inferring surface water equilibrium calcite δ18o during the last deglacial period from benthic foraminiferal records implications for ocean circulation the ocean circulation modifies mixed layer ml tracer signals as they are communicated to the deep ocean by advection and mixing we develop and apply a procedure for using tracer signals observed upstream by planktonic foraminifera and downstream by benthic foraminifera to constrain how tracer signals are modified by the intervening circulation and by extension to constrain properties of that circulation a history of ml equilibrium calcite 18o 18oc spanning the last deglaciation is inferred from a least squares fit of eight benthic foraminiferal 18oc records to green s function estimated for the modern ocean circulation disagreements between this history and the ml history implied by planktonic records would indicate deglacial circulation changes no deviations from the modern circulation are diagnosed because the two estimates of ml 18oc agree within their uncertainties over the deglaciation but diagnostics suggest data collection and modeling procedures useful for inferring circulation changes uncertainties of benthic derived ml 18oc are lowest in the high latitude regions chiefly responsible for ventilating the deep ocean additional high resolution planktonic records constraining these regions are of particular utility benthic records from the southern ocean where data are sparse appear to have the most power to reduce uncertainties in benthic derived ml 18oc understanding the spatiotemporal covariance of deglacial ml 18oc will also improve abilities of 18oc records to constrain deglacial circulation

teleseismic detection of a slow precursor to the great 1989 macquarie ridge earthquake low frequency spectra for the 1989 macquarie ridge earthquake magnitude 8 2 show an amplitude increase and a phase delay decrease below 6 millihertz that require a short term slow precursor this earthquake can be modeled as a compound event in which a fast rupturing ordinary earthquake was initiated by an episode of slow smooth deformation that began more than 100 seconds before the main shock the moment released in the slow precursor was large about 3 x 10 20 newton meters equivalent to an event of magnitude 7 6 the data are consistent with the precursor being generated in a region of the oceanic upper mantle below the main rupture

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

local observations of the onset of a large earthquake 28 june 1992 landers california the landers earthquake mw 7 3 of 28 june 1992 had a very emergent onset the first large amplitude arrivals are delayed by about 3 sec with respect to the origin time and are preceded by smaller scale slip other large earthquakes have been observed to have similar emergent onsets but the landers event is one of the first to be well recorded on nearby stations we used these recordings to investigate the spatial relationship between the hypocenter and the onset of the large energy release and to determine the slip function of the 3 sec nucleation process relative location of the onset of the large energy release with respect to the initial hypocenter indicates its source was between 1 and 4 km north of the hypocenter and delayed by approximately 2 5 sec three station array analysis of the p wave shows that the large amplitude onset arrives with a faster apparent velocity compared to the first arrivals indicating that the large amplitude source was several kilometers deeper than the initial onset an ml 2 8 foreshock located close to the hypocenter was used as an empirical green s function to correct for path and site effects from the first 3 sec of the mainshock seismogram the resultant deconvolution produced a slip function that showed two subevents preceding the main energy release an mw 4 4 followed by an mw 5 6 these subevents do not appear anomalous in comparison to simple moderate sized earthquakes suggesting that they were normal events which just triggered or grew into a much larger earthquake if small and moderate sized earthquakes commonly detonate much larger events this implies that the dynamic stresses during earthquake rupture are at least as important as long term static stresses in causing earthquakes and the prospects of reliable earthquake prediction from premonitory phenomena are not improved

evolution of the deep atlantic water masses since the last glacial maximum based on a transient run of ncar ccsm3 during the last deglaciation from approximately 21 to 11 thousand years ago the high latitudes of the atlantic ocean underwent major changes besides the continuous warming the polar and subpolar ocean surface received a large amount of meltwater from the retracting ice sheets these changes in temperature and salinity affected deep waters such as the antarctic bottom water aabw and the north atlantic deep water nadw which are formed in the southern ocean and in the northern north atlantic respectively in this study we present the evolution of the physical properties and distribution of the aabw and the nadw since the last glacial maximum using the results of a transient simulation with ncar ccsm3 in this particular model scenario with a schematic freshwater forcing we find that modern nadw with its characteristic salinity maximum at depth was absent in the beginning of the deglaciation while its intermediate version glacial north atlantic intermediate water gnaiw was being formed gnaiw was a cold and relatively fresh water mass that dominated intermediate depths between 60 and 20 n at this time most of the deep and abyssal atlantic basin was dominated by aabw within the onset of the bolling allerod period at nearly 15 thousand years ago ka gnaiw expanded southwards when the simulated meridional overturning circulation overshoots the transition between gnaiw and nadw ocurred after that when aabw was fresh enough to allow nadw to sink deeper in the water column when the nadw appears 11 ka aabw retracts and is constrained to lie near the bottom

slip partitioning along major convergent plate boundaries along plate boundaries characterized by oblique convergence earthquake slip vectors are commonly rotated toward the normal of the trench with respect to predicted plate motion vectors consequently relative plate motion along such convergent margins must be partitioned between displacements along the thrust plate interface and deformation within the forearc and back arc regions the deformation behind the trench may take the form of strike slip motion back arc extension or some combination of both we observe from our analysis of the harvard moment tensor catalog that convergent arcs characterized by back arc spreading specifically the marianas and new hebrides are characterized by a large degree of slip partitioning however the observed rates directions and location of back arc spreading are not sufficient to account for degree of partitioning observed along the respective arcs implying that the oblique component of subduction is also accommodated in part by shearing of the overriding plate in the case of the sumatran arc where partitioning is accommodated by strike slip faulting in the overriding plate the degree of partitioning is similar to that observed along the marianas but the result is viewed with caution because it is based on a predicted plate motion vector that is based on locally derived earthquake slip vectors in the case of the alaskan aleutian arc where back arc spreading is also absent the degree of partitioning is less and rotation of slip vectors toward the trench normal appears to increase linearly as a function of the obliquity of convergence if partitioning in the alaskan aleutian arc is accommodated by strike slip faulting within the upper plate the positive relationship between obliquity of convergence and the rotation of earthquake slip vectors to the trench normal may reflect that either 1 the ratio of the depth extent of strike slip faults behind the trench z s to the subduction thrust z t increases westward along the arc 2 the dip of the subduction thrust increases westward along the arc or 3 the strength of the subduction thrust decreases westward along the arc

where and how long ago was water in the western north atlantic ventilated maximum entropy inversions of bottle data from woce line a20 1 a maximum entropy me method is used to deconvolve tracer data for the joint distribution of locations and times since last ventilation the deconvolutions utilize world ocean circulation experiment line a20 repeat hydrography for cfc 11 potential temperature salinity oxygen and phosphate as well as global ocean data analysis project glodap radiocarbon data combined with surface boundary conditions derived from the atmospheric history of cfc 11 and the world ocean atlas 2005 and glodap databases because of the limited number of available tracers the deconvolutions are highly underdetermined leading to large entropic uncertainties which are quantified using the information entropy of relative to a prior distribution additional uncertainties resulting from data sparsity are estimated using a monte carlo approach and found to be of secondary importance the me deconvolutions objectively identify key water mass formation regions and quantify the local fraction of water of age or older last ventilated in each region ideal mean age and radiocarbon age are also estimated but found to have large entropic uncertainties that can be attributed to uncertainties in the partitioning of a given water parcel according to where it was last ventilated labrador irminger seawater l water is determined to be mostly less than 40 a old in the vicinity of the deep western boundary current dwbc at the northern end of a20 but several decades older where the dwbc recrosses the section further south pointing to the importance of mixing via a multitude of eddy diffusive paths overflow water lies primarily below l water with young waters 40 a at middepth in the northern part of a20 and waters as old as 600 a below 3500 m

