic the significant reheating episode during the late paleocene to eocene may be related to the heat flux derived from asthenosphere upwelling and igneous activity

seismic velocity structure and event relocation in kazakhstan from secondary p phases abstract three component seismic data from a set of presumed explosions recorded by stations at bayanaul and karkaralinsk in kazakhstan were analyzed in order to model the crustal structure of the region and to examine the use of the arrival times of secondary p phases primarily pmp in regional event location polarization analysis aided in the identification of the secondary phases low pass filtered data 4 hz corner from the first 5 to 10 sec of 13 presumed explosions were modeled with the reflectivity method the two chemical explosions in 1987 provided a check on accuracy as their locations and origin times are accurately known a good fit to the arrival times and amplitudes in the first 5 sec of the p wave pn pg and pmp was obtained in the epicentral distance range of 100 to 300 km beyond 300 km the simple layered model was not adequate to model the pmp arrival the crustal p wave velocity model we derived has an upper crustal velocity increasing fairly rapidly from 4 5 km sec near the surface to 6 5 km sec at 15 km depth then increasing more slowly to 7 05 km sec at 50 km depth the observed difference in the arrival times of the phases pg pmp and pn in the range between 100 and 250 km distance required a relatively sharp transition at the crust mantle boundary the model is generally similar to previous estimates of p velocity structure in the region though with a gentler gradient in the upper crust and a steeper gradient in the lower crust we used the derived crustal model and the primary and secondary p wave arrival times to relocate events in the kazakhstan region inclusion of the phase pmp substantially decreases the focal depth uncertainty for many of the events all but one of the events analyzed are concluded to be surface explosions the identity of the remaining event is uncertain

imaging of crustal heterogeneous structures using a slowness weighted back projection with effects of scattering modes 2 application to the nagamachi rifu fault japan area 1 this is the second paper in a two part series on a newly developed imaging approach for small scale heterogeneities 1 km in the crust with effects of scattering modes in the present paper we estimated a detail three dimensional spatial distribution of small scale heterogeneities around the nagamachi rifu fault northeastern japan in a frequency range of 2 16 hz using the imaging approach presented in the first paper we used seismograms recorded by dense three component seismic arrays that were deployed in this region for 15 explosion sources as one of our important results there are concentrations for only p s but not p p scatterers near the surface trace of the fault p s scatterers in a frequency range of 8 16 hz are localized near the surface trace of the fault implying the possibility of strong heterogeneities with a size of 0 08 km there in the shirasawa caldera region the characteristics of seismic scatterers seem to convert from large p s to large p p relative scattering coefficients with its transition depth range of 5 8 km this feature implies that the materials composed of seismic scatterers may show a systematic variation with depth finally the strength of scattering coefficients is rather weak in the coseismic area of the 15 september 1998 earthquake with a magnitude of 5 2 the largest recent event in this area this result suggests that this coseismic area is rather homogeneous and can hold local stress larger than in the surrounding portions of the fault system

palaeomagnetic rock magnetic and mineralogical investigations of the lower triassic vardebukta formation from the southern part of the west spitsbergen fold and thrust beltk dudzisz and otherspalaeomagnetism of triassic rocks spitsbergen

seismological mapping of fine structure near the base of the earth s mantle the earth s core mantle boundary cmb juxtaposes liquid iron and crystalline silicates and is a region of large vertical thermal gradients the d region which extends up to 200 300 km above the cmb often has elevated shear wave velocity and suggestions of lateral variations in structure1 recent improvements in our ability to assemble and analyse records from regional seismic networks have allowed us to examine long profiles of travel times amplitudes and waveforms from more than a thousand short period seismometers2 we observe across canada and the united states p waves that have grazed the cmb from the powerful nuclear test in lop nor china on 21 may 1992 first arrival travel times and large secondary arrivals are consistent with a 1 5 compressional velocity increase with depth 130 km above the cmb about half the thickness of d in this locality3 our observations together with evidence for the absence of such a thin fast layer in neighbouring regions suggest the presence of lateral heterogeneity in composition or phase at the base of the mantle

