microseismic event location for monitoring co2 injection using double difference tomography increases in pore pressure and volume caused by co2 injection will perturb the ambient stress field and possibly trigger brittle failure on small fractures or faults resulting in microearthquakes monitoring this induced seismicity can help understand the migration of the co2 front and the stress and strain changes in the reservoir miyazawa et al 2008 as part of research efforts of the southwest regional partnership on carbon sequestration supported by the u s department of energy and managed by the national energy technology laboratory a large number of microseismic events have been recorded since early 2008 using a geophone string cemented into a well for monitoring co2 injection at the aneth oil field in utah we use double difference tomography to obtain high precision locations of microseismic events and improve the spatial resolution of the velocity structure simultaneously the double difference seismic tomography method was developed for improving earthquake source locations and tomograph

three dimensional p and s velocity structure in the coalinga region california the coalinga earthquake sequence of 1983 provided a unique opportunity to perform a three dimensional velocity and hypocenter inversion in an area of complex three dimensional structure dominated by folding and blind thrusts additionally since other varied geological and geophysical studies have been completed in this area the three dimensional inversion solution could be compared to other interpretations inversion of 7696 p and 1511 s first arrivals from earthquakes and 696 p first arrivals from refraction shots produced a three dimensional velocity model with grid spacing of 1 2 km in the hypocentral area the overall shape and location of velocity features correspond well to the mapped surface geology the three dimensional inversion yields details of folds where the resolution is good and the general shape where resolution is lower the amounts of structural relief inferred for the local folds are similar to values inferred from geologic data the three dimensional velocity solution has several distinctive features there is a linear high velocity body 6 1 6 5 km s about 25 km long from 6 to 8 km depth that may represent a fragment of coast range ophiolite a shallow low velocity zone lvz which extends for 20 km along the fold axis at about 6 km depth and correlates with lvzs observed in both the refraction and the reflection data may indicate high pore pressure caused by lateral compressive strain deeper lvzs occur within inferred franciscan material and are characterized by horizontal or southwest dipping zones of varied thickness 4 8 km wide and 5 10 km long these lvzs may represent multiple thrust faults their locations and geometry are consistent with thrust faults inferred with seismic reflection data the three dimensional velocity solution compares well to prior two dimensional seismic reflection and refraction models and observed gravity the shape of the inferred sedimentary section agrees well with the reflections from the cenozoic strata compared to the refraction model the three dimensional solution has similar velocities and similar locations of velocity features but is more detailed in the hypocentral zone where it uses more data the gravity computed from the three dimensional velocities is similar to the observed gravity in both shape and amplitude both a simple one dimensional initial model and a complex initial model derived from the refraction interpretation were tried a simple starting model gave the best results the s velocity solution has different resolution than the p velocity solution because it uses a different set of stations and it has lower resolution because it uses fewer arrival times while the general patterns of velocity variation are similar for both vp and vs the vs solution tends to have more smearing of velocity features and can have somewhat different locations of velocity features

impact of distance dependent location dispersion error on interpretation of microseismic event distributions along with horizontal drilling and hydraulic fracturing technology monitoring induced microseismic activity during hydraulic fracturing has played a significant role in the economic development of today s prolific tight gas and shale resource plays many microseismic fracture monitoring papers in the current literature discuss the equipment how to acquire and process microseismic data and how to determine an appropriate velocity model unfortunately for an operating company s completion engineers and geophysicists charged with interpreting these data and integrating them with other data sets only a few papers discuss the impact of artifacts and location uncertainties on the interpretation while microseismic data have been very useful in the economic development of resource plays there are a number of significant pitfalls and interpretational issues that if not understood will potentially lead to erroneous interpretations this is especially important for any real time decisions during a hydraulic

tilted ti models in surface microseismic monitoring abstracta recently published analytic technique for computing locations of microseismic events jointly with velocities of homogeneous isotropic media was extended to surface microseismic monitoring and transversely isotropic models with tilted symmetry axes tti the analysis of traveltimes of the direct p sv and sh waves conducted under the assumptions of homogeneity and weak anisotropy indicated that the sv wave data acquired in modern wide azimuth surface microseismic surveys yield uniquely solvable joint inverse problems for an arbitrary symmetry axis tilt whereas the tilts should be close to 90 from the vertical for the p waves propagating in anelliptically anisotropic media and strictly equal to 90 for the sh waves to maintain the uniqueness of the joint inversion these theoretical findings confirmed on ray tracing synthetic were applied to a field microseismic data set the p waves excited by microseismic events were found to exhibit significantly flatter moveouts and better focused st

inferring maximum geostrophic current velocities in the norwegian greenland sea from settling velocity measurements of sediment surface samples methods application and results abstract critical shear velocities and geostrophic current velocities were calculated from measured grain sizes and settling velocities using a shields type threshold curve of particle movement and assuming bedload transport for particles that were found to be transported by currents because this reconstruction of the current velocities is based on the coarsest transported component that could be recognized in the sediment the resulting current velocities represent a maximum flow speed that was effective during deposition of sediment the shape of the settling velocity distributions identifies the dominant type of deposition that controlled the accumulation of sediment residual well sorted or low energy depositional the resulting pattern of reconstructed maximum geostrophic current velocities in the norwegian greenland sea reflects the known features of present day thermohaline circulation which is influenced by the bathymetry at many places reconstructed current velocities are higher in areas where the cross section of meridional flows is restricted by topographic barriers the distribution of the depositional characters shows that residual and well sorted sediment types are generally associated with higher current velocities in areas of strong geostrophic currents e g fram strait and east greenland continental slope the low energy depositional sediment type is widespread in the basins of the eastern norwegian greenland sea where lower reconstructed maximum current velocities occur the high reconstructed velocities on the spitsbergen bank can be attributed to the disturbance from storm waves in shallower water depths

detecting temporal shoreline changes and erosion accretion rates using remote sensing and their associated sediment characteristics along the coast of north sinai egypt the variation during 15 years in the shoreline along the north sinai coast has been determined by analysing tm and etm true colour landsat images from 1986 to 2001 the analyses identified erosion and accretion patterns along the coast the shoreline has advanced west of el bardawil inlet1 el bardawil inlet2 and el arish harbour where the wave induced littoral transport has been halted by jetty construction and beach growth rates are 20 681 69 855 and 20 160 m2 year respectively on the downdrift side of the constructed jetties to the east the shoreline is retreating and beaches erode at rates of 71 710 69 968 and 11 760 m2 year respectively sedimentological analyses of beach sediment samples have indicated selective transport of heavy minerals according to their densities and grain sizes a general correspondence has been found between variation in grain size sorting and heavy mineral content of beach sand and the patterns of shoreline changes