a new method of quickly estimating epicentral distance and magnitude from a single seismic record rapid estimation of the epicentral distance and magnitude is of fundamental importance for early earthquake detection and warning systems we present a novel method of estimating the epicentral distance from a single seismic record in a short amount of time in order to quantitatively evaluate the difference in observed seismic waveforms we introduced a simple function with the form of bt exp at and determined a and b in terms of the least squares method by fitting this function to the initial part of the waveform envelope we have found that log b is inversely proportional to log where is the epicentral distance this relation holds true regardless of the earthquake magnitude by using this relation we can roughly estimate the epicentral distance nearly immediately after the p wave arrival then we can readily estimate the magnitude from the maximum amplitude observed within a given short time interval after the p wave arrival by using an empirical magnitude amplitude relation that includes the epicentral distance as a parameter manuscript received 11 january 2002

initial rupture of earthquakes in the 1995 ridgecrest california sequence close examination of the p waves from earthquakes ranging in size across several orders of magnitude shows that the shape of the initiation of the velocity waveforms is independent of the magnitude of the earthquake a model in which earthquakes of all sizes have similar rupture initiation can explain the data this suggests that it is difficult to estimate the eventual size of an earthquake from the initial portion of the waveform previously reported curvature seen in the beginning of some velocity waveforms can be largely explained as the effect of anelastic attenuation thus there is little evidence for a departure from models of simple rupture initiation that grow dynamically from a small region the results of this study indicate that any precursory radiation at seismic frequencies must emanate from a source region no larger than the equivalent of a m0 5 event i e a characteristic length of 10 m the size of the nucleation region for magnitude 0 to 5 earthquakes thus is not resolvable with the standard seismic instrumentation deployed in california

the nucleation and rupture process of the 1981 gulf of corinth earthquakes from deconvolved broad band data summary source parameters of the largest three normal faulting earthquakes ms6 6 6 3 6 4 in the 1981 gulf of corinth greece sequence are determined using deconvolved broad band data recorded by arrays and single stations and a 2 d finite source model such a model enables the spatial extent rupture velocity and stress drop of the earthquakes to be determined and geological observations of surface slip can be included as a further constraint on the waveform modelling all three earthquakes were shallow lo km with low stress drops 30 bars and exhibited source complexity the correspondence between the complexity of the earthquake sources and that of the mapped fault breaks implies that the segmentation of surface faulting in greece is representative of faulting at depth tiny initial pulses which correlate across the arrays are seen in the seismograms from the gulf of corinth earthquakes at most stations these initial subevents 1 per cent of the total moment are interpreted as the breaking of small asperities which initiated the main rupture and are used to constrain the attenuation correction t 0 2 s

examination of scaling between proposed early signals in p waveforms and earthquake magnitudes summary we examine seismic waveforms generated by clusters of similar earthquakes and additional events located on the karadere duzce branch of the north anatolian fault to investigate possible scaling between proposed early signals in the p waveforms and magnitudes the employed signals can be divided into two classes the first is associated with ellsworth beroza and iio type signals that are assumed to reflect signatures of seismic nucleation phases the second are associated with nakamura type period and wu zhao type maximum displacement in the early waveforms that are related to measures of the corner frequency and local magnitude respectively the impact on each signal of artefacts generated by the finite impulse response fir filter is also examined the use of repeating event clusters reduces considerably the variability of path and site effects compared to previous regional and global studies this allows us to focus more accurately on variations of the potential scaling parameters with the source magnitude however the magnitude range of the employed events is limited to about 4 units the fir filter artefacts can generate spurious signals prior to the p wave arrival potentially affecting the ellsworth beroza type signals but are not a significant influence on any of the other measurements the results indicate that candidates for the ellsworth beroza signals associated with identification of a weak arrival before the main p phase exist in less than 20 per cent of the cases the iio type signals can be determined by definition on all waveforms both of these potential signatures of nucleation phases show little or no scaling with the final event size in the examined data in contrast the nakamura and wu zhao type signals in the first few seconds which can also be determined by definition on all waveforms appear to scale with the final event magnitude albeit with large scatter the relatively large time windows used in the analysis compared to the rupture time of the employed events implies that the results provide little if any information on the physical process of the rupture initiation the scaling of the nakamura and wu zhao type early signals with the final event size may be explained by statistical tendency of stronger initial rupture phases to propagate larger distances

the rupture process and asperity distribution of three great earthquakes from long period diffracted p waves moment tensor source mechanisms for the largest twenty nine shallow earthquakes from 1981 m s 6 5 have been determined using long period rayleigh wave spectra and p wave first motions recorded by the international deployment of accelerograms ida global digital seismic network gdsn and world wide standardised seismic network wwssn these are compared with the centroid moment tensor mechanisms presented by dziewonski and woodhouse which were obtained in a contrasting manner in most cases the source is well represented by a single double couple mechanisms with good agreement of scalar moment and orientation between the two studies apparent mismatches for other cases may be interpreted in terms of source complexity we emphasise the objectivity of comparing moment tensor elements apparent discrepancies in the moment and orientation of double couple fault plane solutions are usually due to differences in the mxz and myz moment tensor elements which are poorly constrained by long period data

an excellent fossil wood cell texture with primary uranium minerals at a sandstone hosted roll type uranium deposit nw china abstract a fossil wood cell texture with pitchblende and coffinite found at a sandstone hosted roll type uranium deposit xinjiang nw china is first reported here for the country in the mineralized sandstone detrital grains consisting of quartz feldspar rock fragments carbonaceous trashes mica and accessory minerals were deposited in early turassic time and were cemented by clays and minor authigenic calcite and quartz pitchblende and minor coffinite are principal ore minerals at the deposit and selectively replaced carbonized fossil wood remnants or filled fossil wood cells an excellent fossil wood cell texture with primary uranium minerals formed replacement of organic debris by primary uranium minerals may be due to a local reducing environment resulting from the production of ch 4 h 2 s or h 2 so 3 in its decomposition and a biochemical reaction indicated by the common presence of framboidal pyrite

mapping proterozoic unconformity related uranium deposits in the rockhole area northern territory australia using landsat etm abstract the rockhole area northern territory australia hosts a number of proterozoic unconformity related uranium deposits the geology of the area features within paleoproterozoic rocks of the pine creek orogen near the unconformity with overlying platform cover sandstone of the paleo to mesoproterozoic mcarthur basin landsat enhanced thematic mapper plus etm data was used in the rockhole area to assist in mapping geological structures and lithology and to identify anomalous concentrations of ferrous minerals the product of alteration which can be indicators of buried uranium mineralization several image processing procedures were applied to the etm data to identify isolate and enhance mineralogical information as simple and complex false color composites etm 754 shown as red green and blue respectively was the best simple image overall complex images based on principal component analysis proved to be the most useful products sandstone shale and siltstone the target lithologies koolpin formation the target stratigraphic unit and bleaching pattern due to the removal of iron ii compounds the target alteration pattern were confidently mapped to provide information required by the mineral emplacement model which ultimately identified areas of likely uranium mineralization thus the contrasting behavior of the two principle oxidation states of uranium and iron can be utilized to map delineate bleached alteration zones associated with economic concentrations of uranium using multispectral sensors like landsat or better hyperspectral sensors

the mean age of ocean waters inferred from radiocarbon observations sensitivity to surface sources and accounting for mixing histories a number of previous observational studies have found that the waters of the deep pacific ocean have an age or elapsed time since contact with the surface of 700 1000 yr numerical models suggest ages twice as old here the authors present an inverse framework to determine the mean age and its upper and lower bounds givenglobaloceandataanalysisproject glodap radiocarbonobservations andthey show that the potential range of ages increases with the number of constituents or sources that are included in the analysis the inversion requires decomposing the world ocean into source waters which is obtained here using the total matrix intercomparison tmi method at up to 2 83 28 horizontal resolution with 11 113 surface sources the authors find that the north pacific at 2500 m depth can be no younger than 1100 yr old which is older than some previous observational estimates accounting for the broadness of surface regions where waters originate leads to a reservoir age correction of almost 100 yr smaller than would be estimated with a two or three water mass decomposition and explains some of the discrepancy with previous observational studies a best estimate of mean age is also presented using the mixing history along circulation pathways subject to the caveats that inference of the mixing history would benefit from further observations and that radiocarbon cannot rule out the presence of extremely old waters from exotic sources the deep north pacific waters are 1200 1500 yr old which is more in line with existing numerical model results