p and s wave reflections from anomalies in the lowermost mantle summary seismic p and s waves recorded at the grf array in germany are used to study the inhomogeneous structure of the boundary layer d at the base of the mantle the use of the seismic array allows the detection of small scale anomalies in the lowermost mantle the lateral resolution attainable is about 100 to 400 km i e more than 10 times better than with tomographic methods the analysis of 13 years of grf broad band array data yields 255 events with high signal to noise ratio which are used to map the lowermost mantle 74 of these events show anomalous p waves pdp which arrive 3 6 s after the direct p wave and have a slowness 0 7 0 8 s smaller than the slowness of direct p other events close to the events with pdp do not have such an anomalous phase using slowness traveltime amplitude and waveform information it is demonstrated that pdp is caused by an anomalous lower mantle velocity structure below the turning point of the p wave if the pp phase i e a p wave first reflected at the free surface near the source is used together with the p phase distinct and well sepaarted p velocity anomalies can be determined under the nansen basin the kara sea and northern siberia the areas of the bounce points of pdp in the lower mantle have a lateral extension of about 100 by 200 km but this is not the size of the anomaly since the resolution of p waves at 1 hz in this depth is 130 km by 260 km fresnel zone the accuracy to which the depth of the reflector 2612 km under the nansen basin and 2605 km under northern siberia respectively can be determined for 1 d models is 10 to 20 km the velocity contrast at the lower mantle discontinuity is about 3 per cent 1 to 1 5 per cent areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas 2 d models of the anomalies reveal the range of adequate models and possible trade offs if the lateral extension of the anomaly is about 7 the reflector has to be 40 km deeper than in the 1 d model if the dip of the reflector in the lowermost mantle is only about 1 5 it is difficult to resolve if the reflector is tilted towards the source or towards the receiver for the anomaly under the nansen basin deviations from the great circle path are observed for pdp indicating 3 d effects below the three anomalies the core mantle boundary cmb can be located by pcp with a depth in agreement with standard earth models under northern siberia has a region where p and s velocity anomalies coincide but also a region where only an s velocity anomaly but no p velocity anomaly is observed this results in changes of the poisson ratio from 5 9 per cent to 4 8 per cent 1 5 per cent across the discontinuity in the lowermost mantle for the regions studied the analysis of the s waves s sds and scs reveals two s velocity anomalies in the lowermost mantle under northern siberia the first anomaly coincides with the p velocity anomaly under northern siberia and can be explained by a 1 d model with a reflector depth of 2610 km 15 km and a velocity contrast of 2 3 per cent the second s velocity anomaly is in an area where no p velocity anomaly can be detected the corresponding 1 d s velocity model has a reflector depth of 2575 km 15 km and an s velocity contrast of 2 6 per cent the smallest structures that can be resolved with the s waves in this depth are about twice as large as for the p waves i e 230 km by 460 km fresnel zone the joint analysis of p and s waves therefore shows a region with a p velocity anomaly together with weak indications for an s velocity anomaly nansen basin and a second region with a p but no s anomaly kara sea the lowermost mantle

tracking of velocity variations at depth in the presence of surface velocity fluctuations we tested a small scale experiment that is dedicated to the study of the wave separation algorithm and to the velocity variations monitoring problem itself it handles the case in which velocity variations at depth are hidden by near surface velocity fluctuations using an acquisition system that combines an array of sources and an array of receivers coupled with controlled velocity variations we tested the ability of beam forming techniques to track velocity variations separately for body waves and surface waves after wave separation through double beam forming the arrival time variations of the different waves were measured through the phase difference between the extracted wavelets finally a method was tested to estimate near surface velocity variations using surface waves or shallow reflection and compute a correction to isolate target velocity variations at depth