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

spatial and temporal variability of titanomagnetite placer deposits on a predominantly black sand beach abstract titanomagnetite forms rich placer deposits along the northwest coast of new zealand these deposits were sampled along 3 shore normal transects spaced over the southern 2 km of a dissipative high energy beach on the west coast in 5 field campaigns covering one year the percentage of opaque minerals mainly titanomagnetite was extremely high in the upper 30 m of the beach face extending seaward where these opaque minerals were gradually replaced with variable amounts of lighter augite hornblende and plagioclase the pattern appeared to be divided into two regions a lower seaward and an upper landward region separated by a point where either marine dominated over aeolian processes or where swash dominated over breaking processes in the seaward region the percentage of opaques increased and particle size fined landward as undertow removed the lighter larger particles seaward in the landward region the percentage of opaques and particle size were more constant or even showed the reverse pattern as wind transported the lighter material shoreward or swash asymmetry transported the heavier material seaward the similarity of settling velocities over the whole beach face suggests that sorting by size rather than weight plays a dominant role in separating the mineral assemblages considerable variations existed between transects this could be explained by the spatial changes in surfzone waves and currents that were associated with proximity to the southern headland and various rip current channels that characterised this dissipative site surprisingly the percentage of opaques decreased when the wave conditions of the day of sampling were more energetic in contrast to many other placer deposits these deposits are abundant on the beach face forming an armouring layer during lower wave energy conditions during higher wave conditions the surface layer erodes allowing lighter augite plagioclase and hornblende to be released from the sediments below

tomographic inversion of pn travel times in china 1 pn travel times are useful for studying crustal and uppermost mantle structure and regional tectonics because they are affected by crustal velocity and thickness as well as uppermost mantle velocity and anisotropy we obtained 57 740 pn travel time picks from 5433 earthquakes and 307 stations from chinese national and provincial earthquake bulletins and the international seismological center bulletins to invert for pn velocity variation and anisotropy and station delays in china our inversion reveals significant features that correlate with surface geology the main results are as follows 1 the pn velocities show a mosaic of very fast and very slow anomalies mirroring the heterogeneous geology of china at the surface the pn velocities are high beneath the major basins in the west sichuan qaidam west tarim tulufan and junggar and low in areas of active volcanoes myanmar and western yunnan and quaternary volcanism in northern tibet in seismically active areas in north china and tien shan and in the southern part of south china the hainan plume 2 the pn anisotropy beneath the major basins in the west is generally weak strong anisotropy is found beneath high deformation regions the tibetan plateau southeastern margin of the tibetan plateau western tien shan and part of north china suggesting the anisotropy is likely related to recent large scale tectonic activity 3 a large area of north china shows prominent low pn velocity beneath archean basement with thin crust our observations are consistent with rifting lithospheric thinning and mantle upwelling in the region the pn anisotropy is consistent with a dextral simple shear in the nne direction in the lithosphere mantle during the last and ongoing major deformation period 4 the pn velocity in northern tibet is generally lower than that in the south southern tibet has significant e w structure low velocity anomalies can be traced from northern tibet across southwestern tibet and south central tibet to near the india plate anisotropy is absent beneath much of the himalaya block but consistent anisotropy with e w fast direction is present beneath the lhasa block and large anisotropy up to 4 is observed in low velocity regions of the northern and western tibet complex station delays in the eastern margin of the plateau suggest that the whole crust may be highly deformed the anisotropy pattern in the southeastern margin of the tibetan plateau suggests a mantle lithospheric deformation similar to the clockwise rotation of material observed at the surface 5 crustal thicknesses inferred from our station delays are consistent with previous models which correlate well with surface topography

on the dynamics of extensional basin geological and geophysical data from the north china bohai basin and basin and range province were examined and compared they are similar to each other in many respects surficial geological structures are characterized by a series of half grabens with their one flank constituted by normal fault those extensional structures usually extend to a depth of 6 8 km therefore the stress condition in the upper 8 km can be written as sigma 2 sigma x sigma y where x y denote the directions of maximum compression and maximum tension on the horizontal plane while z signifies the vertical direction

s wave tomography of the crust and uppermost mantle in china 1 we use 280 000 local and regional arrival times from 13 105 local earthquakes recorded by 220 seismic stations to determine a three dimensional s wave velocity structure of the crust and uppermost mantle under china and surrounding regions we use a traveltime tomography code that accommodates a heterogeneous crust and varying moho depth the grid for tomographic inversion is 1 1 in the horizontal direction and 10 km in depth our results show that large velocity variations of more than 6 exist in the crust and upper mantle in the china region the velocity image of the upper crust correlates with surface geological features crustal heterogeneity is clearly observed and velocity changes are visible across some of the large fault zones in tibet low velocities exist in the thick crust and high sn velocities exist in the southern and eastern parts low velocity zones beneath volcanic sites and the rifts are clearly observed in our tomographic results under the tengchong hainan and changbai volcanic areas strong low velocity zones are visible down to the upper mantle indicating the existence of magma chambers beneath the volcanoes

lithospheric structure of tibet and western north america mechanisms of uplift and a comparative study we seek to determine the mantle lid thickness of the lithosphere and to constrain the mechanisms that are responsible for the cenozoic uplift of the tibetan plateau located behind the himalaya collision zone and the western united states located behind the san andreas transform convergent plate boundary selected first p arrivals of international seismological centre isc data at distances less than 22 are analyzed by the two station method to constrain the upper mantle structure of the basin and range br colorado plateau cp great plains gp and tibetan plateau tp the data are presented as plots of apparent velocity versus distance to the second station we show that these observations of apparent velocities versus distance as well as those that concern the attenuation of sn provide estimates of the mantle lid thicknesses we obtain the following mantle lid thicknesses br 20 40 km cp 35 50 km gp 150 195 km and tp 135 180 km sn is not observed for paths beneath the br and cp while sn efficiently propagates beneath the gp sn is also observed beneath most of tibet except in the north central part of the tp a possible interpretation of our observations is that indian continental lithosphere underthrusts only about the southern 2 3 of tibet

pn velocities beneath continental collision zones the turkish iranian plateau summary tomographic imaging of the uppermost mantle velocity in the middle east shows normal pn velocities 8 0 8 2 kms beneath the black sea and the southern caspian sea in contrast low pn velocities 7 9 km s i are found beneath the aegean sea and much of the turkish iranian plateau a region of exceptionally low pn velocities c7 6 km s beneath the lesser caucasus mountains and along the turkish iranian border coincides with a region of high sn attenuation and extensive neogene volcanism all these features suggest that near solidus conditions exist within the uppermost mantle beneath the turkish iranian plateau such conditions may result either from the decompression melting associated with upwelling convection cells or by the infiltration of water released from subducted lithosphere into the mantle above the presence of a partially melted uppermost mantle weakens the lithosphere beneath the turkish iranian plateau thus allowing it to become the locus of deformation in the arabian eurasian collision zone

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

reverse modelling for seismic event characterization summary the localization of seismic events is of utmost importance in seismology and exploration current techniques rely on the fact that the recorded event is detectable at most of the stations of a seismic network weak events not visible in the individual seismogram of the network are missed out we present an approach where no picking of events in the seismograms of the recording network is required the observed wavefield of the network is reversed in time and then considered as the boundary value for the reverse modelling assuming the correct velocity model the reversely modelled wavefield focuses on the hypocentre of the seismic event the origin time of the event is given by the time where maximum focussing is observed the spatial extent of the focus resembles the resolution power of the recorded wavefield and the acquisition this automatically provides the uncertainty in the localization with respect to the bandwidth of the recorded data the method is particularly useful for the upcoming large passive networks since no picking is required it has great potential for localizing very weak events not detectable in the individual seismogram since the reverse modelling sums the energy of all recorded traces and therefore enhances the signal to noise ratio similar to stacking in seismic exploration the method is demonstrated by 2 d and 3 d numerical case studies which show the potential of the technique events with a s n ratio smaller than 1 where the events cannot be identified in the individual seismogram of the network are localized very well by the method

estimation of effective anisotropy simultaneously with locations of microseismic events passive seismic tomography in which the event locations and the velocity model are inferred simultaneously is seldom used to process microseismic surveys acquired in the oil and gas industry we discuss advantages of applying tomographic ideas to typical microseismic data recorded in a single nearly vertical well to monitor hydraulic stimulation of a shale gas reservoir microseismic events are conventionally located in the energy industry applications using a velocity model derived from sonic logs and perforation shots instead of fixing the model as is normally done we alter it while locating the events this added flexibility not only makes it possible to accurately predict traveltimes of the recorded p and s waves but also provides a convincing evidence for anisotropy of the examined shale formation while we find that velocity heterogeneity does not need to be introduced to explain the data acquired at each stage of hydraulic fracturing the obtained models are suggestive of possible time laps

