local atmospheric forcing during the coastal ocean dynamics experiment 1 a description of the marine boundary layer and atmospheric conditions over a northern california upwelling region as part of the coastal ocean dynamics experiment code meteorological instruments were deployed on buoys and at coastal stations and instrumented aircraft flights and coastal soundings were made to study the three dimensional structure of the marine boundary layer over the continental shelf off northern california during the 1981 and 1982 upwelling seasons these measurements show that after the atmospheric spring transition the airflow in the marine layer is dominated by the north pacific high and the surface wind field over the shelf is characterized by periods of strong 7 15 m s upwelling favorable alongshelf winds lasting for up to 30 days interrupted by shorter periods of much weaker winds directed either equatorward or poleward these periods of weak or reversed winds typically last several days and are called wind relaxations even though they are primarily associated with coastally trapped perturbations of the marine layer along the central and northern california coast and not with a large scale weakening of the north pacific high the atmospheric boundary layer measurements made in code suggest a simple conceptual model which can explain much of the physiology or structure of the marine layer and associated surface wind field during periods of persistent upwelling favorable winds during these periods which represent the quasi steady state regime during the upwelling season the inversion base of the marine layer drops eastward toward the coast until it intersects the coastal mountain range at a height of several hundred meters and the associated thermal wind produces an alongshelf wind jet which has a maximum speed just below the inversion base turbulent mixing tends to homogenize any stratification in the marine layer beneath the jet and couple the jet to the ocean surface producing strong upwelling favorable winds over the shelf day night heating cooling over the narrow coastal strip beneath the marine layer generates a weak cross coast secondary circulation which causes the core of the alongshelf jet to drop in elevation and shift onshore this diurnal change in the marine layer structure explains both the daytime acceleration of the surface winds observed over and near the coast and its offshore decay and the associated offshore increase in the subdiurnal alongshelf wind thus the quasi steady component of the wind stress has a significant curl over the inner shelf during periods of active upwelling this mean summer atmospheric boundary layer regime is occasionally interrupted by synoptic and or mesoscale events or anomalous conditions analysis of the code observations suggests five types of events two primarily synoptic scale conditions which lead to stronger than normal upwelling favorable winds over the shelf and three primarily mesoscale events which lead to wind relaxation about half of the wind relaxation events observed in 1981 and 1982 are believed to be associated with either coastal trapped gravity currents or internal kelvin waves which propagate north ward in the marine layer along the central and northern california coastal mountain range

diurnal variability in currents and temperature on the continental shelf between central and southern california 1 long time series are used to describe the diurnal variability in the meteorological and astronomical tidal forcing and the temperature and current response on the continental shelf of california between port san luis and port hueneme spectral characteristics are used to attribute the response to each of the forcing mechanisms the diurnal temperature field is decomposed into two dominant vertical modes a surface enhanced mode is coherent with the meteorological forcing suggesting that it is a direct response to local heating and wind forcing the amplitude of the second mode is maximum at middepth spectra of this mode exhibit multiple peaks at frequencies corresponding to the astronomical tidal forcing the phase difference between the middepth mode and the tidally dominated sea level is not consistent across the region we conclude that this mode is a localized baroclinic response to the astronomical tidal forcing the diurnal velocity field is dominated by a clockwise component at the surface this component is coherent across the study region and it is also coherent with the diurnal wind stress suggesting that it is a direct response to the diurnal heating and wind forcing coherence with the surface enhanced temperature mode further confirms this a weaker barotropic counterclockwise component is also present in the diurnal velocity field spectra of this component exhibit multiple peaks at periods corresponding to the astronomical tidal forcing the phase difference between this component and the surface elevation varies little between the observational sites we conclude that this velocity component represents a barotropic response to astronomical forcing that is coherent over the study area

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

anisotropic response of surface circulation to wind forcing as inferred from high frequency radar currents in the southeastern bay of biscay the short term less than 20 days response of surface circulation to wind has been determined in waters of the southeastern bay of biscay using wind impulse response time domain and transfer frequency domain functions relating high frequency radar currents and reanalysis winds the response of surface currents is amplified at the near inertial frequency and the low frequency and it varies spatially the analysis indicates that the response of the ocean to the wind is slightly anisotropic likely due to pressure gradients and friction induced by the bottom and coastline boundaries in this region thus the transfer function at the near inertial frequency decreases onshore due to the coastline inhibition of circularly polarized near inertial motion in contrast the low frequency transfer function is enhanced toward the coast as a result of the geostrophic balance between the cross shore pressure gradient and the coriolis forces the transfer functions also vary with season in summer the current response to wind is expected to be stronger but shallower due to stratification in winter the larger mixed layer depth results in a weaker but deeper response the results obtained are consistent with the theoretical description of wind driven circulation and can be used to develop a statistical model with a broad range of applications including accurate oceanic forecasting and understanding of the coupled atmosphere ocean influence on marine ecosystems

anisotropic response of surface currents to the wind in a coastal region analysis of coastal surface currents measured off the coast of san diego for two years suggests an anisotropic and asymmetric response to the wind probably as a result of bottom coastline boundary effects including pressure gradients in a linear regression the statistically estimated anisotropic response explains approximately 20 more surface current variance than an isotropic wind ocean response model after steady wind forcing for three days the isotropic surface current response veers 42 86 28 to the right of the wind regardless of wind direction whereas the anisotropic analysis suggests that the upcoast onshore wind stress generates surface currents with 10 86 48 71 86 38 to the right of the wind direction the anisotropic response thus reflects the dominance of alongshore currents in this coastal region both analyses yield winddriven currents with 3 5 of the wind speed as expected in addition nonlinear isotropic and anisotropic response functions are considered and the asymmetric current responses to the wind are examined these results provide a comprehensive statistical model of the wind driven currents in the coastal region which has not been well identified in previous field studies but is qualitatively consistent with descriptions of the current response in coastal ocean models

shear wave splitting in the crust in north china stress faults and tectonic implications summary this paper uses data from the capital area seismograph network in north china nc currently the largest regional seismograph network in china to investigate the compressional stress orientations from the polarizations of faster shear waves pfs a parameter of shear wave velocity anisotropy data are restricted to earthquakes deeper than 5 km to reduce the influence of the heterogeneous uppermost few kilometres of the crust the results show that the regional compressional stress direction is ene wsw to nearly ew in nc which agrees with seismic mechanisms and drilling and gps data predominantly pfs are also nearly ew in nc and are influenced by faults geology and tectonics pfs orientations at stations on seismically active faults are consistent with the strike of strike slip faults and indicate large seismically active faults breaking or nearly breaking the free surface pfs orientations are influenced by both regional compressional stress and by nearby faults it is an effective way to study detailed spatial distribution of crustal compressional stress by shear wave anisotropy with dense seismograph network shear wave splitting may also be used to indicate hitherto unknown faults

winds between san diego and san clemente island dense ship observations off san diego were used to capture the diurnal and seasonal wind trends the land breeze regime extends to 40 km off san diego in the winter but goes to less than 10 km when averaged for the winter significant diurnal variations in wind speed occur in all seasons from the coast to beyond 140 km ship observations were averaged over 0 2 of latitude and longitude these have significant structure that is obscured in other studies based upon 1 averages wind speeds decrease by a factor of 2 or more from 140 km offshore to the coast in both the northerly and westerly component the mean flow has a weak summer southerly component this is in opposition to earlier studies which indicated northerly mean monthly flow all year ship observations were compared to san diego airport and san clemente island measurements in order to check the accuracy of allowing land measurements to substitute for over water measurements san diego winds were found to be very representative of the nearest 10 km of ocean in wind speed and direction on the diurnal and seasonal scales san diego would be a good proxy station from which to infer the long term near shore meteorlogical conditions san clemente island wind speeds are systematically less than the ship observed wind speeds in the easterly component but the correspondence between the northerly component at san clemente and ships is irregular in the diurnal and monthly means this station could not be used to infer the northerly component without an uncertain and elaborate correction