strong ground motion from the michoacan mexico earthquake the network of strong motion accelerographs in mexico includes instruments that were installed under an international cooperative research program in sites selected for the high potenial of a large earthquake the 19 september 1985 earthquake magnitude 8 1 occurred in a seismic gap where an earthquake was expected as a result there is an excellent descripton of the ground motions that caused the disaster

global distribution and long term fate of anthropogenic 129i in marine and surface water reservoirs 1 since the advent of the nuclear age in the mid 1940s the mass of radioactive 129i t1 2 15 7 myr circulating in the earth s hydrosphere has increased nearly fortyfold from its natural background level of 140 kg nuclear fuel reprocessing has been by far the major contributor responsible for releasing 5400 kg of 129i primarily into the north atlantic ocean regional and global trends in the distribution of the 129i inventory are elucidated from an examination of more than 600 determinations of 129i in environmental samples from around the world because the major point sources are located in europe and the united states more than 99 of the present 129i reservoir is distributed in the northern hemisphere where both 129i concentrations and 129i i ratios in rivers lakes and shallow seawater are several orders of magnitude above the preanthropogenic background downwelling in the north atlantic presently provides a major sink for marine 129i however marine upwelling along the margins of the pacific will eventually return part of this anthropogenic input to the ocean surface where it will find its way back into surface waters and the atmosphere iodine 129 has a long half life 15 7 myr and consequently there is also the possibility that climate change will influence the dynamics of iodine transfer in surface reservoirs we model the effect of a collapse in thermohaline circulation and project a concentration increase of more than 3 orders of magnitude in shallow oceans over the 10 000 years that follow if nuclear reprocessing is to continue at the present rate

stable isotope constraints on the role of graphite in the genesis of unconformity type uranium deposits the key lake unconformity type uranium deposit occurs in a shear zone where it intersects the unconformity between archean and aphebian gneisses and the overlying proterozoic athabasca group sandstones the roots of the key lake and many other unconformity type uranium deposits in the athabasca basin are close to gneisses rich in graphite and most deposits have small amounts of carbonaceous materials bitumen and hydrocarbon buttons within and around altered basement and sandstone ore zones in many athabasca uranium deposits hydrothermal fluids have destroyed graphite disseminated in the strongly altered gneisses in the vicinity of the deposits prompting some to suggest that graphite was converted to ch4 which reduced and precipitated the uranium and partially condensed to form bitumen the 13c values of graphite collected from unaltered and altered gneisses around the key lake deposit have a limited range 25 5 and are not a function of distance from the mineralization or the intensity of alter

enhanced arabian sea intermediate water flow during glacial north atlantic cold phases abstract during the last glacial period polar ice cores indicate climate asynchrony between the poles at the millennial time scale yet surface ocean circulation in large parts of the globe varied in tune with greenland temperature fluctuations suggesting that any anti phase behavior to a substantial degree must lie in the deeper global ocean circulation which is poorly understood outside the atlantic ocean here we present data from the north western indian ocean which indicate that the timing of maxima in northward extensions of glacial antarctic intermediate water gaaiw coincides with dramatically reduced thermohaline overturn in the north atlantic associated with the heinrich ice surge events he the repeated expansion of the gaaiw during hes recorded far north of the equator in the arabian sea suggests that southern hemisphere driven intermediate water mass variability forms an integral part of the inter hemisphere asynchronous climate change behavior at the millennial time scale

genesis of the sanbaqi deposit a paleokarst hosted uranium deposit in china the sanbaqi uranium deposit in hunan province south china is the largest of a group of paleokarst hosted uranium deposits in lower carboniferous limestone mineralization is localized in cavities and fault breccias formed by dissolution of carbonates four episodes of karst formation are recognized late triassic early jurassic late jurassic early cretaceous cretaceous tertiary and recent field relations indicate that the main uranium mineralization is related to the second karst episode this is supported by isotopic ages of two pitchblende samples at 129 ma and 134 ma as indicated by their nearly concordant data points on concordia plot these ages are in the time range of the early yanshanian tectonic movements that affected southern china and the faulting related to the movements likely triggered the mineralization process at the sanbaqi deposit associated minerals include pyrite millerite ullmannite niccolite molybdenite chalcopyrite sphalerite galena calcite and dolomite fluid inclusion studies on calcite reveal that temperature of ore deposition was from 181 to 150 c the 18o and d values of the ore fluids range from 1 5 to 7 9 per mil and from 30 4 and 54 8 per mil respectively the mineralogical fluid inclusion and isotopic data indicate that the minerlization took place in episodic pulses of hydrothermal fluids that were introduced along a set of ring faults mobilization and redeposition of earlier formed ore minerals in an open system added to the complexity of the paragenetic sequence younger episodes of mineralization occurred during the later karst events as suggested by the geological and additional pitchblende u pb isotopic data during the cretaceous tertiary late yanshanian tectonic movements and recently finally a comparison of the sanbaqi uranium deposit with the uranium deposits hosted by solution collapse breccia pipes of the colorado plateau usa shows that they have many similarities

dynamically and observationally constrained estimates of water mass distributions and ages in the global ocean abstracta data constrained ocean circulation model is used to characterize the distribution of water masses and their ages in the global ocean the model is constrained by the time averaged temperature salinity and radiocarbon distributions in the ocean as well as independent estimates of the mean sea surface height and sea surface heat and freshwater fluxes the data constrained model suggests that the interior ocean is ventilated primarily by water masses forming in the southern ocean southern ocean waters including those waters forming in the antarctic and subantarctic regions make up about 55 of the interior ocean volume and an even larger percentage of the deep ocean volume in the deep north pacific the ratio of southern ocean to north atlantic waters is almost 3 1 approximately 65 of interior ocean waters make first contact with the atmosphere in the southern ocean further emphasizing the central role played by the southern ocean in the regulation of the earth s climate results of the a

do big and little earthquakes start differently can the occurrence of earthquakes be predicted this question has profound social and scientific implications but little is known about what triggers earthquakes or what precursor events might indicate one is about to happen in his perspective vidale reviews recent findings that attempt to resolve the question of whether large earthquakes are initiated by different mechanisms than smaller ones

a composite source model for computing realistic synthetic strong ground motions a composite source model is presented for convolution with synthetic green s functions in order to synthesize strong ground motions due to a complex rupture process of a large earthquake subevents with a power law distribution of sizes are located randomly on the fault each subevent radiates a displacement pulse with the shape of a brune s pulse in the far field at a time determined by a constant rupture velocity propagating from the hypocenter thus all the input parameters have a physical basis we simulate strong ground motions for event station pairs that correspond to records obtained in mexico by the guerrero accelerograph network the synthetic accelerations velocities and displacements have realistic amplitudes durations and fourier spectra

transit time distributions in a global ocean model abstract results from a simulation of the ocean transit time distribution ttd for global and regional ocean surface boundary conditions are presented based on a 5000 yr integration using the parallel ocean program ocean general circulation model the ttd describes the probability that water at a given interior point in the ocean was at some point on the ocean surface a given amount of time ago it is shown that the spatial distribution of ocean ttds can be understood in terms of conventional wisdom regarding time scales and pathways of the ventilated thermocline and the thermohaline circulation driven deep ocean circulation the true mean age from the model the first moment of the ttd is demonstrated to be very large everywhere because of very long tailed distributions regional ttd distributions are presented for distinct surface boundary subregions and it is shown how these can help in the interpretation of the global ttd the spatial structure of each regional ttd is shown to become essential