crustal rheology of the himalaya and southern tibet inferred from magnetotelluric data the collision between the indian and asian continents which began about 70 million years ago involved a convergence of at least 1 400 km and raised the tibetan plateau since crustal material is too buoyant to be transported downwards into the earth s interior the mechanisms at work in plateau formation are important in understanding the evolution of the continents yet still a matter of debate geophysical observations in the himalaya and tibet using ultra low frequency radio waves now reveal that a partially molten layer is present along the entire length of the himalaya this weak layer implies that widespread crustal flow is occurring and explains many features of plateau evolution the cenozoic collision between the indian and asian continents formed the tibetan plateau beginning about 70 million years ago since this time at least 1 400 km of convergence has been accommodated1 by a combination of underthrusting of indian2 and asian lithosphere crustal shortening3 horizontal extrusion4 and lithospheric delamination5 rocks exposed in the himalaya show evidence of crustal melting1 6 and are thought to have been exhumed by rapid erosion and climatically forced crustal flow7 8 magnetotelluric data can be used to image subsurface electrical resistivity a parameter sensitive to the presence of interconnected fluids in the host rock matrix even at low volume fractions here we present magnetotelluric data from the tibetan himalayan orogen from 77 e to 92 e which show that low resistivity interpreted as a partially molten layer is present along at least 1 000 km of the southern margin of the tibetan plateau the inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in southern tibet

feasibility of joint 1d velocity model and event location inversion by the neighbourhood algorithm using a set of synthetic p and s wave onsets computed in a 1d medium model from sources that mimic a distribution of microseismic events induced by hydrofrac treatment to a monitoring geophone array s we test the possibility to invert back jointly the model and events location we use the neighbourhood algorithm for data inversion to account for non linear effects of velocity model and grid search for event location the velocity model used is composed of homogeneous layers derived from sonic logging results for the case of one and two monitoring wells are compared these results show that the velocity model can be obtained in the case of two monitoring wells if they have optimal relative position the use of one monitoring well fails due to the trade off between the velocity model and event locations

tomographic inversion for three dimensional velocity structure at mount st helens using earthquake data tomographic inversion is applied to 17 659 p phase observations at 21 stations from 2023 earthquakes in the vicinity of mount st helens to study the three dimensional velocity structure block size for the inversion is 2 km horizontally and 2 km or more vertically locations of hypocenters are assumed known and are based on a reference one dimensional layered velocity structure a conjugate gradient technique lsqr is used to invert the large sparse system of equations augmented by regularization with a laplacian roughening matrix resolution is estimated by computing the impulse response of the inversion for various critical locations and uncertainties of the estimates are determined by a jackknife approach the results of the inversion show a remarkable correlation with known geological and geophysical features the spirit lake and spud mt plutons are characterized by high velocity regions extending to approximately 9 km depth the st helens seismic zone a band of diffuse seismicity extending nnw from the volcano is evident as a prominent low velocity lineation the change in character of the velocity anomalies south of st helens corresponds well with the near cessation of seismic activity there a low velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations

understanding heavy mineral enrichment using a three dimensional numerical model layered deposits of relatively light and heavy minerals can be found in many aquatic environments quantification of the physical processes which lead to the fine scale layering of these deposits is often limited with flumes or in situ field experiments therefore the following research questions were addressed i how can selective grain entrainment be numerically simulated and quantified ii how does a mixed bed turn into a fully layered bed and iii is there any relation between heavy mineral content and bed stability herein a three dimensional numerical model was used as an alternative measurement to study the fine scale process of density segregation during transport the three dimensional model simulates particle transport in water by combining a turbulence resolving large eddy simulation with a discrete element model prescribing the motion of individual grains the granular bed of 0 004 m in height consisted of 200 000 spherical particles d50 500 m five suites of experiments were designed in which the concentration ratio of heavy 5000 kg m to light particles i e 2560 kg m was increased from 6 15 35 60 to 80 all beds were tested for 10 s at a predefined flow speed of 0 3 m s analysis of the particle behaviour in the interior of the beds showed that the lighter particles segregated from the heavy particles with increasing time the latter accumulated at the bottom of the domain forming a layer whereas the lighter particles were transported over the layer forming sweeps particles below the heavy particle layers indicated that the layer was able to armour the particles below consequentially enrichment of heavy minerals in a layer is controlled by the segregation of a heavy mineral fraction from the light counterpart which enhances current understanding of heavy mineral placer formation this article is protected by copyright all rights reserved

cross double difference inversion for simultaneous velocity model update and microseismic event location microseismic monitoring is an approach for mapping hydraulic fracturing detecting the accurate locations of microseismic events relies on an accurate velocity model the 1d layered velocity model is generally obtained by model calibration from inverting perforation data however perforation shots may only illuminate the layers between the perforation shots and the recording receivers with limited raypath coverage in a downhole monitoring problem some of the microseismic events may occur outside of the depth range of these layers to derive an accurate velocity model covering all of the microseismic events and locating events at the same time we apply the cross double difference cdd method for the simultaneous inversion of a velocity model and event locations using both perforation shots and microseismic data the cdd method could provide accurate locations in both the relative and absolute sense utilizing cross traveltime differences between p and s phases over different events at the downhole monitoring scale the number of cross traveltime differences is sufficiently large to constrain events locations and velocity model as well in this study we assume that the layer thickness is known velocities of p and s wave are inverted different simultaneous inversion methods based on the geiger s dd and cdd algorithms have been compared with the same input data synthetic and field data experiments suggest that combining both perforation shots and microseismic data for the simultaneous cdd inversion of the velocity model and event locations is available for overcoming the trade offs in solutions and producing reliable results this article is protected by copyright all rights reserved

studies on settling traction and entrainment of larger benthic foraminiferal tests implications for accumulation in shallow marine sediments settling and traction velocities were measured on optimally preserved tests of larger foraminifera using a settling tube and flume tank within larger foraminifera with porcelaneous tests the peneroplids peneroplis antillarum p planatus p pertusus and dendritina cf d zhengae are distinguished by low test densities ca 1 2 that do not change with growth buoyancy is high because of low reynolds numbers and increases in large individuals because of the allometric change of test shape the fusiform alveolinella quoyi with test densities ca 1 6 is characterized by high reynolds numbers inducing the weakest buoyancy within porcelaneous larger foraminifera the highest buoyancy was recorded for the three soritids parasorites orbitolitoides sorites orbiculus and amphisorus hemprichii because of their low test densities ca 1 25 and the extremely flat biconcave plate like shape flat tests however reduce traction and entrainment from smooth surfaces within hyaline larger foraminiferat the amphisteginids show thick lenticular amphistegina lobifera a radiata to thin lenticular tests a bicirculata a papillosa influencing buoyancy here high test densities ca 1 8 decrease with growth in a lobifera a lessonii and a bicirculata and remain constant in a radiata and a papillosa minimum velocities required for entrainment are lower for thick lenticular tests and higher for thin lenticular tests test densities remain constant with growth in the calcarinid baculogypsina sphaerulata 1 78 and decrease slightly in calcarina gaudichaudii and neorotalia calcar starting at 1 85 all living under extreme hydrodynamic conditions density decreases the most in baculogypsinoides spinosus starting at 1 8 resulting in higher buoyancy through low reynolds numbers traction is promoted in spherical tests of baculogypsina and baculogypsinoides within nummulitids the thick lenticular palaeonummulites venosus test density decreasing with size starting at 1 78 is less buoyant expressed in high reynolds numbers but easily entrained thick lenticular juveniles and extremely flat adults distinguish operculinella cumingii heterostegina depressa and the giant cycloclypeus carpenteri test densities increase during growth starting from ca 1 6 and attaining a maximum of 1 8 buoyancy is low in small tests and high in large tests while entrainment velocities are reduced as the tests flatten high buoyancy is also a characteristic of the entirely flat tests in operculina ammonoides from deeper regions and planostegina operculinoides which is expressed in the lowest reynolds numbers within larger foraminifera