west florida shelf response to upwelling favorable wind forcing kinematics the barotropic responses of the west florida continental shelf to idealized upwelling favorable alongshore and offshore wind stresses are studied using the three dimensional time dependent primitive equation princeton ocean model pom when forced with uniform winds the shelf circulation evolves quickly to a quasi steady state a southeastward alongshore wind lowers sea level along the coast and drives a southeastward coastal jet with a relatively weak northwestward return flow farther offshore a southwestward offshore wind lowers sea level along the west florida coast and raises sea level along the panhandle coast two independent circulation gyres are set up in association with these regionally different sea level distributions an anticyclonic gyre off the west florida coast consisting of a southeastward coastal jet and a broad return flow over the middle shelf and a cyclonic gyre off the panhandle coast consisting of a strong northwestward coastal jet and an adjacent narrow southeastward undercurrent these gyres are separated by the big bend region in both cases alongshore and offshore wind forcing the circulations are fully three dimensional with opposing surface and bottom boundary layer flows accounting for the across shelf transports the shapes of the coastline and the isobaths are important determinants of the shelf wide responses several locally maximum upwelling regions are identified for geometrical reasons these include the panhandle coast south of apalachicola bay the west florida coast south of tampa bay along the florida keys and at the shelf break where topographic rossby waves are evident

palaeomagnetism of flood basalts in the pilbara craton western australia late archaean continental drift and the oldest known reversal of the geomagnetic field 1 a late archaean circa 2775 2715 ma succession of terrestrial continental flood basalts mafic tuffs felsic volcanic rocks and clastic sedimentary rocks in the nullagine synclinorium and meentheena centrocline of the east pilbara basin western australia has been sampled for a palaeomagnetic study over 500 oriented mostly basalt drill cores were collected from the supracrustal succession and associated dykes thermal and alternating field demagnetization revealed two distinct components positive fold conglomerate and reversal tests confirm that the primary natural remanent magnetization nrm is still preserved the secondary component is interpreted as the record of remagnetization during a major thermal event possibly in the early proterozoic analysis of the primary nrm directions results in a magnetostratigraphy and an apparent polar wander path apwp for the 60 myr interval covered by the sampled succession assuming a geocentric axial dipole during this time interval the apwp shows that the pilbara craton was drifting during the late archaean and that drift rates probably varied significantly in particular a mean 27 2 shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry this study suggests that continents moved horizontally during the late archaean and that the rates of movement were significantly faster than in the phanerozoic in addition a reversed polarity interval with a positive reversal test is recorded we argue that it documents the oldest known geomagnetic reversals of the geomagnetic field

archean upper crust transition from mafic to felsic marks the onset of plate tectonics the archean eon witnessed the production of early continental crust the emergence of life and fundamental changes to the atmosphere the nature of the first continental crust which was the interface between the surface and deep earth has been obscured by the weathering erosion and tectonism that followed its formation we used ni co and cr zn ratios in archean terrigenous sedimentary rocks and archean igneous metaigneous rocks to track the bulk mgo composition of the archean upper continental crust this crust evolved from a highly mafic bulk composition before 3 0 billion years ago to a felsic bulk composition by 2 5 billion years ago this compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks suggesting the onset of global plate tectonics at 3 0 billion years ago

the energy flux from the wind to near inertial motions in the surface mixed layer abstract time series of wind stress computed from long term meteorological buoy data off north america are used to examine the forcing of surface inertial currents by the wind a simple damped slab model of the mixed layer is used to compute h the average flux of energy from the wind to mixed layer inertial currents in a mixed layer of fixed depth h the forcing of mixed layer inertial motions is highly intermittent most of the forcing occurs during the winter months with a few dozen events accounting for typically over half the total energy flux major forcing events are usually associated with translating cold fronts or small lows with scales of about 100 km the larger synoptic scale features have little energy at the inertial frequency and thus result in only weak forcing of inertial currents a strong seasonal signal exists in the inertial forcing at ows p 50 n 145 w 50 m is largest from october to february and is a factor of 12 above the june and july values if seasonally varying

near surface measurements of quasi lagrangian velocities in open water abstract near surface water velocities have been measured in the coastal zone of lake huron and cape cod bay by tracking drifters and drogues using acoustic travel time and compass sighting techniques the near surface current defined as the velocity of near surface drifters and drogues relative to a drogue set at 1 8 m varied on the depth scale on the order of 1 m and was directed nearly parallel to the wind and to predominant wave propagation velocity velocity profiles were logarithmic with depth to order 1 m depth and realistic values of stress were calculated using a law of the wall formula and a karman s constant of 0 4 inferred roughness lengths were of the order of 30 cm anomalously high values of wind stress were inferred from velocity profiles observed during conditions of light wind and steady swell these may be due to the similarity of stokes drift distribution to turbulent shear flow profiles

hf radar comparisons with moored estimates of current speed and direction expected differences and implications the validation of estimates of ocean surface current speed and direction from high frequency hf doppler radars can be obtained through comparisons with measurements from moored near surface current meters acoustic doppler current profilers or drifters expected differences between current meter cm and hf radar estimates of ocean surface vector currents depend on numerous sources of errors and differences such as instrument and sensor limitations sampling characteristics mooring response and geophysical variability we classify these sources of errors and differences as being associated exclusively with the current meter as being associated exclusively with the hf radar or as a result of differing methodologies in which current meters and hf radars sample the spatially and temporally varying ocean surface current vector field in this latter context we consider three geophysical processes namely the stokes drift ekman drift and baroclinicity which contribute to the differences between surface and near surface vector current measurements the performance of the hf radar is evaluated on the basis of these expected differences vector currents were collected during the high resolution remote sensing experiment ii off the coast of cape hatteras north carolina in june 1993 the results of this analysis suggest that 40 60 of the observed differences between near surface cm and hf radar velocity measurements can be explained in terms of contributions from instrument noise collocation and concurrence differences and geophysical processes the rms magnitude difference ranged from 11 to 20 cm s 1 at the four mooring sites the average angular difference ranged between 15 o and 25 o of which about 10 o is attributed to the directional error of the radar current vector estimates due to the alignment of the radial beams

impact driven subduction on the hadean earth impacts could have driven transient subduction events on the hadean earth according to numerical simulations the scenario reconciles evidence for tectonic activity with that for an otherwise tectonically stagnant early earth

modelling the compliance of crustal rock ii response to temporal changes before earthquakes summary there have been several claims that seismic shear waves respond to changes in stress before earthquakes the companion paper develops a stress sensitive model ape for the behaviour of low porosity low permeability crystalline rocks containing pervasive distributions of fluid filled intergranular microcracks and this paper uses ape to model the behaviour before earthquakes modelling with ape shows that the microgeometry and statistics of distributions of such fluid filled microcracks respond almost immediately to changes in stress and that the behaviour can be monitored by analysing seismic shear wave splitting the physical reasons for the coupling between shear wave splitting and differential stress are discussed in this paper we extend the model by using percolation theory to show that large crack densities are limited at the grain scale level by the percolation threshold at which interacting crack clusters lead to pronounced increases in rock matrix permeability in the simplest formulation the modelling is dimensionless and almost entirely constrained without free parameters nevertheless ape modelling of the evolution of fluid saturated rocks under stress reproduces the observed fracture criticality and the narrow range of shear wave azimuthal anisotropy in crustal rocks it also reproduces the behaviour of temporal variations in shear wave splitting observed before and after the 1986 m 6 north palm springs earthquake southern california and several other smaller earthquakes the agreement of ape modelling with a wide range of observations confirms that fluid saturated crystalline rocks are stress sensitive and respond to changes in stress by critical fluid rock interactions at the microscale level this means that the effects of changes in stress and other parameters can be numerically modelled and monitored by appropriate observations of seismic shear waves