rupture process of an extended earthquake sequence teleseismic analysis of the chilean earthquake of march 3 1985 broadband displacement and velocity records of p waves recorded at teleseismic distances are analyzed to determine the static and dynamic source parameters of the chilean earthquake of march 3 1985 ms 7 8 and seven large mb 5 6 aftershocks besides the usual parameters of depth moment and focal mechanism the analysis provides estimates of radiated energy associated stresses and source dimension for seven of the eight modeled earthquakes to insure that the parameters are describing the same physical aspect of the rupture process the parameters for each shock are computed from the time window in which most of the seismic energy is radiated the hypocenters of an additional 149 small foreshocks and aftershocks of 4 5 mb 5 6 are computed with the method of joint hypocenter determination the spatial and temporal variation of source parameters and earthquake locations are used to infer a description of the rupture process before during and after the main shock the main shock and modeled aftershocks occurred as thrust faulting on the interface between the nazca and south american plates a cluster of foreshocks in the 10 days preceding the main shock also involved thrust faulting on the plate interface the main shock itself was a complex rupture consisting of three events the first two events denoted as ms1 and ms2 released minor amounts of energy and they occurred on the periphery of the small zone defined by the foreshocks the major release of energy occurred with the third event denoted by ms which nucleated downdip of ms1 and ms2 within the time window of major energy release rupture extended approximately 90 km to the south from the point at which ms nucleated the size of the early aftershock zone which far exceeds our inferred dimensions for the major shock and the strong frequency dependence of scalar moment imply that substantial slow slip occurred on the plate interface that was not associated with major energy release the dip of the seismically active interface increases landward from about 15 at 20 km depth to about 35 at 40 km depth where the seismogenic interface reaches its maximum observed depth the vast majority of small and moderate aftershocks occurred in the shallower part of the interface as did the foreshocks ms1 and ms2 it appears however that the asperities that control the rupture of the largest earthquakes are the asperities on the deeper interface ms and the largest 1985 aftershock that of april 9 ms 7 2 occurred on the deeper interface the chilean earthquake of july 9 1971 ms 7 5 which occurred just north of the 1985 earthquake also involved main shock rupture on the deeper interface and was followed by aftershocks on the shallower interface asperities on the deeper interface appear on average to be stronger larger and more uniform in size than asperities on the shallow interface a greater average strength for deeper asperities is suggested by the observation that stress drops and apparent stresses of the main shock and the after shock of april 9 ms 7 2 are higher than those of the large shallow aftershocks and is consistent with arguments that the maximum friction on a fault zone should be near the base of the seismogenic zone one deep interface aftershock with low stress drop and apparent stress may have been situated below the point of maximum friction on the interface the inferred larger and more uniform sizes of the deeper asperities are suggested by the higher proportion of large to small shocks on the deeper interface the downdip change in the asperity size distribution may reflect increased friction on the plate interace or the coalescence of small asperities into large asperities with increased subduction the radiated energies of the main shock and large aftershocks that are computed directly from the broad bandwidth data are smaller than those predicted by standard empirical formulas by factors ranging from 4 to 20

ventilation of the deep ocean constrained with tracer observations and implications for radiocarbon estimates of ideal mean age abstract ocean ventilation is the process that transports water and climatically important trace gases such as carbon dioxide from the surface mixed layer into the ocean interior quantifying the dominant source regions and time scales remains a major challenge in oceanography a mathematically rigorous approach that accounts for the multiplicity of transport pathways and transit times characteristic of an eddy diffusive flow such as the ocean is to quantify ventilation in terms of a probability distribution that partitions fluid parcels according to the time and location of their last surface contact here we use globally gridded radiocarbon data in combination with other transient cfcs and hydrographic temperature salinity phosphate and oxygen tracer data to estimate the joint distribution of age and surface origin of deep ocean waters our results show that 40 and 26 of the global ocean was last in contact with the southern ocean and north atlantic respectively some 80 of the global deep ocean below 1500 m is ventilated from these high latitude regions however contrary to the classical description of the deep ocean as a roughly equal mixture of northern and southern source waters we find a significantly higher contribution from the southern ocean relative to the north atlantic we estimate the mean transit time from the surface to the deep north pacific at 1360 350 y intermediate between two widely used radiocarbon based estimates to reconcile our estimate of the ideal mean age with ventilation age estimates based on radiocarbon we apply the estimated distribution function to construct a 3 dimensional distribution of the water mass fraction weighted surface initial radiocarbon concentration that can serve as an accurate reservoir age radiocarbon ages corrected for this initial reservoir age are found to be in good agreement within 5 with our ideal age estimate demonstrating that it is essential to take into account the spatially variable surface radiocarbon field when computing ventilation ages using radiocarbon a wide spectrum of ages contributes to the mean age providing evidence for the fundamentally eddy diffusive nature of the large scale general circulation of the ocean

transit time and tracer age distributions in geophysical flows transport in the atmosphere and in the ocean is the result of the complex action of time dependent and often highly turbulent flow a useful diagnostic that summarizes the rate at which fluid elements are transported from some region to a point or the reverse via a multiplicity of pathways and mechanisms is the probability density function pdf of transit times the first moment of this pdf often referred to as mean age has become an important transport diagnostic commonly used by the observational community this paper explores how to probe the flow with passive tracers to extract transit time pdf s as a foundation the literal tracer age is defined as the elapsed time since tracer was injected into the flow and the corresponding tracer age distribution z as the fractional tracer mass in a given interval of tracer age the distribution z has concrete physical interpretation for arbitrary sources but is only equivalent to a tracer independent transit time pdf of the flow in special cases the transit time pdf is a propagator g9 of boundary conditions the age spectrum of t m hall and r a plumb applied over a control surface v the propagator g9 is shown to be the flux into v resulting from a unit mass injected into the time reversed flow through explicit construction of the transit time pdf using the concept of tracer age the special cases for which z and g9 coincide are established this allows a direct physical demonstration of g9 and its adjoint g9 as the pdf s of transit times since fluid at point r had last contact with v and until fluid at r will have first contact with v respectively in the limit as v is shrunk to a point point to point transit time pdf s are well defined but their mean transit time and higher order moments become infinite several concrete geophysical examples are considered to illustrate under what conditions characteristics of tracer age and transit time pdf s can be inferred from observations in the atmosphere or the ocean

ages of major uranium mineralization and lead loss in the key lake uranium deposit northern saskatchewan canada uranium and lead isotope ratios were measured on nine ore samples and four mineral separates from the deilmann orebody of the key lake uranium nickel deposit the time of primary mineralization is the oldest reported for proterozoic unconformityrelated uranium deposits in the athabasca basin data obtained in this study show that lead loss or uranium gain has occurred in highgrade rocks and that lead gain or uranium loss has occurred in low grade rocks lead loss rather than uranium gain in high grade ore is suggested by the petrologic work of cumming and rimsaite and the presence of excess radiogenic lead in the overlying sandstone documented by curtis and gancarz some of the results obtained in this study provide evidence of preferential movement of sup 238 u intermediate daughters out of uranium minerals and into the matrix

propagation pathways of classical labrador sea water from its source region to 26 n 1 more than two decades of hydrography on the abaco line east of the bahamas at 26 n reveals decadal variability in the salinity of classical labrador sea water clsw despite the long distance from its source region in the north atlantic ocean hydrographic time series from the labrador sea and from the abaco line show a pronounced step like decrease in salinity between 1985 and 1995 in the labrador sea and between 1995 and 2010 at the abaco line suggesting a time lag between the two locations of approximately 9 years the amplitude of the anomaly at the abaco line is 50 of the amplitude in the labrador sea a similar time lag and reduction of amplitude is found in the high resolution ofes model in which salinity anomalies can be observed propagating through the deep western boundary current as well as through a broad interior pathway on its way south to the abaco line the clsw becomes 8 standard deviations saltier due to isopycnal mixing with mediterranean outflow water mow climatological data in the north atlantic suggests that the mixing ratio of mow to clsw at the abaco line is 1 4 and that no variability in mow is required to explain the observed variability at the abaco line the data studied here suggest that decadal clsw anomalies stay relatively coherent while getting advected despite the important role of interior pathways

the age of water and ventilation timescales in a global ocean model abstract the age of water in the world ocean is studied using a passive age tracer introduced into a global ocean model additional information is derived from a transient dye tracer that tracks the time dependent spreading of surface waters into the model ocean interior of particular interest is the nature of ocean ventilation over the 10 100 yr timescale as well as the simulated age of deep and bottom water masses in the upper model levels young water is found to correspond with regions of convergence and downwelling in the surface ekman layer upwelling and convection are both shown to age the upper ocean by entraining older waters into the surface mixed layer in the deep model levels water age varies greatly between oceans with young water found in convectively active regions in the north atlantic and in the ross and weddell seas and old water found in the deep north pacific the oldest water mass mixture located at 2228 m depth in the western pacific ocean is dated at 1494 years made