magnetic mineral transport and sorting in the swash zone northern lake erie canada a combined field and laboratory study in northern lake erie has provided new insights into the origin and dynamics of heavy mineral placer deposits on beaches consisting primarily of non magnetic sediment work was conducted on the cross shore and longshore transport of heavy magnetic minerals using magnetic susceptibility and fluorescent paints to trace the movement in the field of samples of magnetic magnetite and non magnetic quartz and calcite grains respectively laboratory experiments examined how the burial of small dense magnetic minerals is affected by the grain size of the non magnetic host material and how grain burial affects magnetic susceptibility measurements at the surface the field experiments demonstrated that the magnetic mineral tracers were buried rapidly beneath coarser non magnetic grains under low to moderate wave conditions and subsequently were unable to move in the longshore or cross shore directions the laboratory experiments showed that the magnetic susceptibility rapidly decreased with the rate and depth of burial of the magnetic minerals and that magnetic grain burial was most effective beneath coarser rather than finer non magnetic sand and for the latter sediments under less rather than more energetic conditions the results imply that magnetic mineral concentrations develop in this area through magnetic grain burial under fairly mild conditions and subsequent settling exposure and concentration in the upper swash zone during more energetic periods when the non magnetic grains are eroded it is probably during these erosional periods when the magnetic minerals are exposed in fairly homogeneous deposits that longshore and cross shore transport takes place

the north china basin an example of a cenozoic rifted intraplate basin the north china basin was a stable continental region that has undergone several distinct phases of rifting and subsidence during the mesozoic and cenozoic eras the last major phase featuring block faulting rapid subsidence and widespread calc alkaline basaltic volcanism began during early tertiary time this intraplate rifting appears to be the result of an approximately 30 extension and thinning of the lithosphere beneath the northeastern china by the late tertiary period active rifting had slowed and postrift thermal subsidence had begun this regional extension and subsidence resulted in the present day north china basin a large saucer shaped oil producing basin with thinner crust and lower upper mantle p and s wave velocities than the surrounding regions during the quaternary period however the subsidence rate increased in the north china basin the frequent occurrence of destructive earthquakes and observations of relatively high heat flow suggest that the quaternary tectonic activity differs from the late tertiary simple thermal subsidence pattern and may indicate a new phase of rifting

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

passive seismic tomography using induced seismicity at a petroleum field in oman a borehole network consisting of five monitoring wells monitored the induced seismicity at a producing petroleum field for about 11 months nearly 5400 microseismic events were analyzed and used to image the reservoir based on a new double difference dd seismic tomography the dd tomography method simultaneously solved for event locations and vp vs and vp vs models using absolute and differential p s and s p arrival times microseismicity in the field was caused primarily by compaction of the reservoir in and above the gas bearing formation and was distributed along the two major northeast southwest faults in the field the model resolution analysis based on the checkerboard test and the resolution matrix showed that the central part of the model was resolved relatively well for the depth range of 0 7 to 1 1 km clear velocity contrasts were imaged across most parts of the two northeast southwest faults the vp vs ratio estimates from the tomographic inversion were low 1 75 in the shallow

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

probabilistic earthquake location in complex three dimensional velocity models application to switzerland 1 we present a new approach to determine precise and reliable hypocenter locations in the tectonically complex region of switzerland a three dimensional 3 d p wave velocity model to be used for earthquake relocation is obtained by simultaneously inverting arrival times of local earthquakes for hypocenter locations and 3 d p wave velocity structure a 3 d p wave velocity model derived from controlled source seismology css is used as an initial reference model the final 3 d model thus combines all available information from both css and local earthquake data the probabilistic nonlinear formulation of the earthquake location problem includes a complete description of location uncertainties and can be used with any kind of velocity model in particular the combination of nonlinear global search algorithms such as the oct tree importance sampling with probabilistic earthquake location provides a fast and reliable tool for earthquake location the comparison of hypocenter locations obtained routinely by the swiss seismological service sed to those relocated in the new 3 d velocity model using a probabilistic approach reveals no systematic shifts but does exhibit large individual shifts in some epicenter locations and focal depths we can attribute these large shifts in part to large uncertainties in the hypocenter location events with a low number of observations 8 and no observation within the critical focal depth distance typically show large location uncertainties improved hypocentral locations particularly for mine blasts and earthquakes whose routine hypocenter locations had been questionable confirm that improved velocity model and probabilistic earthquake location yield more precise and reliable hypocenter locations and associated location uncertainties for switzerland

2 5 dimensional tomography of uppermost mantle beneath sichuan yunnan and surrounding regions abstract we examine the pn and sn travel time data with long and short epicentral distance ranges separately and obtain 2 5 dimensional tomographic models for the uppermost mantle velocity and the vp vs ratio beneath the sichuan yunnan and adjacent regions the tomographic models show that the average vp and vs of the upper layer are approximately 8 10 km s and 4 60 km s and those of the deeper layer are 8 19 km s and 4 65 km s respectively the average velocity ratio is 1 76 under the study area a comparison of the pn and sn velocity distribution reveals that the high and low velocity anomalies are consistent high velocity and low vp vs ratios exist in the sichuan basin and low velocity and high ratios distribute in the myanmar yunnan songpan ganzi and hainan regions respectively our models provide more evidence for the subduction of the indian plate beneath the myanmar yunnan region and the existence of the hot material upwelling beneath the hainan region in our velocity model there is a significant difference between the southern and the northern parts of the yunnan region i e north and south of 26 n we infer that these regions are controlled by different dynamic processes the velocity distributions at different depths are mainly similar in most of the regions of the study area and the differences between the upper layer and the deeper layer beneath the yunnan region indicate that the heat flow from the upper mantle has complex characteristics

selective entrainment and transport of mixed size and density sands flume experiments simulating the formation of black sand placers a process based model for the selective entrainment of grains from sands of mixed sizes and densities has been tested by measuring threshold stresses for a range of relative grain sizes in a laboratory flume the model was found to yield good predictions for the selective entrainment of different grain size fractions it predicts that a minimum threshold stress exists for intermediate grain sizes of a mixed size sediment while higher threshold stresses are required for the movement of both smaller and coarser size fractions this prediction is in part confirmed by our flume experiments and it also agrees with results of experiments by other investigators we also conducted flume tests with settling equivalent sediment mixtures to examine grain sorting leading to the concentration of heavy minerals the experiments yielded systematic increases in grain sizes and decreasing heavy mineral concentrations in the transport direction sorting patterns which imitate those found in our parallel studies of placers on ocean beaches