searching for giant ancient impact structures on earth the mesoarchaean maniitsoq structure west greenland a 100km scale circular region in the archaean north atlantic craton centred at 651150n 511500w near maniitsoq town in west greenland comprises a set of highly unusual geological features that were created during a single event involving intense crushing and heating and are incompatible with crustal orogenic processes th presently exposed features of the maniitsoq structure were buried 20 25 km below the surface when this event occurred at c 3ga during waning convergent orogeny these features include a large aeromagnetic anomaly a central 35x50 km2 large area of comminuted quartzo feldspathic material regional scale circular deformation widespread random fractures with feather like textures intense fracture cleavage amphibolite granite matrix breccias unrelated to faulting or intrusions formation and common fluidisation of microbreccias abundant evidence of direct k feldspar and plagioclase melting superimposed on already migmatised rocks deformation of quartz by oc4 slip formation of planar elements in quartz and plagioclase and emplacement of crustally contaminated ultramafic intrusions and regional scale hydrothermal alteration under amphibolite facies conditions the diagnostic tools employed to identify impacting in the upper crust are inadequate for structures preserved deep within the continental crust nevertheless the inferred scale strain rates and temperatures necessary to create the maniitsoq structure rule out a terrestrial origin of the structure

seasonal variations in the speed factor and deflection angle of the wind driven surface flow in the tsushima strait 1 the speed factor and deflection angle of the wind driven surface flow were estimated for the tsushima strait from long 3 5 years records of surface velocities measured with hf radar geostrophic velocities estimated from sea level differences and analyzed wind velocities large seasonal variations in both the speed factor and deflection angle were found the speed factor which is defined here as the ratio of the wind driven surface flow to the waterside friction velocity is 7 7 9 5 in winter and 11 4 14 3 in summer while the deflection angle is 17 7 27 3 in winter and increases to 48 5 67 3 in summer a likely cause of these seasonal variations is the seasonal variation of the eddy viscosity profile it was also found that the speed factor in winter increases with the friction velocity this dependence is possibly explained by deepening of the wind driven surface flow with increasing friction velocity as the friction velocity increases hf radar whose measurement depth is fixed tends to measure a shallower part of the wind driven surface flow whose speed factor is larger

poleward propagating subinertial alongshore surface currents off the u s west coast 1 the network comprising 61 high frequency radar systems along the u s west coast uswc provides a unique high resolution and broad scale view of ocean surface circulation subinertial alongshore surface currents show poleward propagating signals with phase speeds of o 10 and o 100 300 km d 1 that are consistent with historical in situ observations off the uswc and that can be possibly interpreted as coastally trapped waves ctws the propagating signals in the slow mode are partly observed in southern california which may result from scattering and reflection of higher mode ctws due to curvature of shoreline and bathymetry near point conception california on the other hand considering the order of the phase speed in the slow mode the poleward propagating signals may be attributed to alongshore advection or pressure driven flows a statistical regression of coastal winds at national data buoy center buoys on the observed surface currents partitions locally and remotely wind forced components isolates footprints of the equatorward propagating storm events in winter off the uswc and shows the poleward propagating signals year round

observations of the velocity response to wind forcing in the upper ocean the response of the horizontal velocity field to forcing by the local wind was observed in the upper 150 m of the ocean by profiling with vector measuring current meters from the research platform flip as it drifted off the coast of california in january 1977 in the mixed layer approximately 50 m deep motion at frequencies lower than 0 1 cph was coherent with the wind stress rotary cross spectral analysis showed that the shear in the surface layer was related to the local wind stress and that the structure of the response at low frequencies was similar to but did not agree completely with the predictions of simple linear eddy viscosity models

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 1 presented here are self consistent three dimensional simulations of mantle convection some of which display an approximation of plate tectonic behavior that is continuous in space and time plate behavior arises through a reasonable material description of silicate deformation with a simple yield stress being sufficient to give first order plate like behavior however the required yield strength or fault frictional coefficient is much less than experimentally determined values toroidal poloidal ratios are within geologically observed limits the sensitivity of the system to yield strength and the form of strength envelope is systematically investigated optimum plate character is obtained in a narrow range of yield strength below which diffuse boundaries and above which episodic behavior and eventually a rigid lid are observed models with mobile lids develop very long wavelength horizontal structure the longest wavelength possible in the domain two dimensional models display much greater time dependence than three dimensional models

indication of high pore fluid pressures in a seismically active fault zone summary polarisations of seismic shear wave splitting observed above small earthquakes in iceland are typically approximately ne to sw parallel to the direction of maximum horizontal stress in contrast the polarisations of shear waves at three new stations sited over the husav ik flatey fault a major seismically active transform fault in northern iceland are approximately nw to se orthogonal to the stress aligned polarisations elsewhere modelling suggests that these 90 flips in polarisations are caused by propagation through cracks containing fluids at high pore fluid pressures within one or two mpa of the critical stress these observations suggest that high pore fluid pressures which play a key role in earthquake source mechanisms can be monitored by analysing shear wave splitting above seismically active fault planes

local shear wave observations in joao câmara northeast brazil 1 we present shear wave splitting data from a 10 km aperture eight station digital three component seismograph network which operated from december 1992 to august 1994 near joao c mara 5 33 s 35 51 w to record shear waves from continuing microearthquake activity persisting 6 years after a main shock in november 1986 previous aftershock recording had shown remarkably impulsive simple seismograms and hypocentral errors of typically 200 m in a structure of near uniform wave speed but only one station was three component the pattern of polarization directions shown is consistent with the trend of precambrian foliation observed in the field it is therefore concluded that these shear wave observations are controlled by the seismic anisotropy associated with the precambrian foliation and hence bear the signature of the paleo stress field i e the precambrian deformation regime this conclusion has important implications for the interpretation of shear wave splitting observations made in crustal crystalline rock because other authors have asserted that shear wave splitting normally results from the effect of stress aligned fluid filled cracks extensive dilatancy anisotropy or eda eda is by contrast a signature of the present day stress field in this region there is firm evidence that the present day maximum compressive stress is e w this evidence comes from earthquake focal mechanisms and borehole breakout data and is summarized in the world stress map we therefore conclude that our present data are in maximum conflict with an interpretation in terms of eda our interpretation is supported by the observation that delay times are consistent with those expected from modeling of elastic waves in the crust

decomposing observations of high frequency radar derived surface currents by their forcing mechanisms decomposition techniques and spatial structures of decomposed surface currents 1 surface current observations from a high frequency radar network deployed in southern san diego are decomposed according to their driving forces pure tides and their neighboring off band energy local winds and low frequency several superposed ocean responses are present as a result of the complicated bottom topography and relatively weak winds off southern san diego as opposed to coastal regions where circulation can be explained by a dominant forcing mechanism this necessitates an application of a statistical decomposition approach surface currents coherent with pure tides are calculated using harmonic analysis locally wind driven surface currents are estimated by regression of observed winds on observed surface currents the dewinded and detided surface currents are filtered by weighted least squares fitting assuming white noise and three colored signal bands low frequency band less than 0 4 cycles per day and near tidal peaks at the diurnal k1 and semidiurnal m2 frequencies the spatial and temporal variability of each part of the decomposed surface currents is investigated in terms of ocean response to the driving forces in addition the spatial correlations of individual components exhibit gaussian and exponential shapes with varying decorrelation length scales

organization of stratification turbulence and veering in bottom ekman layers 1 detailed observations of the ekman spiral in the stratified bottom boundary layer during a 3 month period in an upwelling season over the oregon shelf suggest a systematic organization counter clockwise veering in the bottom boundary layer is constrained to the weakly stratified layer below the pycnocline and its height is nearly identical to the turbulent boundary layer height veering reaches 13 4 degrees near the bottom and exhibits a very weak dependence on the speed and direction of the interior flow and the thickness of the veering layer a simple ekman balance model with turbulent viscosity consistent with the law of the wall parameterization modified to account for stratification at the top of the mixed layer is used to demonstrate the importance of stratification on the ekman veering the model agrees reasonably well with observations in the lower 60 70 of the bottom mixed layer above which it diverges from the data due to the unaccounted physics in the interior neglect of stratification in an otherwise identical model results in far worse agreement with the data yielding veering in the bottom ekman layer which is much smaller than measured but distributed over a much thicker layer