on ocean transport diagnostics the idealized age tracer and the age spectrum abstract the idealized age tracer is commonly used to diagnose transport in ocean models and to help interpret ocean measurements in most studies only the steady state distribution the result of many centuries of model integration has been presented and analyzed however in principle the transient solution provides more information about the transport here it is shown that this information can be readily interpreted in terms of the ventilation histories of water masses a simple relationship is derived valid for stationary transport between the transient evolution id r t of the idealized age tracer and the age spectrum g r t the distribution of times t since a water mass was last ventilated namely g r t tt id r t implications of the relationship are discussed and the relationship is illustrated with an idealized model

source time functions of large mexican subduction earthquakes morphology of the benioff zone age of the plate and their tectonic implications we study source parameters of large shallow mexican subduction zone earthquakes 95 w to 106 w which occurred between 1928 and 1986 by modeling the p waves recorded on galitzin wilip seismograph in debilt dbn holland for post 1962 events the source parameters retrieved from dbn seismograms alone agree well with those obtained from long period world wide standardized seismograph network records giving us confidence in our results for pre 1962 events all earthquakes are shallow h 10 to 20 km with few exceptions the sources in oaxaca 95 w to 99 w are very simple to the northwest of 99 w they are simple as well as complex the ratio of surface wave to body wave seismic moment mos mop which is a measure of long to short period radiation is smaller in oaxaca 1 5 0 5 than in the regions northwest of 99 w 3 1 1 3 these results suggest a change in the plate interface characteristics near 99 w the sharp change in the rupture mode and the intersection of the o gorman fracture zone ofz with the trench occur near 99 w two strike slip events offshore close to ofz suggest a segmentation of the subducting plate near 99 w the age of the plate near the trench in oaxaca is not well known it is possible that it does not increase continuously from northwest to southeast in the region but jumps across 99 w if so then the older age of the subducted plate southeast of 99 w may be the cause of the distinct rupture mode of the oaxaca earthquakes the length of the benioff zone which is greatest below oaxaca 400 km and decreases toward the northwest can be explained by the correlation between the length of the subducted slab and the product of the lithosphere age and convergence rate the relative complexity of sources the weaker background seismicity and the lesser number of aftershocks northwest of oaxaca may be explained by a stronger interface coupling resulting from subduction of younger oceanic slabs 5 to 13 m y old in this region this however explains neither larger mos mop values northwest of oaxaca nor the low stress drop estimates obtained from the analysis of near field strong motion data for the michoacan earthquake of 1985 both of which indicate weaker coupling of the interface thus the issue of whether subduction of very young plates 10 m y old results in strong or weak coupling remains unsolved from the presently available mexican data

the calculated solubility of platinum and gold in oxygen saturated fluids and the genesis of platinum palladium and gold mineralization in the unconformity related uranium deposits thermodynamic calculations on the solubility of platinum and gold indicate that saline 1 m nacl fluids saturated with atmospheric oxygen can transport geologically realistic concentrations of platinum group elements pge gold and uranium as chloro complexes a number of calculations involving fluid rock interaction suggest that the oxygen saturated fluids flowing through rocks containing quartz muscovite kaolinite magnetite and hematite initially oxidize any magnetite to hematite allowing subsequent batches of ore fluids to retain their high oxidation state during their migration through the aquifer the oxidizing fluids would move the oxidation reduction interface deeper into the aquifer leaching and redepositing platinum and gold the redissolution of earlier precipitated platinum and gold depends on the fluid rock ratio and the associated increase in the oxidation state therefore lowering of fluid rock ratios and or mixing of the oxidized fluids with a large amount of reduced fluid will precipitate uranium pge and gold it is suggested that this model can explain the genesis of gold and pge mineralization in the unconformity related uranium deposits of the alligator rivers uranium field in the northern territory australia

changes in the hoar isotope composition of clays during retrograde alteration abstract k ar ages of illite alteration associated with middle proterozoic athabasca unconformity type u deposits in saskatchewan range from 414 to 1493 ma the k ar ages correlate with water contents and d values such that illites with young k ar ages have d values as low as 169 and water contents as high as 7 7 wt whereas illites with older ages have d values near 70 and water contents near 4 wt water extracted at 400 c from illites with low d values and high water contents has low d and 18 o values similar to those of modern meteoric water suggesting that some of the illites associated with the original deposition of the ore underwent varying degrees of retrograde alteration the alteration is initiated by hydration of sites in the interlayer region of the illite which results in the partial resetting of the k ar ages and introduction of excess structural water in the form of interlamellar water the interlamellar water is enriched in 18 o by about 7 per mil relative to the water that physically surrounded the clay particle further alteration decreases the d value and increases the 18 o value of the illite by isotopic exchange between the mineral and the interlamellar water although the chemical compositions and xrd patterns of the altered illites indicate that no detectable smectite component is present in the samples the isotopic results suggest that the altered illites may be an early precursor in the formation of mixed layer illite smectite by retrograde alteration of pure illite the wide variation of d values of chlorite and kaolinite from these u deposits is analogous to that of the illite suggesting that retrograde alteration of clays by meteoric water can be substantial the general association of altered clays with areas containing the highest concentrations of u is probably related to localized permeability within the ore zone

the rupture process of the armenian earthquake from broad band teleseismic body wave records summary the destructive earthquake of december 1988 in armenia m 6 9 was recorded on broad band and very long period channels at teleseismic distances by geoscope and gdsn networks these records are well distributed in azimuth and allow a detailed study of the rupture process of this earthquake the average focal mechanism obtained by p and sh wave modelling c i 300 f lo 6 63 f 5 a 100 f 20 is compatible with the mechanism obtained from very long period surface waves and intermediate period single station determinations as well as field observations the mean depth of the rupture is also fixed by waveform modelling between 5 and 7 km which means that the rupture surface extends from the surface to a depth between 10 and 14 km in agreement with aftershock depth distribution the rupture is found to be complex composed of a weak beginning or a small foreshock and two pulses well separated in time the time delay between the two events is estimated for each station by waveform modelling and by spectral analysis the azimuthal variation of this time delay is interpreted in terms of direction and velocity of rupture on the fault plane a more detailed analysis of the source implies the use of additional information coming from aftershock studies and tectonics we use forward modelling to investigate several rupture mechanisms a three source model gives an acceptable fit to the observed records but the western mechanism is at odds with observed tectonics and furthermore rupture propagation is not well simulated a five segment model of the source obtained from field seismotectonic data gives a better waveform fit a time sequence of individual breaks that simulates a rupture propagating away from the hypocentre and subsource mechanisms that are compatible with surface tectonics

seismic source parameters from the guerrero subduction zone abstract a method to systematically determine seismic moment and corner frequency from shear wave spectra has been applied to strong motion data recorded in guerrero mexico the method determines a model spectrum that minimizes the misfit from the observed spectrum after a correction for frequency dependent site response has been applied the calculated moments for larger events in guerrero are consistent with moments determined from teleseismic body waves adjusted for differences in focal depth and consequent assumed differences in the velocity and density structure in the source region the average stress drop for the 82 earthquakes with magnitudes from 3 to 7 is 150 bars the spatial distribution of the stress drops shows a distinctive pattern at the northwest end of the guerrero gap adjacent to the aftershock zones of large earthquakes in 1979 and 1985 there is a mix of low and high stress drop events at the southeast end of the guerrero gap where the most recent large earthquake occurred in 1957 there are predominantly high stress drop events this pattern might be caused by the different relative positions of these two active regions in the earthquake cycle although several other explanations cannot be ruled out