influence of nearshore sediment dynamics on the distribution of heavy mineral placer deposits in sri lanka beach sediments in sri lanka contain industrial grade heavy mineral occurrences samples of both offshore and onshore sediments were collected to examine the provenance mineralogy and geochemical compositions of the heavy mineral occurrences coastal morphodynamic changes along the coastline of sri lanka were analyzed using the time series satellite images these coastal morphodynamic changes were used to identify the prominent directions of monsoon influenced longshore currents coastal sediment accretion and depositional trends and their relationships to the provenance of the heavy minerals results show the concentrations of detrital ilmenite zircon garnet monazite and rutile vary in the onshore and offshore sediments the heavy mineral potential of the northeastern coast is high average contents of about 45 50 in the verugal deposit 70 85 in the pulmoddai deposit and 3 5 5 0 in offshore samples stretching from nilaveli to kokkilai compared to sediments in southwest average content about 10 in onshore sediments and 2 in offshore sediments from the mouth of the gin river therefore no economic grade heavy mineral placers were identified in the offshore environments the high concentrations of heavy minerals in beach sediments and low concentrations in offshore sediments suggest operation of a panning system in the surf zone to form enriched placer deposits major and trace element compositions of beach sediments show marked enrichments of tio2 fe2o3 la ce zr cr nb th and v compared to average upper continental crust ucc values analysis of prominent coastal longshore transport patterns identifies bidirectional sediment transport in the northeast coast of sri lanka in the southwestern coast two transport directions occur with anti clockwise transport from galle to hambantota and clockwise transport from hikkaduwa to wadduwa the heavy minerals in the placers were mainly derived from precambrian metamorphic rocks and transported to the coast through the river systems of sri lanka

tomographic pn and sn velocity beneath the continental collision zone from alps to himalaya 1 we have obtained vp and vs velocity images of the uppermost mantle beneath the continental collision zone from the alps to the himalaya by performing tomographic inversion using both pn and sn travel times 654 999 pn arrivals and 121 838 sn arrivals were selected from the joint database including isc ehb iran bulletin and the annual bulletin of chinese earthquakes average pn and sn velocities are 8 04 km s and 4 60 km s respectively and maximum velocity perturbations are about 6 pn velocity correlates well with topography in general mountains with high elevations show low velocity while the seas basins and plains with low elevations show high velocity because the mountains are collision zones with strong tectonic activity and the low elevation areas are stable plates the large tectonic lines are boundaries between high and low pn velocity such as plate boundaries sutures and faults between plates and orogens sn velocity shows a very similar pattern to pn velocity a geodynamic cartoon is proposed to show the relationship between velocity and tectonics indicating that the sutures are boundaries with high dip angles and the plate boundaries are low angle reverse faults in the region

uppermost mantle seismic velocity and anisotropy in the euro mediterranean region from pn and sn tomography summary in the last 10 15years the number of high quality seismic stations monitoring the euromediterranean region has increased significantly allowing a corresponding improvement in structural constraints we present here new images of the seismic velocity and anisotropy variations in the uppermost mantle beneath this complex area compiled from inversion of pn and sn phases sampling the whole region the method of hearn has been applied to the traveltime arrivals of the international seismological center catalogue for the time period 1990 2010 a total of 579 753 pn arrivals coming from 12 377 events recorded at 1 408 stations with epicentral distances between 220km and 1400km have been retained after applyingstandardqualitycriteria maximumdepth minimumnumberofrecordings maximum residual values our results show significant features well correlated with surface geology and evidence the heterogeneous character of the euro mediterranean lithosphere the station terms reflect the existence of marked variations in crustal thickness consistent with available moho depths inferred from active seismic experiments the highestpnvelocities are observed along a continuous band from the po basin to the northern ionian sea other high velocity zones include the ligurian basin the valencia trough the southern alboran sea and central part of the algerian margin most significant low velocity values are associated to orogenic belts betics pyrenees alps apennines and calabrian arc dinarides hellenides and lowvelocityzonesarealsoidentifiedbeneathsardiniaandthebalearicislands theintroductionof an anisotropic term enhances significantly the lateral continuity of the anomalies in particular in the most active tectonic areas pn anisotropy shows consistent orientations subparallel to major orogenic structures such as betics apennines calabrian arc and alps the sn tomographic image has lower resolution but confirms independently most of the features evidenced in the pn tomography

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

upper mantle velocity structure beneath the tibetan plateau from pn travel time tomography we inverted 1510 p arrival times from regional distances 333 1600 km in and around the tibetan plateau to map the lateral velocity variation within the uppermost mantle previous studies have placed first order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau we improve resolution by using 40 events recorded at stations within the tibetan plateau we combine these data with observations obtained from the international seismological centre isc to extend our coverage by including pn arrivals from 85 additional plateau events relocated in previous studies and recorded at stations in and around the tibetan plateau we use synthetic travel time data to evaluate the resolution of our data set the observations provide good resolution to about 1 over most of the plateau and surrounding regions our results show average pn velocities that are about 3 lower in the northern plateau relative to the southern plateau these variations correlate well with major tectonic features and previous geophysical observations in the qiangtang terrane of the northern plateau an area known to be inefficient for sn propagation pn is slow relative to both the plateau south of the banggong nujiang suture and the tectonically stable tarim basin north of the plateau this is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern tibetan plateau however when laboratory estimates of relationships between temperature velocity and attenuation are applied a relatively small temperature variation 240 to 370 c is required to explain our pn velocity observations when combined with geochemical constraints from volcanics in the northern plateau our results strongly suggest that the mantle lid is intact beneath the northern plateau this result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern tibetan plateau

fast location of seismicity a migration type approach with application to hydraulic fracturing data we propose a new approach for the location of seismic sources using a technique inspired by gaussian beam migration of three component data this approach requires only the preliminary picking of time intervals around a detected event and is much less sensitive to the picking precision than standard location procedures furthermore this approach is characterized by a high degree of automation the polarization information of three component data is estimated and used to perform initial value ray tracing by weighting the energy of the signal using gaussian beams around these rays the stacking is restricted to physically relevant regions only event locations correspond to regions of maximum energy in the resulting image we have successfully applied the method to synthetic data examples with 20 30 white noise and to real data of a hydraulic fracturing experiment where events with comparatively small magnitudes 0 were recorded

three dimensional vp vs variations for the coso region california recent seismological studies of the coso region of southeastern california document both low p wave velocities and abnormal sv attenuation in indian wells valley south of the pleistocene volcanics of the coso range in order to learn more about the physical nature of these colocated anomalies a tomographic inversion for the three dimensional variations of vp vs the ratio of compressional to shear velocity was performed iterative back projection of 2966 shear and compressional wave travel time residuals from local earthquakes recorded on vertical instruments reveals that vp vs is generally high at the surface and decreases systematically to 10 km depth superimposed on this trend are several large anomalies near devil s kitchen in the coso geothermal area vp vs values are very low near the surface consistent with measured values for steam dominated geothermal systems abnormally high values of vp vs are observed in portions of indian wells valley and also below the cactus peak rhyolite dome from 2 to 5 km in depth in indian wells valley the co occurrence of low p velocities low s velocities high vp vs and anomalous sv attenuation are indicative of subsurface partial melt because the inversion is based on vertical data only however the results cannot be considered conclusive only suggestive

imaging poisson s ratio of the uppermost mantle beneath china we have obtained a poisson s ratio image of the uppermost mantle beneath china by performing tomographic inversion of travel time differences between sn and pn the arrival pairs were selected from the annual bulletin of chinese earthquakes from 1985 to 2007 and from the international seismological center isc data set between 1985 and 2005 the data include 58 663 arrival pairs from 10 486 earthquakes recorded at 204 stations the average poisson s ratio is 0 266 the preliminary tomographic results show that 1 the pseudowave velocity is high and the velocity ratio v p v s and poisson s ratio are low in the stable cratons around the tibetan plateau such as the tarim and junggar basins the ordos craton and the southern region of the sichuan basin 2 a low pseudowave velocity and high velocity ratio and poisson s ratio exist in the central and northern tibetan plateau the north south seismic zone and north china and 3 the high velocity ratio and poisson s ratio region in the tibetan plateau extends to the surrounding cratons suggesting that the uplift of the tibetan plateau results from a high poisson s ratio or partially melted rocks beneath the plateau and that the softer rocks have intruded into the upper mantle of surrounding cratons