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

an archean impact layer from the pilbara and kaapvaal cratons the barberton greenstone belt of south africa and the eastern pilbara block of western australia provide information about earth9s surface environments between 3 2 and 3 5 billion years ago including evidence for four large bolide impacts that likely created large craters deformed the target rocks and altered the environment we have obtained identical single zircon uranium lead ages of 3470 2 million years ago for the oldest impact events from each craton these deposits represent a single global fallout layer that is associated with sedimentation by an impact generated tsunami and in western australia is represented by a major erosional unconformity

three dimensional wind driven flow in an elongated rotating basin abstract the wind driven circulation in lakes lagoons estuaries or coastal embayments is described with a linear steady three dimensional barotropic model in an elongated basin of arbitrary depth distribution on an f plane with rotation the vertically averaged velocity scales with the ekman depth rather than the maximum depth h0 as in the case without rotation near the closed ends of the basin the flow turns in viscous boundary layers because the length of the turning areas depends on the sign of the bottom slope and on the ratio of the ekman depth to h0 there is a striking contrast between the turning areas on either side of an observer looking toward the end of the basin in the northern hemisphere the turning area on the left is broad of order 1b where b is the basin half width the turning area on the right is narrow of order b and dynamically equivalent to the western boundary current in models of the wind driven ocean circulation ekman solutions are used to describe the ve

the response of the upper ocean to solar heating ii the wind driven current the current profile generated by a steady wind stress is disturbed by the diurnal variation of mixed layer depth forced by solar heating momentum diffused deep at night is abandoned to rotate inertially during the day when the mixed layer is shallow and then re entrained next night when it deepens the resulting variation of current profile has been calculated with a one dimensional model in which power supply to turbulence determines the profile of eddy viscosity the resulting variations of current velocity at fixed depths are so complicated that it is not surprising that current meter measurements have seldom yielded the classical ekman solution however the progressive vector diagrams do exhibit an ekman like response albeit with superimposed inertial disturbances suggesting that the model might be tested by tracking drifters designed to follow the flow at fixed depths the inertial rotation of the current in the diurnal thermocline leads to a diurnal jet the dynamical equivalent of the nocturnal jet in the atmospheric boundary layer over land the role of inertial currents in deepening the mixed layer is clarified leading to proposals for improving the turbulence parametrizations used in models of the upper ocean the model predicts that the diurnal thermocline contains two layers of persistent vigorous turbulence separated by a thicker band of patchy turbulence in otherwise laminar flow

periodic wind driven circulation in an elongated and rotating basin abstract an idealized model is developed for the three dimensional response of a coastal basin e g lagoon bay or estuary to time periodic wind stress this model handles basins that are deeper and or shallower than an ekman depth with wind forcing frequencies ranging from subinertial to superinertial here the model is used to describe how the response current and sea level of a basin deeper than one ekman depth depends on the wind forcing frequency at low subinertial frequencies the response is similar to the steady wind case and is hence called quasi steady there is a near surface ekman transport to the right of the wind balanced by a return flow at depth lateral bathymetric variations introduce an along basin circulation that decays with increasing frequency and sets the extent of the quasi steady response in the frequency domain at the inertial frequency the wind forces a damped resonant response with large vertical shear and weak depth integrated flow this result is potentially impor

the early geodynamic evolution of mars type planets abstract it is not clear whether mars once possessed active tectonics yet the question is critical for understanding the thermal evolution of mars and the origin and longevity of its early dynamo to address these issues we have coupled mantle flow simulations together with parameterized core evolution models to simulate the early evolution of mars like planets and constrain the influence of early mobile lid tectonics on core evolution we have explored a wide parameter suite encapsulating a range of uncertainties in initial conditions rheological parameters and surface strength we present successful models that experience early mobile lid behaviour with a later transition into a stagnant lid mode which reproduce core dynamo histories similar to the magnetic history of early mars

high frequency response of wind driven currents measured by drifting buoys and altimetry over the world ocean 1 we investigate the wind driven current response over the world ocean using svp drifting buoys ers 1 and ers 2 and topex altimetry data and ecmwf wind stress fields wind driven ageostrophic currents are estimated from drifting buoys by removing altimetry deduced geostrophic currents from observed drifting velocities removing the geostrophic signal enhances the coherence with wind stress at periods longer than 10 days for superinertial periods lower than 20 days a two parameter i e angle and amplitude model is fitted to the isolated ageostrophic signal to study the upper ocean response to surface wind stress in that frequency band the model which can explain up to 30 of the variance in some areas indicates that the currents spiral to the right left of the wind with depth in the northern southern hemisphere in agreement with the ekman theory the absolute value of the angle parameter is found to exhibit significant spatial and seasonal variability the vertical eddy viscosity and the ekman layer depth as deduced from ekman theory are compared to previous results their seasonal variability is also studied model results are then used to compute the mean and standard deviation of ageostrophic currents in 5 boxes over the world ocean the equatorial divergence and the subpolar and subtropical convergence zones are well reproduced

ductile precambrian fabric control of seismic anisotropy in the acu dam area northeastern brazil 1 in the joao c mara area ne brazil seismic anisotropy has been interpreted as the result of either precambrian ductile fabric or stress aligned fluid filled grain boundary caused by high pore fluid pressures the first interpretation is based on the fact that the direction of the fastest s wave splitting agrees with the north south trending precambrian fabric the second interpretation related to the extensive dilatancy anisotropy eda is based on the stress field orientation and on the assumption that the pore fluid pressure in the region is high enough to modify microcrack distribution and cause s wave polarization to become parallel to the north south oriented minimum horizontal stress shmin here we address this question by investigating this ambiguity we have analyzed the seismicity in the acu dam area 150 km west of joao c mara over a 3 year period in the acu area the northeast southwest trending ductile precambrian fabric is not parallel or orthogonal to the east west trending maximum horizontal stress shmax we combined the interpretation of fastest s wave polarizations their time delays and fabric orientation measured in the field with side looking airborne radar imagery taken before the lake impoundment thus we discriminate between 1 stress aligned fluid filled cracks 2 high pore pressure fluids modifying the microcrack distribution and therefore causing 90 flips and 3 fabric or paleostrain as causes for the observed s wave anisotropy our main result indicates that the direction of the fastest s wave polarization in the acu dam area agrees with the northeast southwest trending fabric n40 e eda or 90 flips control of seismic anisotropy if occurring in acu have minor effects

early archean silicate spherules of probable impact origin south africa and western australia layers of sand sized silicate spherules in 3 2 to 3 5 ga archean greenstone belts in south africa and western australia formed by the accumulation of quenched liquid silicate droplets having primary compositions resembling those of immediately underlying rock sequences the layers and particles appear to be unrelated to volcanic activity and we suggest that both occurrences may represent melt droplets formed during meteorite or comet impacts if so these spherules are the oldest known terrestrial impact products if these objects are representative of a larger population of early archean impact features it may be possible to directly estimate the meteorite flux on the early archean earth