how long to oceanic tracer and proxy equilibrium abstract the various time scales for distribution of tracers and proxies in the global ocean are critical to the interpretation of data from deep sea cores to obtain some basic physical insight into their behavior a global ocean circulation model forced to least square consistency with modern data is used to find lower bounds for the time taken by surface injected passive tracers to reach equilibrium depending upon the geographical scope of the injection major gradients exist laterally between the abyssal north atlantic and north pacific and vertically over much of the ocean persisting for periods longer than 2000 years and with magnitudes bearing little or no relation to radiocarbon ages the relative vigor of the north atlantic convective process means that tracer events originating far from that location at the sea surface will tend to display abyssal signatures there first possibly leading to misinterpretation of the event location ice volume glacio eustatic corrections to deep sea 18 o values involving fresh water addition or subtraction regionally at the sea surface cannot be assumed to be close to instantaneous in the global ocean and must be determined quantitatively by modelling the flow and by including numerous more complex dynamical interactions

size dependent microearthquake initiation inferred from high gain and low noise observations at nikko district japan we deployed a temporary seismic station at a low noise site in the nikko district japan in november 1993 and analyzed velocity waveforms of 17 microearthquakes 0 3 m 2 1 with small focal distances 11 km we compared the initial parts of the p wave velocity seismograms on the same absolute scale after correcting for the effects of geometrical spreading and radiation pattern we find that the p wave radiated from an earthquake of a larger final magnitude has a greater amplitude than that from a smaller earthquake even at a very early stage when the p wave from the smallest event is still rising with an increasing slope this observation does not necessarily mean that the initiation of the rupture is dependent on the eventual size because anelastically attenuated waveforms of earthquakes with short time duration could show such an apparent trend hence we computed synthetic seismograms to test a group of source scaling models in which source time function grows independently of its eventual size taking into account anelastic attenuation and instrument response we find that such scaling models do not easily reproduce simultaneously the size dependence of the initial rise and the pulse duration of the observed waveforms though poor data coverage above m1 5 prevents us from deriving a firm conclusion a model with size dependent growth rate is able to match our observations

a stochastic model for fracture growth on a heterogeneous seismic fault summary we present a model for the propagation of seismic fracture on a heterogeneous fault on which the strength and applied stress vary locally along the fault plane in a random fashion the propagation of fracture is controlled by three macroscopic parameters the average of the difference between the applied stress and fault strength a measure of fault heterogeneity and the average coupling between elementary faults evolution of fracture can then be represented by an epidemics model whose domains of slip are obtained from percolation theory the geometry of these domains contains the principal features of both barrier and asperity models

mineralogical and geochemical constraints on the genesis of the granite hosted huangao uranium deposit se china abstract the huangao uranium deposit located in jiangxi province se china is hosted mainly by the indosinian medium to coarse grained porphyritic biotite granite as a batholith sanerer pluton and to a lesser extent by the yanshanian fine grained porphyritic biotite granite as stock the indosinian granite has a whole rock rb sr isochron age of 215 6 ma and the yanshanian granite has a whole rock rb sr isochron age of 155 7 6 4 ma both granites may be formed by anatexis of uranium rich precambrian and cambrian metasedimentary rocks deuteric alteration of primary accessory minerals zircon sphene apatite allanite monazite and xenotime in the granites and erosion of the granites under a hot dry palaeoclimate resulted in the partial liberation of uranium these processes were critical for the formation of the mesothermal to epithermal u deposit uranium in the deposit occurs as pitchblende along tectonic fractures interstitially in granular minerals and as cements of breccias the host rock alteration comprises pre ore alkali metasomatism and ore stage hydrothermal chloritization pyritization hematitization silicification damouritization and carbonatization other minerals present in the ore include pyrite galena sphalerite chalcopyrite hematite microcrystalline quartz chlorite calcite and fluorite fluid inclusion studies on quartz fluorite and calcite reveal that temperatures of the mineralizing fluids range from 110 to 260 c and give salinity values ranging from 11 7 to 21 6 wt nacl equivalent the 18 o and d values of the fluids range from 4 0 to 3 6 and from 81 3 to 56 6 respectively indicating meteoric water mineralogical fluid inclusion and isotopic data are consistent with a mesothermal to epithermal origin for the huangao deposit thermal activity at the huangao deposit was maintained from the emplacement of the indosinian granite at 215 ma until the intrusion of the lamprophyre dike at 105 ma magmatism in the district as well as radioactive decay of k u and th in the granite may have provided an essential or additional heat source for the formation of the hydrothermal solutions the u bearing hydrothermal solution was also heated and deep convective circulation was mainly driven by the regional tectonism this is supported by the u pb isochron age of four pitchblende samples at 48 ma which is in the time range of the late yanshanian regional tectonism that affected south china

implications of a composite source model and seismic wave attenuation for the observed simplicity of small earthquakes and reported duration of earthquake initiation phase an examination of p waves recorded on near source velocity seismograms generally shows that most small earthquakes mw 2 to 3 are simple on the other hand larger earthquakes mw 4 are most often complex the simplicity of the seismograms of mw 2 to 3 events may reflect the simplicity of the source and hence may imply that smaller and larger earthquakes are not self similar or may be a consequence of attenuation of seismic waves to test whether the attenuation is the cause we generated synthetic p wave seismograms from a composite circular source model in which subevent rupture areas are assumed to follow a power law distribution the rupture of an event is assumed to initiate at a random point on the fault and to propagate with a uniform speed as the rupture front reaches the center of a subevent patch all of which are circular a p pulse is radiated that is calculated from the kinematic source model of sato and hirasawa 1973 synthetic p wave seismograms which are all complex are then convolved with an attenuation operator for different values of t the results show that the observed simplicity of small events mw 2 to 3 may be entirely explained by attenuation if t 0 02 sec the composite source model predicts that the average time delay between the initiation of the rupture and the rupture of the largest patch scales as m 1 3 such that log 1 3 log m 8 462 this relation is very similar to that reported by umeda et al 1996 between m and the observed time difference between the initiation of the rupture and the rupture of the bright spot it roughly agrees with the relation between m and the duration of the initiation phase reported by ellsworth and beroza 1995 and beroza and ellsworth 1996 the relation also fits surprisingly well the data on duration of slow initial phase t sip and m reported by iio 1995 one possible explanation of this agreement may be that the composite source model which is essentially the cascade model successfully captures the evolution of the earthquake source process and that the rupture initiation and the abrupt increase in the velocity amplitude observed on seismograms by previous researchers roughly corresponds to the rupture of the first subevent and the breaking of the largest subevent in the composite source model

seismicity on a fault controlled by rate and state dependent friction with spatial variations of the critical slip distance 1 we perform systematic simulations of slip using a quasi dynamic continuum model of a two dimensional 2 d strike slip fault governed by rate and state dependent friction the depth dependence of the a b and l frictional parameters are treated in an innovative way that is consistent with available laboratory data and multidisciplinary field observations various realizations of heterogeneous l distributions are used to study effects of structural variations of fault zones on spatiotemporal evolution of slip we demonstrate that such realizations can produce within the continuum class of models realistic features of seismicity and slip distributions on a fault we explore effects of three types of variable l distributions 1 a depth dependent l profile accounting for the variable width of fault zones with depth 2 uncorrelated 2 d random distributions of l with different degrees of heterogeneity and 3 a hybrid distribution combining the depth dependent l profile with the 2 d random l distributions the first type of l distribution with relatively small l over the depth range corresponding to the seismogenic zone and larger l elsewhere generates stick slip events in the seismogenic zone and ongoing creep above and below that region the 2 d heterogeneous parameterizations generate frequency size statistics with event sizes spanning 4 orders of magnitude our results indicate that different degrees of heterogeneity of l distributions control 1 the number of simulated events and 2 the overall stress level and fluctuations other observable trends are 3 the dependency of hypocenter location on l and 4 different nucleation phases for small and large events in heterogeneous distributions

the bright spot of an earthquake abstract umeda y 1992 the bright spot of an earthquake in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 13 22 field surveys in the source region for some large shallow earthquakes reveal that there is a special spot characterized by high acceleration aftershock gap and parallel faults system this spot is called an earthquake bright spot which occupies a relatively small region compared to the final rupture region at least two kinds of phases p1 and p2 which are responsible for the rupture initiation and the bright spot formation can be identified on the broad band seismograms for large earthquakes duration times of preliminary waves p1 p2 are proportional to the earthquake magnitudes by using this evidence from the field and seismograms a model having three rupture stages is proposed for a growth process of large earthquakes the 1st stage is an initiation and propagation of rupture the 2nd stage is formation of the bright spot which produces the high frequency seismic waves in the 3rd stage smooth dislocations again propagate in bi laterally and radiate the long period seismic waves