velocity variations in the crust and upper mantle beneath the tien shan inferred from rayleigh wave dispersion implications for tectonic and dynamic processes we used data from nuclear explosions and earthquakes recorded by the kyrgyzstan telemetered broadband seismic network to examine crustal structure in the tien shan region of central asia seventy nine measurements of fundamental and first higher mode rayleigh waves were analyzed using the single station multiple filtering technique to determine group velocity dispersions we found that there was a distinct difference in rayleigh wave group velocity values between different regions but the data could be combined in three groups 1 paths from the west and south across the turan platform and western tarim basin 2 paths across the eastern tarim basin and 3 nuclear explosions from the lop nor test site group velocity values were inverted to derive one dimensional shear wave velocity models for the tien shan region results reveal crustal thickness varying from 50 to 60 km with thickness increasing toward the eastern tien shan region in agreement with published crustal thickness maps for the region array analysis indicates that paths along the eastern tien shan and tarim tien shan are affected by lateral refractions and multipathing but we argue that these have a small effect on the measured dispersion curves our results indicate that upper mantle velocities in the eastern tarim and tien shan are significantly lower than those beneath the western tien shan and turan platform our results suggest that the boundary between these two regimes lies somewhere west of the depocenter of the tarim basin and east of kashgar china

accounting for lateral variations of the upper mantle gradient in pn tomography studies 1 the effect of an upper mantle velocity gradient on regional arrival times has been approximated by a cubic distance term which can be extended to two dimensions for use in tomographic studies to demonstrate this we add a laterally varying upper mantle gradient to the standard pn time term tomography technique and apply to a data set from asia compiled using ground truth event location criteria we observe strong lateral variations in the gradient ranging from 0 001 to 0 003 s 1 with high gradients associated with the tethys convergence zone the gradient patterns may reflect lateral variations in the thermal gradient of the mantle lid variance reduction is 63 with respect to pn tomography without gradients adding gradients allows the use of longer path lengths improving velocity image definition in high gradient regions with sparse station distribution such as tibet

planform and mobility in the meaipe maimba embayed beach on the south east coast of brazil the meaipe maimba embayed beach mmeb on the south east coast of brazil has been subject to anthropogenic pressures since the 70 s in this study we discuss the adequacy and contribution of the parabolic planform model to determine the planform and variability of the mmeb taking into consideration variation in wave conditions the role of different controlling conditions on the planform variability is analyzed as well as the morphological and planform mobility mmeb exhibited a new configuration in response to the construction of a harbor which interrupted the longshore sediment transport after four decades three particular morphodynamic sectors have been recognized along the beach the central sector is more exposed to normal wave incidence and cross shore processes predominate the northern and southern sectors are influenced by wave diffraction processes around the headlands and port respectively in the northern sector the presence of secondary headlands and inner islands imposed a geomorphological control on beach morphology and coastal processes the use of the parabolic planform model provided useful insights for the assessment of potential planform mobility since the decadal shoreline evolution combined with beach profiles and sediment characteristics allowed understanding of the beach mobility processes and supported the interpretation of modeling results

experiments on the entrainment threshold of well sorted and poorly sorted carbonate sands few studies have examined the hydrodynamic behaviour of carbonate sediments the data presented here are the result of preliminary research on entrainment in well and poorly sorted carbonate sands experiments were performed using naturally occurring sediments in a tilting recirculating freshwater flume results indicate that when of similar size shape and density the transport threshold of carbonate sands is similar to that of quartz however owing to their lower density and often platy or irregular shape skeletal sands require a lower shear stress to initiate transport because the density of carbonate particles may increasingly vary with grain size the threshold of motion in coarse carbonate grains may differ more markedly from that of quartz in poorly sorted samples results show that the coarse grained constituents move before the finer grained components grain properties and boundary layer dynamics are believed to explain this phenomenon rollability of the larger grains combined with physical trapping and immersion within a low velocity sublayer are believed to prevent finer particles from moving given the appropriate sediments and flow conditions it may therefore be possible to deposit and preserve fine grained sediments in a flow regime typically thought to transport such materials

a currents of removal approach for interpreting carbonate sedimentary processes abstract this study develops a currents of removal methodology to examine and quantify the nature of physical transport processes affecting the formation of bioclastic deposits in the cocos keeling islands an indian ocean atoll this approach is based on the hydraulic settling and threshold of entrainment characteristics of bioclastic deposits and measured current energy potential mobility pm analysis of 157 samples quantifies the proportion of deposits able to be transported in each geomorphic zone results show that under mean energy conditions wave induced currents entrain and transport sediment within the atoll and show that transport of sediment is not solely reliant on storm energy conditions gradients of pm values are used to reconstruct sediment transport pathways from the reef flat 85 100 pm through shallow passages 75 100 pm to sand aprons 50 75 pm and shallow 0 20 pm and deep lagoon 0 pm comparison of settling velocity distributions of bed material and sediment retained in sediment trap experiments show that actual mobility levels correspond with pm estimates potential mobility analysis also identifies the immobile portions of deposits which increase lagoonward constituent analysis of immobile fractions and analysis of settling frequency distributions are used to differentiate the importance of physical and biological processes in the formation of deposits throughout cocos and identify the role of each geomorphic zone in the transport system the pattern of sediment is controlled by physical processes between the reef flat the primary sediment production zone with sediment transported through the shallow passage conduits to the sand aprons the formation of shallow and deep lagoon deposits is controlled by autochthonous sediment production and storm deposition potential mobility analysis is a powerful tool enabling physical transport processes within bioclastic sedimentary environments to be quantified the ability to examine an individual deposit s hydraulic behaviour and development also enables depositional processes to be examined at a much finer resolution than previously attempted using conventional textural approaches

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

upper mantle beneath southeast asia from s velocity tomography 1 we present a three dimensional s velocity model of the se asian western pacific upper mantle with 400 km lateral resolution using the novel automated multimode inversion technique we processed 4038 vertical component seismograms and extracted 22 708 linear equations with uncorrelated uncertainties that constrain upper mantle structure we used time frequency windows to select signal with negligible proportion of scattered energy the windows included the fundamental rayleigh mode and s and multiple s waves the observed range of s velocity variations is the widest 17 18 in the upper 150 km of the mantle high velocity continental roots can reach beyond the present extent of the overlying archean proterozoic crust by 500 km beneath some precambrian units the roots are absent which can be attributed to deformation and gradual destruction of the ancient lithosphere at 120 150 km s velocity beneath some cratons reaches 4 8 km s this can be accounted for by thermal and compositional effects beneath the hainan island area a low velocity anomaly is observed from near the surface to the bottom of our model the hot spot type volcanism here may be caused by the deep mantle hainan plume a low velocity mantle domain underlies the south central sea of japan surrounded on the surface by intraplate volcanoes a deep seismicity gap is present near 40 n in the pacific slab subducting below and may result from a plume slab interaction a high velocity anomaly is present in the transition zone beneath the northern boundary of the yangtze craton we propose that the anomaly corresponds to subducted continental lithosphere stagnant atop the 660 km discontinuity