comment on quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction by richard c aster peter m shearer and jon berger

an archaean heavy bombardment from a destabilized extension of the asteroid belt the late heavy bombardment lasted much longer than previously thought up to 1 7 billion years ago on earth with impacts on the moon and earth coming mostly from the e belt survivor hungaria asteroids the late heavy bombardment was a period of time generally put at about 4 1 billion to 3 8 billion years ago when the inner planets of the solar system were subjected to a high frequency barrage of asteroids this left its mark on the moon but on earth the craters quickly disappeared owing to tectonic processes and erosion in the first of two papers on the bombardment brandon johnson and jay melosh determine the properties of the asteroids by looking at spherule beds layers of debris ejected during the impacts the thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit earth this historical record of impacts indicates that the number of projectiles colliding with earth was substantially higher 3 5 billion years ago than it is today with a gradual decline in the number of strikes after the late heavy bombardment bottke et al modelled the evolution of an asteroid belt that extended farther towards mars than the present one they find that most of the impactors traced by the spherule beds probably originated in this e belt which was disrupted during migrations of some of the giant planets the barrage of comets and asteroids that produced many young lunar basins craters over 300 kilometres in diameter has frequently been called the late heavy bombardment1 lhb many assume the lhb ended about 3 7 to 3 8 billion years gyr ago with the formation of orientale basin2 3 evidence for lhb sized blasts on earth however extend into the archaean and early proterozoic eons in the form of impact spherule beds globally distributed ejecta layers created by chicxulub sized or larger cratering events4 at least seven spherule beds have been found that formed between 3 23 and 3 47 gyr ago four between 2 49 and 2 63 gyr ago and one between 1 7 and 2 1 gyr ago5 6 7 8 9 here we report that the lhb lasted much longer than previously thought with most late impactors coming from the e belt an extended and now largely extinct portion of the asteroid belt between 1 7 and 2 1 astronomical units from earth this region was destabilized by late giant planet migration10 11 12 13 e belt survivors now make up the high inclination hungaria asteroids14 15 scaling from the observed hungaria asteroids we find that e belt projectiles made about ten lunar basins between 3 7 and 4 1 gyr ago they also produced about 15 terrestrial basins between 2 5 and 3 7 gyr ago as well as around 70 and four chicxulub sized or larger craters on the earth and moon respectively between 1 7 and 3 7 gyr ago these rates reproduce impact spherule bed and lunar crater constraints

recently discovered 3 42 3 23 ga impact layers barberton belt south africa 3 8 ga detrital zircons archean impact history and tectonic implications the barberton greenstone belt bgb includes eight known layers containing spherical particles spherules that condensed from rock vapor clouds formed by the impact of large meteorites or asteroids 3 47 3 23 ga previous studies have inferred that the spherules represent bolides at least 20 70 km across spherule beds s1 s4 have been previously characterized in detail we provide here the first detailed analysis of more recently discovered beds s5 s8 all eight beds are composed of the same basic compositional and textural spherule types including nearly pure silica spherules representing nonaluminous melt precursors nearly pure phyllosilicate spherules representing mostly mafic and ultramafic liquids and compositionally mixed spherules evidence of spherule amalgamation and surface corrosion within the rock vapor clouds is developed in some beds bed s6 which occurs within a thick sequence of ultramafic volcanic rocks is overlain by a tsunami layer containing zircons as old as 3811 7 ma suggesting deep possibly impact related crustal uplift and erosion in distant areas the formation of at least 8 major impact layers representing bolides 20 70 km in diameter over an interval of 240 m y suggests impact rates greatly exceeding those of later geologic time and provides direct evidence that terrestrial bombardment by large bolides did not end abruptly at 3 8 ga but waned gradually until 3 0 ga or even later the coincidence of at least four large impact layers and the initiation of bgb deformation at 3 26 3 23 ga suggests that an impact cluster at this time may have disrupted a long lived earlier geodynamic system and triggered the development of a contrasting more modern plate tectonic regime

short term episodicity of archaean plate tectonics by combining geochemical data and geodynamical models evidence is provided to address the existence and style of archaean plate tectonics a topic of vigorous debate for decades using careful analyses of lithostratigraphic archaean assemblages and numerical model results we illustrate that a short term episodic style of subduction was a viable style of tectonics in the early earth modeling results show how due to the low strength of slabs in a hotter earth frequent slab break off events prevented a modern style long lived subduction system and resulted in frequent cessation and re initiation of the subduction process on a typical time scale of a few million years results fit with geochemical observations that suggest frequent alternation of arc style and non arc style volcanism on a similarly short time scale such tectonics could provide the link between early pre plate tectonic style of tectonics or stagnant lid convection and modern style plate tectonics in which short term episodes of proto subduction evolved over time into a longer term more successful style of plate tectonics as mantle temperature decayed

the fine structure of nearshore tidal and residual circulations revealed by h f radar surface current measurements abstract using the ocean surface current radar oscr developed by the rutherford appleton laboratory uk 30 days of synoptic hourly surface current vectors were obtained for 84 locations within a nearshore region some 18 kms square tidal analyses of these data show that the currents associated with the predominant m2 constituent sweep smoothly and regularly through the area unaffected by the finer topographic features moreover contours of the amplitude of the m2 semi major axis are mutually consistent to a precision of better than 0 5 cm s 1 statistical analyses of these data indicate that the standard error of oscr current measurements is less than 4 cm s 1 by contrast the major higher harmonic constituent m4 shows pronounced but ordered spatial variability relating the observed distributions for m4 and m2 is foreseen as an instructive modeling problem that should advance our knowledge of shallow water tidal interaction processes standard relationships between residual surface currents and

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

the scatter of time delays in shear wave splitting above small earthquakes summary measurements of time delays in seismic shear wave splitting above small earthquakes typically display a scatter of often as much as 80 per cent about the mean changes in the average timedelay appear to be related to changes of stress but applications of this potentially powerful tool have been handicapped by the previously inexplicable scatter in time delays above earthquakes in contrast measurements of shear wave time delays in controlled source exploration seismics are typically well controlled and display little scatter previous estimates of possible causes of scatter cannot produce sufficient variation specifically above earthquakes here we show that 90 flips in shear wave polarizations due to fluctuating high pore fluid pressures on seismicallyactive fault planes are the most likely cause of the scatter

review secular tectonic evolution of archean continental crust interplay between horizontal and vertical processes in the formation of the pilbara craton australia the archean pilbara craton contains five geologically distinct terranes the east pilbara karratha sholl regal and kurrana terranes all of which are unconformably overlain by the 3 02 to 2 93 ga de grey superbasin the 3 53 3 17 ga east pilbara terrane ep represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3 53 3 43 3 35 3 29 and 3 27 3 24 ga each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3 72 ga and melting of crust to generate first tonalite trondhjemite granodiorite ttg and then progressively more evolved granitic magmas in each case plume magmatism was accompanied by uplift and crustal extension the combination of conductive heating from below thermal blanketing from above and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust these mantle melting events caused severe depletion of the subcontinental lithospheric mantle making the ep a stable buoyant unsubductable continent by c 3 2 ga extension accompanying the latest event led to rifting of the protocontinent margins at between 3 2 and 3 17 ga after 3 2 ga horizontal tectonic forces dominated over vertical forces as revealed by the geology of the three terranes karratha sholl and regal of the west pilbara superterrane the c 3 12 ga whundo group of the sholl terrane is a fault bounded 10 km thick volcanic succession with geochemical characteristics of modern oceanic arcs including boninites and evidence for flux melting that indicate steep archean subduction at 3 07 ga the 3 12 ga sholl terrane 3 27 ga karratha terrane and c 3 2 ga regal terrane accreted together and onto the ep during the prinsep orogeny this was followed by development of the de grey superbasin an intracontinental sag basin and widespread plutonism 2 99 2 93 ga as a result of orogenic relaxation and slab break off craton wide compressional deformation at 2 95 2 93 ga culminated with 2 91 ga accretion of the 3 18 ga kurrana terrane with the ep this compression caused amplification of the dome and keel structure in the ep final cratonization was effected by emplacement of 2 89 2 83 ga post tectonic granites