evidence for normal and inhomogeneous lowermost mantle and core mantle boundary structure under the arctic and northern canada summary seismic p waves from a total of about 200 nuclear explosions in the usa the former ussr and china observed at 10 arrays and four networks in europe canada and the usa are used to analyse the structure at the base of the mantle and the core mantle boundary cmb the simple waveforms and well controlled source parameters of nuclear explosions allow one to use the events as source arrays in addition to the usual receiver array configuration a new array technique double beamforming kruger et al 1993 integrating both concepts is applied which increases the slowness resolution considerably a total of 56 source receiver combinations i e reflection points in the lower mantle or on the cmb could be analysed in five regions anomalous arrivals pdp with slowness and arrival times between those of p and pcp are observed one of these five areas svalbard region shows short period pcp p amplitude ratios which are about three times higher than those predicted by standard earth models in the severnaya zemlya region where pdp and pcp precursors were observed previously kruger et al 1993 pcp shows azimuth deviations of up to 10 for some other regions deviations of the pcp waveform from the direct p waveform are also observed these anomalous phases and the pcp waveform distortions cannot be explained with standard 1 d earth models they are probably produced by inhomogeneities in the lowermost mantle the observed variations in the waveforms are strong indications of a laterally heterogeneous structure in two depth ranges the first is the cmb and its immediate vicinity of a few tens of kilometres the second region is the depth range between about 200 and 300 km above the cmb maps of the north pole region giving the distributions of inhomogeneities in the lower mantle and on the cmb are presented these maps show evidence of strong heterogeneity of the d boundary layer and possibly also of the cmb in the same area

late mesozoic cenozoic exhumation history of northern svalbard and its regional significance constraints from apatite fission track analysis the late mesozoic cenozoic was a time of profound tectonic activity in the arctic with incipient spreading in the arctic ocean baffin bay labrador sea and north atlantic as well as the northward movement of the greenland microplate leading to collision and deformation in greenland arctic canada and svalbard eurekan orogeny it is however still unclear how northern svalbard situated at the northwestern edge of the barents shelf was affected by these processes furthermore northern svalbard has been proposed to have been a cretaceous cenozoic sediment source to surrounding regions because it lacks a post devonian sedimentary cover when erosion took place and how that related to the tectonic history of the arctic is yet unresolved in order to reconstruct the erosion history of northern svalbard we constrained its thermal evolution using apatite fission track aft thermochronology our data reveal aft ages between 62 5 and 214 10 ma recording late mesozoic early paleogene exhumation our data show that northern svalbard was emergent and experienced erosion from the early jurassic and presumably through the cenozoic although total exhumation was restricted to 6 km pronounced exhumation took place during jurassic cretaceous time probably linked to the extensional tectonics during the opening of the amerasian basin arctic ocean in contrast cenozoic ocean basin formation and the eurekan deformation did not cause significant erosion of northern svalbard nonetheless aft data show that late cretaceous early paleocene fault related exhumation affected some parts of northern svalbard fault zones were reactivated due to the reorganization of arctic landmasses during an early phase of the eurekan deformation which implies that this episode commenced 20 m y earlier in svalbard than previously understood

reactivation history of the long lived billefjorden fault zone in north central spitsbergen svalbard new geological mapping has revealed further details of the tectonic and stratigraphic effects of devonian and later reactivations of the billefjorden fault zone one of a number of important north south trending lineaments in svalbard analysis of offsets along the many steeply dipping faults within the zone and effects on the subsidence and deformation of the adjacent crustal blocks is presented as a series of tectonic maps from the late devonian through to the tertiary late devonian sinistral transpression suggested previously cannot be ruled out and carboniferous reactivation was dominated by extension with possibly a slight dextral strike slip component after late carboniferous to early cretaceous platform subsidence during which the fault zone had little effect on sedimentation development of the tertiary west spitsbergen fold belt related to the opening of the arctic ocean involved compressive and transpressive reactivation of basement seated structures further east including the billefjorden fault zone in the billefjorden austfjorden area this produced a large monoclinal fold across the fault zone which was later cross cut by extensional structures to produce the present day billefjorden syncline this localized late extension is related to a slight variation in the trend of the billefjorden fault zone through this area