uppermost mantle velocities beneath china and surrounding regions 1 seismic p wave velocity variations in the uppermost mantle beneath china are tomographically imaged using pn wave travel time data the average velocity of the mantle lid beneath china is 8 05 km s with a range from 0 30 km s to 0 35 km s eastern china has slower mantle velocities and a thinner crust than western china different inversions performed for shorter ray path and longer ray path data sets separated the shallow and deep structure within the mantle lid velocities in the lower lid are no more than 0 2 km s faster than at the top the fastest velocities are beneath cratons which remained relatively undeformed despite orogenic events around their margins the pn velocity beneath southern tibet is faster than that beneath northern tibet suggesting that this contrast delineates the northern edge of the indian lithosphere on the southeastern margin of the tibetan plateau the panxi rift slow pn velocity exists in the lithosphere due to the transextensional tectonic environment in eastern china extension of the north china block produced a series of nne ssw trending rifts that cause slow mantle lid velocities these slow velocities extend to the base of the mantle lid suggesting that late cenozoic pacific subduction drove the rifting both high temperature and hydration may contribute to these low uppermost mantle velocities the shanxi rift along the east margin of the ordos platform is characterized by a slow mantle velocity in contrast the yinchuan rift on the west margin does not show any uppermost mantle velocity anomaly

upper mantle seismic velocities and anisotropy in china determined through pn and sn tomography 1 we have obtained velocity images of the uppermost mantle beneath china by performing tomographic inversion of both pn and sn traveltimes from the annual bulletin of chinese earthquakes 99 139 pn arrivals and 43 646 sn arrivals were selected pn anisotropy was also obtained simultaneously with pn velocity average pn and sn velocities are 8 05 and 4 55 km s respectively and maximum velocity perturbations are about 3 4 the pn and sn velocities are low in eastern china and high in western china particularly high velocities are associated with old basins for example tarim junggar turpan hami qaidam and sichuan and stable craton for example ordos low sn velocities are found mainly throughout north china in addition velocities are relatively low beneath the central tibetan plateau and the north south seismic zone along 103 e in tarim and central china where we observe strong anisotropy the fast pn velocity directions are consistent with the directions of maximum principal compressive stress as well as directions of crustal movement determined from global positioning system beneath the india eurasia collision zone the pn anisotropy direction is parallel to the collision arc and nearly perpendicular to both the direction of maximum compression and crustal movement resulting from pure shear deformation both the velocity variations and anisotropy indicate that the tibetan plateau was extruded and the mantle material beneath the plateau has flowed around the east himalaya syntax while the remaining material has diverted northwestward beneath the tarim basin

magnetic mineral and sediment porosity distribution on a storm dominated shelf investigated by benthic electromagnetic profiling bay of plenty new zealand abstract magnetic susceptibility and electric conductivity within the uppermost meter of the seafloor were measured with high resolution along 33 coast normal profiles in the bay of plenty new zealand using the recently developed electromagnetic benthic profiler marum neridis iii these parameters are used to determine magnetic mineral concentration and porosity of mostly volcanoclastic sediments between 2 and 35 m water depth to investigate distribution and formation of magnetic mineral enrichments on a storm dominated shelf in general magnetic mineral concentration susceptibility is inversely correlated to porosity conductivity along profiles susceptibility maxima and conductivity minima were commonly found on bathymetrical elevations such as dune and ripple crests cores and grab samples show the highest degree of enrichment in the uppermost 20 cm of the seafloor complementing petromagnetic analysis indicates a spectrum of feti oxides in the sediment ranging from magnetite and hematite over titanomagnetite tm40 and tm60 to titanohematite likely th80 and th95 three distinct zones of magnetic mineral enrichment could be identified a coast parallel structure with low porosity in recent fine sand between 5 and 20 m water depth a widespread structure with low porosity in older transgressionally reworked coarse sand in up to 30 m water depth and third structure coarse sand with high porosity located mostly below 30 m while the fine sand structure is interpreted as the result of an active process the other two appear to be relics sorting by grain size selective entrainment appears to be the major formation mechanism this study demonstrates how electromagnetic benthic profiling in combination with environmental magnetic laboratory analysis provides reliable highly interpretable data that allow insight into sorting processes within the boundaries of established lithofacies that are difficult to detect using a classical sedimentological approach

lateral variations in compressional velocities beneath the tibetan plateau from pn traveltime tomography summary a back projection algorithm is applied to 353 p traveltime measurements to image the lateral variations in compressional velocity in the uppermost mantle in and around tibet prior to the tomographic inversion we refined the apparent p slowness measurements based on a recently developed relocation procedure zhao helmberger 1991 and mathematical formulations of zhao 1993 these refinements are crucial in obtaining a high quality velocity image we obtained an average p velocity value for the uppermost mantle of 7 93 kms l and an average mantle p velocity gradient of 3 1 x lo s for tibet the final 2 d p velocity image in the uppermost mantle is characterized by a large 7 to 9 in dimension low velocity down to about 7 78 km s region in north central tibet and by large high velocity up to 8 11 km s i regions in the western and eastern flanks of tibet the crustal thickness is greater than 70 km inside the tibetan plateau it decreases toward the southern eastern and western boundaries of the plateau at and beyond these boundaries the crustal thickness is similar to those beneath the eurasian shield rigorous analyses of resolution and error suggest that 1 there is a pronounced difference between the tomographic velocity image obtained with and without the refinements to the p slowness measurements indicating a significant improvement in the quality of our inversion by these refinements 2 random errors in measuring earthquake origin times and locations and in the p arrival times should not cause errors of more than 0 1 km s in the velocity image 3 the resolution is generally within 6 to 10 the velocity image obtained in this study is consistent with previous s p traveltime studies and geothermal petrological studies and is consistent with a mantle convection model in which a convective upwelling is occurring in north central tibet and downwellings occurring in the surrounding regions molnar 1988 1990

anisotropic pn tomography in the western united states pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy to investigate uppermost mantle anisotropy a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy and it was applied to pn travel time data from the western united states results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations a detailed resolution study examined the trade off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage pn anisotropy beneath the western united states has maximum amplitudes of 0 3 km s 7 5 when resolved on a length scales of around 3 the orientations of pn anisotropy often correlate well with those inferred from shear wave splitting results this correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions the orientation of the pn anisotropy can be correlated with some of the tectonic processes which occur within the western united states for example the fast direction of pn anisotropy parallels the northeast direction of subduction of the juan de fuca plate beneath the northwest pacific coast suggesting that there the anisotropy is related to subduction driven deformation the fast direction of pn anisotropy also parallels the strike of the san andreas fault system in central california indicating that shear strains along the plate boundary may be responsible for the anisotropy there within the great basin the pn anisotropy varies substantially and both partial melting and small scale convection within the uppermost mantle could be responsible for these anisotropy variations

uncertainties in passive seismic monitoring the use of passive seismic techniques to monitor oil field completion and production processes is on the rise stress changes induced by such reservoir activities as hydraulic fracturing water injection or fluid extraction will often result in failure of the rocks with a concurrent release of seismic energy in the form of compressional p and shear s waves passive seismic monitoring is based on recording these emitted waves and then using their arrival times to estimate the location of the failure events

analytical modelling of wave refraction and convergence on coral reef platforms implications for island formation and stability abstract an analytical model was constructed to simulate the refraction of waves on coral reef platforms comprising an idealised bathymetry of uniform depth and vertical reef faces model simulations were conducted to investigate the influence of key parameters such as reef shape and depth as well as wave period on the propagation behaviour of incident waves results of the refraction analysis demonstrate that different reef shapes produce characteristic patterns of wave convergence on reef surfaces the location and stability of focal zones and hence wave convergence is largely controlled by the shape of platforms platform configuration further controls the distribution of wave energy across platform surfaces and determines the influence of incident wave forcing on different reef sections results have significant implications for sedimentation processes and hence the formation and stability of islands on reef platforms wave propagation patterns define sediment transport vectors and subsequently control the transport and deposition of different sized material platforms which promote marked wave convergence behaviour such as elliptical and circular reefs are more likely to retain sediment on reef surfaces whereas narrow linear structures have a higher potential for the off reef evacuation of sediment over leeward reef margins and the subsequent infill of deeper lagoonal areas the study provides a physical basis for future investigations of reef hydrodynamics and platform sedimentation processes