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

the 1992 nicaragua earthquake a slow tsunami earthquake associated with subducted sediments the 1992 nicaragua earthquake was a tsunami earthquake that is it generated tsunamis disproportionately large for its surface wave magnitude m s the moment magnitude m w determined from long period 250 s surface waves was 7 6 significantly larger than the 20 s m s of 7 this m s m w disparity is also characteristic of tsunami earthquakes the nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms we conclude that the nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the cocos and north american plates and because of the absence of sediments on the trench floor offshore of nicaragua the slip propagated up dip all the way to the ocean bottom exciting large tsunamis the occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction zone thrust earthquakes our results reinforce the idea that tsunami warning systems using long period 100 s waves are necessary to reduce the hazard from this type of earthquake

site response attenuation and source spectra of s waves along the guerrero mexico subduction zone acceleration spectra from nine earthquakes and up to 11 stations per earthquake are inverted to find the source spectra the site response and the quality factor q of s waves this method is applied to moderate earthquakes 4 0 0 8 hz does not follow a simple linear relation as observed in previous studies of attenuation in other regions when the source spectra are interpreted in terms of brune s model the stress drops obtained are less than 20 bars for the four larger events analyzed the site functions obtained indicate that although all the stations used are on rock the effect of the near surface geology caused important amplifications at some stations

beginnings of earthquakes in the mexican subduction zone on strong motion accelerograms abstract the guerrero mexico accelerograph network has recorded at short distances earthquakes with magnitudes from 3 0 to 8 1 in this article the initial 3 5 sec of the p waves on the vertical component are compared in addition to unfiltered accelerograms several causal bandpass filters are applied we do not see significant differences in either original or filtered records between the initial e g 0 5 sec seismograms of moderate and large earthquakes in the original and 2 to 5 hz passband the main differences among events with magnitudes over about 4 5 is in the duration of shaking the initial ground motions of all the large events have very small amplitudes which gradually get larger at lower frequencies seismograms of the largest events show a complex series of multiple pulses assumed related to failure of numerous asperities in our lowest frequency band 0 2 to 0 5 hz most of the large events that we examined begin with rupture of asperities that are not necessarily larger and are sometimes smaller than the asperities that fail during smaller events the observations are consistent with a model of faulting in which the largest asperities are located at random on the eventual fault plane and fail when the rupture front propagating from the hypocenter reaches them this allows the largest asperities to occasionally be at the hypocenter and to fail immediately but for a large fault it is more likely that they will be located elsewhere and fail later

characterizing transport between the surface mixed layer and the ocean interior with a forward and adjoint global ocean transport model the theory of first passage time distribution functions and its extension to last passage time distribution functions are applied to the problem of tracking the movement of water masses to and from the surface mixed layer in a global ocean general circulation model the first passage time distribution function is used to determine in a probabilistic sense when and where a fluid element will make its first contact with the surface as a function of its position in the ocean interior the last passage time distribution is used to determine when and where a fluid element made its last contact with the surface a computationally efficient method is presented for recursively computing the first few moments of the first and last passage time distributions by directly inverting the forward and adjoint transport operator this approach allows integrated transport information to be obtained directly from the differential form of the transport operator without the need to perform lengthy multitracer time integration of the transport equations the method which relies on the stationarity of the transport operator is applied to the time averaged transport operator obtained from a three dimensional global ocean simulation performed with an ogcm with this approach the author i computes surface maps showing the fraction of the total ocean volume per unit area that ventilates at each point on the surface of the ocean ii partitions interior water masses based on their formation region at the surface and iii computes the three dimensional spatial distribution of the mean and standard deviation of the age distribution of water

strong ground motion from the uttarkashi himalaya india earthquake comparison of observations with synthetics using the composite source model the uttarkashi earthquake of 19 october 1991 ms 7 0 occurred in the greater himalayan region north of the main central thrust at an estimated depth of 12 km the fault plane solution indicates a low angle thrust mechanism striking northwest consistent with the tectonic pattern of thrusting in the region aftershocks define a belt parallel to and north of the surface trace of the main central thrust roughly 10 km wide and 30 km long the mainshock is located at the northeast edge of this zone the earthquake was recorded on 13 strong motion accelerographs at distances ranging from 25 to 150 km from the epicenter one station at bhatwari peak horizontal acceleration of 272 cm sec 2 is above the aftershock zone the maximum peak horizontal acceleration was about 313 cm sec 2 at uttarkashi at an epicentral distance of 36 km the amplitudes and frequency content of the strong ground motions are more or less consistent with expectations for an earthquake of this magnitude in california synthetics generated using the composite source model and synthetic green s functions zeng et al 1994a b are successful in producing acceleration velocity and displacement with a realistic appearance and the correct statistical properties of the two accelerograms recorded nearest the fault bhatwari and uttarkashi to produce these we introduced trial and error modifications of the layered medium velocity model within uncertainties at more distant stations we first used the velocity structure that worked for the two nearest stations differences emphasize the large potential role of unknown site and wave propagation effects for the station at tehri we explored different velocity models and found one there that was also quite successful we then used these two velocity models to predict strong ground motions at bhatwari and tehri from a potential magnitude 8 5 earthquake filling part of the seismic gap along the himalayan frontal faults the synthetics show peak accelerations that are only somewhat larger than those in the uttarkashi event but much longer durations and increased amplitudes of response spectra at long periods

examination of scaling between earthquake magnitude and proposed early signals in p waveforms from very near source stations in a south african gold mine 1 we analyze near source seismograms recorded in a deep south african mine for scaling between signals early in p waveforms and the magnitude m of the events the data consist of recordings at 26 stations of 122 events with 1 5 0 implying that the final earthquake size is affected statistically by the initial rupture process if these correlations hold for larger earthquakes as suggested by other studies the early predominant period and peak displacement are useful for early warning systems

a generalized transport theory water mass composition and age abstract a general theory to describe and understand advective and diffusive ocean transport is reported it allows any passive tracer field with an atmospheric source to be constructed by superposing sea surface contributions with a generalized green s function called the boundary propagator of the passive tracer equation the boundary propagator has the interpretation of the joint water mass and transit time distribution from the sea surface the theory thus includes the classical oceanographic idea of water mass analysis and extends it to allow for a distribution of transit times from the sea surface the joint water mass and transit time distribution contains complete information about the transport processes in the flow it captures this information in a more accessible way than using velocity and diffusivity fields however at least for the case of sequestration and transport of dissolved material by the ocean circulation the boundary propagator is thus the natural quantity to consider when discus

the age of unconformity related uranium mineralization in the athabasca basin northern saskatchewan age data are presented for major athabasca basin uranium deposits at cigar lake cluff lake collins bay dawn lake eagle point mcarthur river midwest and rabbit lake as well as for several minor or undeveloped deposits including hughes lake and nisto the best constrained data indicate that almost all the deposits formed in a restricted time interval between about 1330 and 1380 ma this range of ages is believed to be real and not the result of uncertainties in the calculation of ages based on discordant data the one major exception is the recently discovered nias free deposit at mcarthur river for which a well determined age of 1514 18 ma 2 has been obtained even this deposit yields an age in the1330 1380 ma range for some material periods of reworking redeposition occurred at 1280 1000 575 and 225 ma these may be basin wide affecting to some degree all the deposits that we have studied other times of redeposition are less well determined but may be present as well no ages tha