a giant circumferential dyke swarm associated with the high arctic large igneous province halip abstract in this study we identify a giant circumferential mafic dyke swarm associated with the 135 75 ma high arctic large igneous province halip previously a halip giant radiating mafic dyke swarm with portions scattered across the canadian high arctic islands northern greenland svalbard and franz josef land was recognized in a pre drift plate tectonic reconstruction of the arctic region the radiating swarm has been interpreted to focus above a mantle plume responsible for halip magmatism the newly recognized halip giant circumferential swarm has a centre that is near the focus of the halip radiating system and hence is likely related to the halip plume elements of the circumferential swarm are located in each of the four regions where the radiating system is found the circumferential swarm has a quasi circular or slightly elliptical geometry an outer diameter of 1600 km and an arc of 220 it is one of the largest giant circumferential dyke swarms recognized on earth and could be linked to the outer edge of the flattening plume head it is also the first such swarm to have been identified by means of a plate tectonic reconstruction although giant circumferential dyke swarms appear to be relatively rare on earth possible analogues are common on venus and are also found on mars on venus giant circular or elliptical tectono magmatic features termed coronae are characterized by an annulus of graben or fissures and prominent topography some coronae include a radiating graben fissure system both radiating and circumferential graben may be underlain by dykes if so coronae could be analogues for terrestrial giant circumferential dyke swarms such as observed in the case of the halip

further evidence for the lay discontinuity beneath northern siberia and the north atlantic from short period p waves recorded in france abstract we analyze data from the french laboratoire de detection geophysique ldg network of digital short period seismic stations the p waves from several earthquakes in the kuril okhotsk region display clear secondary arrivals in the 75 85 range which are not explained by smooth lower mantle models the move out of the arrivals across the network indicates a deep mantle origin we interpret them as waves reflected off or transmitted at the lay discontinuity about 300 km above the core mantle boundary the bounce points at this discontinuity are located beneath northern siberia slightly to the northwest of the region for which a similar structure has been reported we propose tools for investigating the lateral variations of the d structure on the scale 100 1000 km the method we propose is based on the deconvolution of the data or convolution of synthetics by a source time function derived from the data we show that the method helps identification of the waves that interact with the lay discontinuity and enables a more objective mapping of its lateral variations evidence for bumps a few kilometers high on the lateral scale of a few tens of kilometers is proposed we also probe a spot of the lower mantle beneath the north atlantic a detailed analysis of the waveforms from a chiapas earthquake suggests that the lay discontinuity is also present in that part of the world

the influence of paleo bathymetry on total organic carbon distribution tested in the cretaceous hammerfest basin barents sea abstract in basin modelling and petroleum system analysis geometries during deposition of sediments paleo water depth and distribution of organic matter are initial parameters used for further interpretation or modelling this paper presents an approach combining backstripping with a probabilistic forward sedimentary model to calibrate paleo water depth pwd the stochastic pwd results serve as an input for organic facies models and the study demonstrates how pwd will influence models for total organic carbon distribution in a sedimentary basin for the early cretaceous hammerfest basin mainly shelfal to upper bathyal bathymetries average pwd values vary between 118 and 318 m result in average total organic carbon toc varying between 0 47 and 5 24 wt across the basin for the different models basin averaged toc values are similar but vertical and lateral distribution pattern change significantly especially towards the shallow end member pwd the results indicate that pwd uncertainties propagate non linearly into source rock distributions

lateral variations in lowermost mantle shear wave anisotropy beneath the north pacific and alaska s waves recorded by long period world wide standardized seismograph network and broadband stations in north america for deep northwest pacific subduction zone earthquakes provide evidence for anisotropy in the lowermost mantle shear velocity structure beneath the north pacific and alaska systematic delays of up to 4 s are observed between longitudinal components sv and transverse components sh of motion for core reflected scs waves as well as for core grazing and diffracted s waves the absence of significant splitting for s waves that have turning points more than a few hundred kilometers above the core mantle boundary indicates that anisotropy is localized within the d region the lowermost portion of the mantle sv sh differential arrival times for both scs and sdiff along with path length estimates assuming a 250 km thick d region indicate spatial variations in the strength of shear wave anisotropy the strongest anisotropy 1 1 5 is found in the eastern part of the study area with systematic reduction in magnitude toward the west a transverse isotropy model can explain the data with the velocity structure for horizontally polarized waves vsh having a 2 3 discontinuous shear velocity increase at the top of d as proposed in earlier studies of the region and a similar structure for s wave particle motion in the direction normal to the core mantle boundary vsv but with the velocity jump at the top of d and the velocity within d being reduced from that for vsh by 0 5 1 5 km s large uncertainties exist for velocity gradients above and below the velocity jump but the requirement of a reduced vsv relative to vsh in d is clear synthetic waveforms calculated by using separate isotropic structures for sh and sv match the observations well and constrain the basic anisotropic structure because the shear waves all traverse the region with near grazing geometries the study area exhibits strong lateral variations in lower mantle shear velocity structure and variable thickness of the d layer topography of the d layer is not well resolved because of trade off with volumetric heterogeneity