impact spherules as a record of an ancient heavy bombardment of earth the fossilized remnants of vaporized asteroids called spherules can be used to infer that the flux of asteroid impacts on earth 3 5 billion years ago was much greater than it is now the late heavy bombardment was a period of time generally put at about 4 1 billion to 3 8 billion years ago when the inner planets of the solar system were subjected to a high frequency barrage of asteroids this left its mark on the moon but on earth the craters quickly disappeared owing to tectonic processes and erosion in the first of two papers on the bombardment brandon johnson and jay melosh determine the properties of the asteroids by looking at spherule beds layers of debris ejected during the impacts the thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit earth this historical record of impacts indicates that the number of projectiles colliding with earth was substantially higher 3 5 billion years ago than it is today with a gradual decline in the number of strikes after the late heavy bombardment bottke et al modelled the evolution of an asteroid belt that extended farther towards mars than the present one they find that most of the impactors traced by the spherule beds probably originated in this e belt which was disrupted during migrations of some of the giant planets impact craters are the most obvious indication of asteroid impacts but craters on earth are quickly obscured or destroyed by surface weathering and tectonic processes1 earth s impact history is inferred therefore either from estimates of the present day impactor flux as determined by observations of near earth asteroids or from the moon s incomplete impact chronology2 3 4 asteroids hitting earth typically vaporize a mass of target rock comparable to the projectile s mass as this vapour expands in a large plume or fireball it cools and condenses into molten droplets called spherules5 for asteroids larger than about ten kilometres in diameter these spherules are deposited in a global layer spherule layers preserved in the geologic record accordingly provide information about an impact even when the source crater cannot be found1 here we report estimates of the sizes and impact velocities of the asteroids that created global spherule layers the impact chronology from these spherule layers reveals that the impactor flux was significantly higher 3 5 billion years ago than it is now this conclusion is consistent with a gradual decline of the impactor flux after the late heavy bombardment

high pore fluid pressures at bhuj inferred from 90 flips in shear wave polarizations summary shear wave splitting above local earthquakes in the aftershock zone of the 2001 m w7 6 bhuj earthquake display a spatial variation of anisotropy related to crack orientation at most seismic stations the fast polarization directions are approximately nne ssw parallel to the inferred directions of maximum horizontal stress suggesting the dilatancy induced anisotropy resulting from approximately stress aligned parallel vertical microcracks in contrast the fast polarization directions at station samkhyali skl very close to the epicentre of the main shock are fault parallel approximately ew and almost orthogonal to the stress aligned polarizations elsewhere ape modelling and observations elsewhere suggest that these are 90 flips in shearwave polarizations caused by propagation through microcracks containing fluids at critically high pore fluid pressures surrounding the seismically active fault plane the presence of high pore fluid pressures on all seismogenic faults also explains the high scatter in shear wave time delays the average path normalized delay time varies from 2 to 4 ms km 1 except at station skl where higher delays of up to 13 ms km 1 are found the large scatter in measured time delays is caused by cracks saturated with high pressure pore fluids these observations can be explained by the presence of both normal stress aligned and 90 flipped shear wave polarizations at bhuj this suggests that the kutch mainland fault is a major tectonic feature

west florida shelf response to local wind forcing april 1998 we compare west florida shelf velocity and sea level data with a model simulation for april 1998 responses for three upwelling and three downwelling favorable wind events are documented along shelf jets accompanied by oppositely directed upper and lower layer across shelf flows with connecting vertical velocity comprise the fully three dimensional inner shelf responses which are sensitive to stratification with an initial density field representative of april 1998 the model simulates velocity and sea level variations in general agreement with the observations whereas substantial mismatches occur without stratification despite the winds being the primary motive agent for the inner shelf the stratification dependence requires that model density fields be maintained through a combination of adequate initial conditions surface offshore and land derived buoyancy inputs and data assimilation dynamical analyses define the inner shelf as the region where the surface and bottom boundary layers are important in the momentum balance kinematically this is where surface ekman layer divergence fed by the bottom ekman layer convergence or conversely sets up the across shelf pressure gradient stratification causes a response asymmetry wherein the offshore scale and magnitude of the upwelling responses are larger than those for the downwelling responses this asymmetry is attributed to thermal wind effects across the bottom ekman layer buoyancy torque by isopycnals bending into the bottom adds constructively destructively with planetary vorticity tilting under upwelling downwelling favorable winds and this may have important implications for nutrients and other material property distributions on the shelf

alongshore wind forcing of coastal sea level as a function of frequency abstract the amplitude of the frequency response function between coastal alongshore wind stress and adjusted sea level anomalies along the west coast of the united states increases linearly as a function of the logarithm log10 of the period for time scales up to at least 60 and possibly 100 days the amplitude of the frequency response function increases even more rapidly at longer periods out to at least 5 yr at the shortest periods the amplitude of the frequency response function is small because sea level is forced only by the local component of the wind field the regional wind field which controls the wind forced response in sea level for periods between 20 and 100 days not only has much broader spatial scales than the local wind but also propagates along the coast in the same direction as continental shelf waves hence it has a stronger coupling to and an increased frequency response for sea level at periods of a year or more observed coastal sea level fluctuations are not only forced b

peel back controlled lithospheric convergence explains the secular transitions in archean metamorphism and magmatism abstract the preserved archive of continental crust suggests that various secular geologic and geochemical transitions took place during the mesoarchean and neoarchean 3 2 2 5 ga these transitions which are imprinted in the metamorphic and magmatic rock record include the emergence of paired metamorphism an increasing depth of tonalite trondhjemite granodiorite ttg formation the widespread appearance of granites and changes in the composition of mafic rock previous studies have argued that these secular transitions reflect secular cooling of the mantle and herald a gradual transition from pre plate tectonic to plate tectonic regimes however their tectonic driver remains elusive in part due to a lack of detailed understanding of lithospheric dynamics prevalent during this transitional period when the mantle was warmer compared to the present day here we demonstrate that lithospheric convergence driven by peel back peeling process under warmer mantle conditions termed as peel back convergence may explain the late archean secular transitions this tectonic phenomena features large scale peeling a form of delamination but not dripping of the mantle lithosphere with or without lower crustal rocks during convergence we simulated numerical models of peel back convergence under archean crust mantle conditions and predicted the pressure temperature p t conditions of crustal metamorphism and melting the evolution of peel back convergent setting features juxtaposition of a colder compressional regime with thickened crust that forms at the site of peeling and a warmer extensional regime with thinned crust forming behind it the metamorphic and magmatic p t conditions prevalent in these two tectonothermal sites replicate the features of the late archean rock record including the appearance of coeval high t p and intermediate t p metamorphic rocks greater abundance of high t p relative to intermediate t p metamorphic rocks dominant formation of ttgs at higher pressures where garnet rutile is stable in the residue reworking of pre existing felsic rocks sediments to produce potassic granites and interaction of crustal melts and mantle to form hybrid granitoids thus peel back convergence can explain the thermobaric bimodality of late archean rocks within an asymmetric tectono thermal framework however unlike modern convergent plate boundaries the asymmetry is controlled by lithospheric peeling