analytic solutions to the joint estimation of microseismic event locations and effective velocity model obtaining hypocenters of microseismic events a primary task in mining geothermal and hydraulic fracturing applications of induced seismicity requires a velocity model for computing those hypocenters in our paper relying on a notion that information provided by microseismic events themselves enables one to construct a velocity model and calculate the event hypocenters in that model we derive exact analytic solutions to the joint velocity estimation event location problem for downhole microseismic data acquired in homogeneous isotropic media we show that traveltimes and polarization vectors of the direct p and s waves excited by a microseismic event and recorded by a string of receivers placed in one or two vertical wells not only uniquely constrain the event location and the medium velocities but also entail a straightforward analysis of the uncertainties of those estimates caused by the presence of noise in the data although the p and s wave velocities calculated analytically under the assumption of the medium homogeneity cannot fully absorb the complexity of heterogeneous subsurface models they become the proper effective velocities for a given microseismic event and the corresponding event location which is no longer exact even for noise free data might serve as a useful initial guess for more sophisticated event location techniques that account for the velocity heterogeneity

placer formation in gravel bedded rivers a review the literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections however there are few if any comprehensive reviews of the literature surprisingly given the economic importance of placers the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory in this wide ranging review the history of the development of principles of placer formation is explored as far as possible in a non technical fashion a consideration is given to the hydrodynamics of physical grain sorting above lower stage and upper stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed finally examples of the depositional environment of diamond tin and gold placers are considered

hydrodynamic behaviour of taupo 1800a pumice implications for the sedimentology of remobilized pyroclasts large scale explosive eruptions from silicic caldera volcanoes can generate huge volumes of pyroclastic material in terrestrial and marine environments on land erosion remobilization and redeposition of this debris is predominantly carried out by running water in the form of precipitation run off conversely in the submarine realm both primary emplacement and subsequent remobilization are influenced by the presence of water as a transporting medium despite this and the number of studies devoted to volcaniclastic sedimentation relatively little attention has been paid to the hydrodynamic behaviour of the particles themselves which ought to underpin any assessment of transport or depositional process this is crucial as many volcanic particles exhibit variable density according to composition and as functions of differing degrees of vesiculation and the extent to which pore space is filled by water and or gaseous phases during transport and deposition investigation of the physical and hydrodynamic properties of taupo 1800a pumice with reference to sedimentary facies developed during the eruption aftermath shows that although buoyant when dry when sufficiently waterlogged cool pumice clasts will sink and behave more like quartzo feldspathic material saturation is apparently achieved by a combination of rapid capillary flooding of large interconnected vesicles and slower diffusional air water exchange in smaller pores low saturated pumice densities result in lower settling velocities and easier entrainment by tractional currents than those for equivalent sized quartzo feldspathic or crystal lithic particles fine grained pumice is conversely harder to entrain because of the frictional interlocking of angular particles these unusual properties of temporary buoyancy variable saturation low density and size dependent cohesion complicate interpretations of the depositional setting and energy of pumiceous sediments and give rise to several unique facies these findings have implications not only for the analysis of remobilized pyroclastic facies in terrestrial and marine environments but also for primary depositional processes during subaqueous explosive volcanism

uppermost mantle structure beneath eastern china and its surroundings from pn and sn tomography the pn and sn residuals from regional events provide strong constraints on the structure and lithological characteristics of the uppermost mantle beneath eastern china and its surroundings with the dense chinese digital seismic network in eastern china separate pn and sn tomographic inversions have been exploited to obtain p and s velocities at a resolution of 2 2 or better the patterns of p velocities are quite consistent with the s velocities at depth of 50 and 60 km but the amplitude of p wave speed anomalies are a little larger than those of s wave speed the low p wave speed high s wave speed and low vp vs ratio beneath the northern part of ordos basin are related to upwelling hot material abrupt changes in material properties are indicated from the rapid variations in the vp vs ratio

crustal structure of china and surrounding regions from p wave traveltime tomography 1 a three dimensional 3 d p wave velocity model is developed for the crust and uppermost mantle of china and the surrounding area by applying the tomography method of zhao et al using 500 000 high quality p wave first arrivals extracted from the annual bulletin of chinese earthquakes abce this tomographic method can accommodate velocity discontinuities such as the moho in addition to smooth velocity variations the spatial resolution is 1 1 in the horizontal direction and 10 km in depth the velocity images of the upper crust correspond well with the surface geologic features such as basins and the tibetan plateau high velocity anomalies are found in the lower crust beneath the precambrian regions tarim basin ordos basin sichuan basin and western half of songliao basin the highest velocity anomaly is beneath the sichuan basin high and low velocity anomalies imaged beneath the bohai gulf are associated with the presence of a major cenozoic rift system in the lower crust beneath the south china block p wave velocities are lower in the north than in the south the indochina block shows low velocities both in the crust and in the uppermost mantle due to volcanism the pn velocities in the tibet area are higher than those in other areas largely due to thicker crust tomographic model significantly reduces the traveltime residuals tests conducted by relocating large explosions and earthquakes validate the 3 d velocity model

wavefield extrapolation of body waves for 3 d imaging of earthquake sources summary reverse time imaging of earthquake source parameters is extended from two to three spatial dimensions and from two to three component recordings provided that the recording aperture is sufficiently large and the data are not spatially aliased source radiation patterns in both time and space can be reconstructed by elastic reverse time propagation of body waves such reconstructions are necessarily partial as only the energy that was recorded is available for reconstruction for a point source the origin time and the 3 d spatial location of an event can be reconstructed by extracting the time and position of the best focused energy from the backward propagating wavefield for a spatially and temporally extended source biased estimates of fault position and rupture slip time history can be estimated if the recording aperture is sufficient for smaller recording apertures the time history can still be estimated if the fault location and geometry are known a priori the latter is viable for a reasonable number 200 of three component recordings and so is potentially applicable to real data all these processes assume that a smoothed representation of the 3 d velocity distribution in the volume containing the source and receivers is available

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

data acquisition design for microseismic monitoring microseismic monitoring of hydraulic well treatments is one of the most useful seismic techniques that can aid the development of tight gas and shale gas fields the main challenge in these fields is an extremely low formation permeability which lies in the microdarcy or even nanodarcy range and requires thousands of closely spaced wells to drain the reservoir for example the barnett field in texas usa has more than 12 000 producing wells depending on the specifics of a given reservoir it might be developed via vertical or slanted or horizontal wells or any combination thereof such varied well geometries and relatively small well spacing provide ample opportunities for microseismic surveys

a low velocity belt beneath northern and eastern tibetan plateau from pn tomography 1 new pn tomography on tibetan plateau tp and neighboring region is conducted with data assembled from various sources the improved pn tomographic image delineates several distinct geophysical regimes that are approximately separated by geological boundaries at surface dense distribution of the stations in the eastern half of the plateau allows us to infer a 3d view of crustal thickness variation of the region the most striking feature is the bended low velocity belt lvb which extends from west to east beneath qiangtang block and songpan ganzi fold belt sgfb in northern tp and then turns consistently southward with sgfb beneath eastern tp and further south to indo china the consistency between geological boundaries on surface and sharp velocity contrast boundaries in uppermost mantle may suggest strong deformation coupling between crust and at least uppermost mantle if not whole lithosphere a north south striking lvb is also observed cutting through the southern tp coinciding with the yadong gulu rift at surface our tomographic results are generally consistent with dynamic models that involve with underthrusting in the southern tp shortening in the northern tp and extrusion to the east and the southeast furthermore our results suggest significant east west variations in the structure and dynamics