seismic heterogeneous structure in the lowermost mantle beneath the southwestern pacific 1 the p and s wave velocity structure of the d layer beneath the southwestern pacific was investigated by using short period data from 12 deep events in the tonga fiji region recorded by the j array and the hi net two large aperture seismic arrays in japan reflected wave beam forming rwb and a migration method were used to extract weak signals originating from heterogeneities in the lowermost mantle in order to acquire high resolution a double array method was applied to the data the results of the rwb method indicate that seismic energy is reflected at discontinuities near the depths of 2520 and 2650 km which have a negative p wave velocity contrast of 1 at the most in addition there is a positive seismic discontinuity at a depth of 2800 km in the case of the s wave reflected energy is produced almost at the same depth 2550 km depth an apparent depth shift 50 km of the discontinuity at the depth of 2850 km may indicate that the s wave velocity reduction in the lowermost mantle is 2 3 times stronger than that of p a two dimensional cross section constructed with the rwb method suggests that the observed discontinuities can be characterized as intermittent lateral heterogeneities whose lateral extent is a few hundred kilometers the migration shows weak evidence of scattering objects which belong to the seismic discontinuities detected by the rwb method these anomalous structures may represent a part of hot plume generated beneath the southwestern pacific in the lowermost mantle

dinocyst stratigraphy of the valanginian aptian rurikfjellet and helvetiafjellet formations on spitsbergen arctic norway

origin of near surface hydrocarbon gases bound in northern barents sea sediments abstract in the southern part of the norwegian barents sea extensive hydrocarbon exploration and drilling has led to several oil and gas discoveries yet little is known on the petroleum systems and potential hydrocarbon reservoirs in the northern norwegian barents sea if hydrocarbons generated by thermogenic processes deep in the subsurface migrate to the surface traces of these hydrocarbons can remain in the near surface sediments and provide indirect information on potential petroleum systems near surface hydrocarbon prospecting is a method frequently used in hydrocarbon exploration of frontier areas where no direct geochemical information from drilling is available to improve knowledge of subsurface structures and the evolution of potential petroleum systems in the northern barents sea seismic data and near surface sediment samples were collected analysis of bound hydrocarbon gases extracted from these sediments revealed concentrations significantly above background in areas along the hornsund knollega fault complex as well as near margins of the olga basin generally the compositional and stable carbon isotope signatures of bound gases indicate thermogenic origins from source rocks of oil window maturity for near surface gases with anomalous high concentrations whereas for sediments with low concentration of bound gases mixture of thermogenic and microbial gas is indicated amount and composition of bound gas extracted from source rock samples from spitsbergen indicate that contribution of transported material may have influenced the bound gas in near surface marine sediment of the barents sea in areas where presence of mature organic matter is indicated in the knolegga fault complex near the western barents sea margin high concentrations of thermogenic gas in near surface sediments are associated with fault bound basins and most likely originate from the paleocene eocene torsk formation in the olga basin higher bound gas concentrations occur near the southern border of the basin corresponding to sub cropping late jurassic early cretaceous shales whereas elevated concentrations in the northern olga basin are associated with reactivated faults reaching close to the surface sediments above the center of the basin show significantly lower bound gas concentrations these observations indicate that the jurassic shales act as a regional seal for hydrocarbons and that reactivated faults at the basin margin represent pathways for migration to the surface 1d basin and petroleum systems modeling for the olga basin indicates that early to middle triassic sediments reached oil window maturity and represent the most likely source for thermogenic near surface gas in that area