the late heavy bombardment heavily cratered surfaces on the moon mars and mercury show that the terrestrial planets were battered by an intense bombardment during their first billion years or more but the timing sources and dynamical implications of these impacts are controversial the late heavy bombardment refers to impact events that occurred after stabilization of the planetary lithospheres such that they could be preserved as craters and basins lunar melt rocks and meteorite shock ages point toward a discrete episode of elevated impact flux between 3 5 and 4 0 4 2 ga and a relative quiescence between 4 0 4 2 and 4 4 ga evidence from precambrian impact spherule layers suggests that a long lived tail of terrestrial impactors lasted to 2 0 2 5 ga dynamical models that include populations residual from primary accretion and destabilized by giant planet migration can potentially account for the available observations although all have pros and cons the most parsimonious solution to match constraints is a hybrid mode

observations of seasonal variation in the ekman layer abstract the seasonal variation in the ekman layer is examined using observations from the long term upper ocean study lotus including surface meteorology current and temperature the near surface current is found to be coherent with the wind at low frequencies periods greater than one day using a wind relative averaging method the authors find the observed near surface volume transport to be consistent with ekman transport the mean current profile has a spiral shape that is flat compared to the classic ekman spiral in that it rotates less with depth than does the ekman spiral the mean current e folds over 12 m in the summer and 25 m in the winter with a decay in amplitude that implies eddy viscosities of 60 10 4 and 250 10 4 m2 s 1 respectively diurnal cycling is the dominant mode of variability in the summer and it determines the vertical structure of the spiral in the winter diurnal cycling is almost nonexistent due to greatly reduced solar insolation and stronger winds a persisten

shear wave anisotropy spatial and temporal variations in time delays at parkfield central california summary shear wave splitting is analysed on data recorded by the high resolution seismic network hrsn at parkfield on the san andreas fault central california during the three year period 1988 1990 shear wave polarizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress at some 70 to the fault strike whereas at station mm in the immediate fault zone shear wave polarizations are aligned approximately parallel to the fault normalized time delays at this station are found to be about twice as large as those in the rock mass either side this suggests that fluid filled cracks and fractures within the fault zone are elastically or seismically different from those in the surrounding rocks and that the alignment of fault parallel shear wave polarizations are associated with some fault specific phenomenon temporal variations in time delays between the two split shear waves before and after a ml 4 earthquake can be identified at two stations with sufficient data mm within the fault zone and vc outside the immediate fault zone time delays between faster and slower split shear waves increase before the ml 4 earthquake and decrease near the time of the event the temporal variations are statistically significant at 68 per cent confidence levels earthquake doublets and multiplets also show similar temporal variations consistent with those predicted by anisotropic poroelasticity theory for stress modifications to the microcrack geometry pervading the rock mass this study is broadly consistent with the behaviour observed before three other earthquakes suggesting that the build up of stress before earthquakes may be monitored and interpreted by the analysis of shear wave splitting

the timeline of the lunar bombardment revisited abstract the timeline of the lunar bombardment in the first gy of solar system history remains unclear basin forming impacts e g imbrium orientale occurred 3 9 3 7 gy ago i e 600 800 my after the formation of the moon itself many other basins formed before imbrium but their exact ages are not precisely known there is an intense debate between two possible interpretations of the data in the cataclysm scenario there was a surge in the impact rate approximately at the time of imbrium formation while in the accretion tail scenario the lunar bombardment declined since the era of planet formation and the latest basins formed in its tail end here we revisit the work of morbidelli et al 2012 that examined which scenario could be compatible with both the lunar crater record in the 3 4 gy period and the abundance of highly siderophile elements hse in the lunar mantle we use updated numerical simulations of the fluxes of asteroids comets and planetesimals leftover from the planet formation process under the traditional assumption that the hses track the total amount of material accreted by the moon since its formation we conclude that only the cataclysm scenario can explain the data the cataclysm should have started 3 95 gy ago however we also consider the possibility that hses are sequestered from the mantle of a planet during magma ocean crystallization due to iron sulfide exsolution o neil 1991 rubie et al 2016 we show that this is likely true also for the moon if mantle overturn is taken into account based on the hypothesis that the lunar magma ocean crystallized about 100 150 my after moon formation elkins tanton et al 2011 and therefore that hses accumulated in the lunar mantle only after this timespan we show that the bombardment in the 3 4 gy period can be explained in the accretion tail scenario this hypothesis would also explain why the moon appears so depleted in hses relative to the earth we also extend our analysis of the cataclysm and accretion tail scenarios to the case of mars the accretion tail scenario requires a global resurfacing event on mars 4 4 gy ago possibly associated with the formation of the borealis basin and it is consistent with the hse budget of the planet moreover it implies that the noachian and pre noachian terrains are 200 my older than usually considered

kinematics and correlation of the surface wind field in the south atlantic bight surface winds observed at charleston south carolina and at an offshore buoy located some 300 km to the east of charleston are analyzed to determine operational transformations for estimating the offshore winds from the coastal winds both the statistics describing the wind field at each station along with those describing the correlation between stations are found to be seasonally modulated ensemble averaged by season transfer functions computed by using the first 3 years of available data are applied to a fourth independent data year for assessing their predictive utility the transfer functions and predictability are both seasonally modulated and the transformation matrix is nondiagonal each offshore wind velocity component depends upon both coastal components

processing modelling and predicting time lapse effects of overpressured fluid injection in a fractured reservoir summary time lapse seismology is important for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs we analyse two 4 d 3c onshore surveys from vacuum field new mexico usa where the reservoir of interest is a fractured dolomite in phase vi a time lapse survey was acquired before and after a pilot tertiary recovery programme of overpressured co2 injection which altered the fluid composition and the pore fluid pressure phase vii was a similar time lapse survey in the same location but with a different lowerpressure injection regime applying a processing sequence to the phase vi data preserving normal incidence shearwave anisotropy time delays and polarization and maximizing repeatability interval time analysis of the reservoir interval shows a significant 10 per cent change in shear wave velocity anisotropy and 3 per cent decrease in the p wave interval velocities a 1 d model incorporating both saturation and pressure changes is matched to the data the saturation changes have little effect on the seismic velocities there are two main causes of the time lapse changes any change in pore fluid pressures modifies crack aspect ratios additionally when there are overpressures as there are in phase vi there is a 90 change in maximum impedance directions and the leading faster split shear wave instead of being parallel to the crack face as it is for low pore fluid pressures becomes orthogonal to the crack face the anisotropic poro elasticity ape model of the evolution of microcracked rock calculates the evolution of cracked rock to changing conditions ape modelling shows that at high overburden pressures only nearly vertical cracks to which normal incidence p waves are less sensitive than s waves remain open as the pore fluid pressure increases ape modelling matches the observed time lapse effects almost exactly demonstrating that shear wave anisotropy is a highly sensitive diagnostic of pore fluid pressure changes in fractured reservoirs in this comparatively limited analysis ape modelling of fluid injection at known pressure correctly predicted the changes in seismic response particularly the shear wave splitting induced by the high pressure co2 injection in the phase vii survey ape modelling also successfully predicted the response to the lower pressure injection using the same phase vi model of the cracked reservoir the underlying reason for this remarkable predictability of fluid saturated reservoir rocks is the critical nature and high crack density of the fluid saturated cracks and microcracks in the reservoir rock which makes cracked reservoirs critical systems

near resonant ocean response to sea breeze on a stratified continental shelf abstract the spatial structure and temporal characteristics of sea breeze and the associated coastal ocean response in the northwest gulf of mexico are investigated using moored instruments hydrographic stations and wind measurements near the study area of 30 n motions in the diurnal inertial band dib may be significantly enhanced by a near resonant condition between local inertial and diurnal forcing frequencies wavelet analysis is used to quantify the results results indicate that diurnal sea breeze variability peaks in summer and extends at least 300 km offshore with continuous seaward phase propagation the maximum dib oceanic response occurs in june when there is a shallow mixed layer strong stratification and an approximately 10 day period of continuous sea breeze forcing dib current variance decreases in july and august as the consequence of the deepening of the mixed layer and a more variable phase relationship between the wind and current river discharge varies interannually and can s

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

on the interaction between internal tides and wind induced near inertial currents at the shelf edge 1 numerical calculations are performed at the shelf edge to examine the role of nonlinear processes in transferring energy from the m2 tide and wind induced currents close to the inertial frequency f into a wave at the sum of their frequencies termed the fm2 frequency a numerical model of the hebrides shelf edge represented by a cross section initially with idealized topography and subsequently with realistic topography is used in these calculations results show that in the near coastal ocean currents at the fm2 frequency are primarily due to coupling between wind induced inertial oscillations and the m2 internal tide a major source is associated with vertical shear in the inertial oscillations and the vertical velocity due to the internal tide a secondary source is due to the nonlinear momentum advection term in the case in which eddy viscosity is computed from a turbulence energy model shear across the thermocline is larger than when a constant viscosity is used the reduction in shear with a constant viscosity reduces the role of the nonlinear term involving vertical shear and hence the magnitude of the fm2 current in the region of the thermocline increasing the wind stress leads to a deeper thermocline and hence the location of maximum fm2 current in the water column changing the vertical stratification influences the intensity of the inertial oscillations in the surface layer and the distribution of the internal tide and hence changes the pattern of fm2 currents results with realistic topography confirm the major conclusions

seismic anisotropy of the shallow crust in the umbria marche italy region abstract small magnitude earthquake data recorded during the 1997 umbria marche italy seismic crisis have been used to study the shear wave splitting in the central apennine using the method of diagonalization of the covariance matrix a set of 1234 s waves impinging the earth at an angle less than the critical angle has been selected the cross correlation method was used to estimate the splitting time between the two horizontal s components and the direction of the symmetry axis of the anisotropic structure a further analysis based on visual inspection of corrected traces allowed us to estimate the anisotropic parameters for 884 s waves an average splitting time 32 3 4 ms has been estimated splitting times do not generally show a clear pattern with distance which is an indication of possible spatial variations of seismic anisotropy a minimum 2 km depth of the anisotropic layer is estimated the trend of normalized splitting time vs depth roughly indicates a maximum 4 5 km depth of the anisotropic layer the distribution of the fast symmetry axis indicates a main symmetry axis oriented along the direction of the maximum compressive stress in the area and a secondary symmetry axis almost orthogonal to the first one the stereographic plots of fast axis direction seem exclude a depth dependence of anisotropic parameters a clear spatial correlation between the directions of the fast symmetry axes at the considered stations and the orientations of the nearest fault systems and thrust fronts is found which confirms the spatial variability in the anisotropic properties of the area as inferred by the analysis of splitting times based also on previous studies results are interpreted in terms of the extensive dilatancy anisotropy eda model the estimated crack density is always smaller than the critical coalescence value enforcing the hypothesis that fluid filled separated crack systems control the polarization of s waves

measurements of upper ocean surface current shear with high frequency radar estimates of the variation with depth of both the speed and direction of near surface ocean currents within the top meter have been made using time series data collected from a dual frequency high frequency radar located on the california coast south of monterey long term averages derived from this data set over 3 different years consistently revealed an expected reduction in the speed of the near surface current as a function of depth consistent with a logarithmic vertical current profile an unexpected and previously unmeasured clockwise rotation with increasing depth of the near surface current was also observed hypotheses are suggested for the apparent rotation

decomposing observations of high frequency radar derived surface currents by their forcing mechanisms locally wind driven surface currents 1 the wind impulse response function and transfer function for high frequency radar derived surface currents off southern san diego are calculated using several local wind observations the spatial map of the transfer function reflects the influence of the coast on wind current dynamics near the coast within 20 km from the shoreline the amplitudes of the transfer function at inertial and diurnal frequencies are reduced due to effects of coastline and bottom bathymetry meanwhile the amplitude of low frequency currents increases near the coast which is attributed to the local geostrophic balance between cross shore pressure gradients against the coast and currents locally wind driven surface currents are estimated from the data derived response function and their power spectrum shows a strong diurnal peak superposed on a red spectrum similar to the spectra of observed winds current magnitudes and veering angles to a quasi steady wind are typically 2 5 of the wind speed and vary 50 90 to the right of the wind respectively a wind skill map is introduced to present the fractional variance of surface currents explained by local winds as a verification tool for wind data quality and relevance moreover the transfer functions in summer and winter are presented to examine the seasonal variation in ocean surface current response to the wind associated with stratification change

the influence of fetch on the response of surface currents to wind studied by hf ocean surface radar the momentum transfer from wind to sea generates surface currents through both the wind shear stress and the stokes drift induced by waves this paper addresses issues in the interpretation of hf radar measurements of surface currents and momentum transfer from air to sea surface current data over a 30 day period from hf ocean surface radar are used to study the response of surface currents to wind two periods of relatively constant wind are identified one for the short fetch condition and the other for the long fetch condition results suggest that the ratio of surface current speed to wind speed is larger under the long fetch condition while the angle between the surface current vector and wind vector is larger under the short fetch condition data analysis shows that the stokes drift dominates the surface currents under the long fetch condition when the sea state is more mature while the stokes drifts and ekman type currents play almost equally important roles in the total currents under the short fetch condition the ratios of stokes drift to wind speed under these two fetch conditions are shown to agree well with results derived from the empirical wave growth function these results suggest that fetch and therefore sea state significantly influences the total response of surface current to wind in both the magnitude and direction by variations in the significance of stokes drift furthermore this work provides observational evidence that surface currents measured by hf radar include stokes drift it demonstrates the potential of hf radar in addressing the issue of momentum transfer from air to sea under various environmental conditions

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

on the vertical structure of wind driven sea currents abstract the vertical structure of wind driven sea surface currents and the role of wind wave breaking in its formation are investigated by means of both field experiments and modeling analysis of drifter measurements of surface currents in the uppermost 5 m layer at wind speeds from 3 to 15 m s 1 is the experimental starting point of this study the velocity gradients beneath the surface are found to be 2 to 5 times weaker than in the wall boundary layer surface wind drift identified via drift of an artificial slick with respect to 0 5 m depths is about 0 7 which is even less than the velocity defect over the molecular sublayer in the wall boundary layer at a smooth surface to interpret the data a semiempirical model describing the effect of wave breaking on wind driven surface currents and subsurface turbulence is proposed the model elaborates on the idea of direct injection of momentum and energy from wave breaking including microscale breaking into the water body momentum and energy tran

changes in shear wave splitting at anza near the time of the north palm springs earthquake changes in shear wave splitting are observed at knw before and after the m 6 north palm springs earthquake of july 8 1986 knw is a station of the anza seismic network monitoring the anza seismic gap on the san jacinto fault southern california the gradual increase in the delays between the split shear waves over 3 years at knw reported by peacock et al 1988 ended in june 1986 the further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the north palm springs earthquake 33 km north of knw peacock et al demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress aligned fluid filled inclusions and speculated that this increase might be the result of a buildup of stress before an impending earthquake the new data appear to confirm this speculation but the temporal variations require a more complex interpretation although they still suggest that the changes in shear wave splitting are caused by earthquake induced stress changes to the fluid filled inclusions throughout the rockmass central to interpretation of temporal changes in shear wave splitting is the well established existence throughout at leastmore the uppermost 10 to 20 km of the crust of small fluid filled cracks microcracks and pores the existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock the authors term these distributions of inclusions extensive dilatancy anisotropy or eda and the individual inclusions eda cracks because although they may include a wide range of shapes many of the seismic properties can be simulated by distributions of thin parallel cracks less