asperity distributions and large earthquake occurrence in subduction zones abstract ruff l j 1992 asperity distributions and large earthquake occurrence in subduction zones in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 61 83 plate tectonics and the seismic gap hypothesis provide the framework for long term earthquake forecasting of plate boundary earthquakes unfortunately detailed examination reveals that earthquake recurrence times and rupture length vary between successive earthquake cycles in the same subduction zone furthermore larger coseismic slip is commonly associated with larger rupture length hence large earthquake occurrence in subduction zones is characterized by variability in 1 recurrence times 2 rupture length and 3 coseismic slip these facts plus many other observations indicate that there are significant spatial variations in the strength of the plate interface one simple description of these variations and their role in the earthquake cycle is the asperity model where the large strong regions of the plate interface are called asperities and the large earthquakes occur when the large asperities break the asperity model of earthquake occurrence is able to qualitatively explain several features of large plate boundary earthquakes to go beyond general qualitative notions i pose the following scientific test are the observed asperity distributions and a simple model of their interaction self consistent with the above three observed features of large earthquake occurrence the distribution of the major asperities along plate boundary segments has now been determined for several subduction zones rupture process studies of adjacent large and great earthquakes have provided reliable estimates of the along strike asperity lengths and separations for several adjacent asperities in the kurile islands colombia and peru subduction zones the simplest mechanical model for asperity interaction is to idealize two adjacent asperities as frictional sliders that are connected by main springs to the upper plate by a coupling spring to each other and maintain frictional contact with a conveyer belt the lower plate that moves with a constant velocity an earthquake occurs when the net force on the asperity frictional slider reaches some specified level the failure force and spring constants are determined by the observed asperity distribution and simple models of elastic interaction two different macroscopic failure criteria are used this simple mechanical model displays a remarkable range of behavior from simple to complex when the two asperities are identical in all their properties sequences of identical earthquakes are produced for the more realistic case of non identical asperities earthquake sequences show great variety using system variables from the observed asperity distributions the earthquake sequences typically display 1 variable recurrence times 2 variable rupture length i e a combination of single asperity and double asperity failures and for one of the failure criteria 3 larger coseismic slip for double asperity failures statistical summaries of thousands of simulated earthquake sequences for asperity pairs in the kuriles colombia and peru subduction zones are broadly consistent with the observed features of large earthquake occurrence in these subduction zones the main conclusion is that the asperity model provides a self consistent explanation for fault zone heterogeneity the rupture process and recurrence times and rupture mode of large earthquake sequences via a simple model for adjacent asperity interaction in addition a conclusion independent of any particular model for fault zone heterogeneity is that simple deterministic models of fault zone interaction can explain complex patterns of large earthquake occurrence in subduction zones

seismicity in the western great basin apparently triggered by the landers california earthquake 28 june 1992 within 24 hr after the landers earthquake there were three magnitude 3 4 events in western nevada and an unexpected widespread increase in the rate of small events based on combined catalogs for northern nevada southern nevada and southern california and a model that assumes statistical independence of events in these regions the probability of this happening in a 24 hr period by random chance is less than 10 12 per day therefore there is high statistical confidence that the increased seismicity was triggered by the landers event based on the statistical model we develop a list of 227 earthquakes in the first 83 days following the landers earthquake each of which has no more than 10 probability of occurring by random chance the suspect events are broadly distributed in regions that correlate with historical activity in the great basin the events are not uniquely associated with known volcanic activity or with zones that were previously active with microearthquakes or aftershock sequences the magnitudes of the largest triggered events appear to decrease with distance with time the number of suspect events decreases at a rate comparable to the rate of decrease of aftershocks of the landers and big bear earthquakes three of the most significant triggered events that that occurred were as follows mina 500 km from landers m 4 36 min after landers smith valley 590 km from landers m 3 4 56 min after landers and little skull mountain 280 km from landers m 5 6 22 3 hr after landers the evidence for triggering is particularly strong in the case of the little skull mountain event where an increased rate of microseismicity was evident as soon as small events could be identified in the coda of the landers earthquake we evaluate the seismic history for the past 3 yr to understand what was unique about the landers earthquake this includes identifying the previous earthquakes most likely to have caused the strongest shaking in various frequency bands based on a simple screening model the strains from the landers earthquake were uniquely large at low frequencies but at high frequencies they were exceeded frequently by small more local events thus we hypothesize that the cause of triggering is low frequency dynamic strains at periods of about 10 sec or greater and that there is a regional threshold that must be exceeded since previous events with calculated strains that were almost as strong were not causes of widespread triggering this mechanism for triggering satisfies the criterion of being a relatively rare phenomenon since it is likely to occur only when a large long period wave is radiated into an area where strain has been building slowly toward the point where faults are unstable

depth dependence of earthquake frequency magnitude distributions in california implications for rupture initiation statistics of earthquakes in california show linear frequency magnitude relationships in the range of m2 0 to m5 5 for various data sets assuming gutenberg richter distributions there is a systematic decrease in b value with increasing depth of earthquakes we find consistent results for various data sets from northern and southern california that both include and exclude the larger aftershock sequences we suggest that at shallow depth 0 to 6 km conditions with more heterogeneous material properties and lower lithospheric stress prevail rupture initiations are more likely to stop before growing into large earthquakes producing relatively more smaller earthquakes and consequently higher b values these ideas help to explain the depth dependent observations of foreshocks in the western united states the higher occurrence rate of foreshocks preceding shallow earthquakes can be interpreted in terms of rupture initiations that are stopped before growing into the mainshock at greater depth 9 15 km any rupture initiation is more likely to continue growing into a larger event so there are fewer foreshocks if one assumes that frequency magnitude statistics can be used to estimate probabilities of a small rupture initiation growing into a larger earthquake then a small m2 rupture initiation at 9 to 12 km depth is 18 times more likely to grow into a m5 5 or larger event compared to the same small rupture initiation at 0 to 3 km

formation and evolution of the cigar lake uranium deposit based on u pb and k ar isotope systematics a systematic geochronological study was conducted on the cigar lake uranium deposit u pb and pb pb systematics were applied to different types of uranium ore and various associated sheet silicates were dated by the k ar method these two approaches define a widespread retrograde metamorphic event in the basement which occurred at about 1780 ma during the hudsonian orogeny and a four stage evolutionary model for the ore deposit this model is compatible with the history of other unconformity type uranium deposits in the athabasca basin the first stage in the evolution of the cigar lake uranium deposit is marked by uraninite crystallization at 1341 17 ma which in association with arsenides and sulpharsenides constitutes the mineralization of economic interest main orebody the second stage characterized by a fe kaolinite and fe illite paragenesis occurred locally at about 900 ma the third stage involved uranium mobilization at about 325 ma and is characterized by i preferential loss of radi

importance of mesoscale eddies and mean circulation in ventilation of the southern ocean this study examines the relative importance of the mean advection and mesoscale currents in the property exchange between the southern ocean mixed layer and downstream in the upper 2000 meters this exchange is referred to as ventilation a new highly efficient offline tracer model employed here uses pre calculated velocities to advect dynamically passive tracers two idealized tracers are considered the boundary impulse response bir tracer which helps to determine the ventilation pathways and time scales and the transient surface tracer tst which is relevant to transient atmospheric tracers the importance of eddies is isolated by contrasting the control simulation with a simulation without mesoscale currents the analysis reveals complex three dimensional ventilation pathways controlled by the interplay between the mean advection and eddy induced spreading the mean currents carry the tracers eastward within acc and contribute to the formation of the antarctic intermediate water aaiw in the south pacific and south atlantic the main effect of eddies is to disperse tracers away from the mean pathways and this dispersion acts to retain the bir tracer in the atlantic and indian sectors and reduce the upstream influence of these regions on the south pacific in addition the eddy induced along isopycnal spreading within acc increases the ventilated depth and the inventory of tst the results can be used to interpret distribution of tracers in the ocean in numerical simulations and observations this article is protected by copyright all rights reserved

a slow seismic event recorded in pasadena a prominent long period wave with a duration of 2000 sec or longer was recorded with a very broadband system in pasadena on june 18 1988 this wave was not observed elsewhere and is considered of local origin the acceleration amplitude is 2 5 10 5 cm sec in the northwest direction with the vertical component less than 10 of the horizontal the horizontal acceleration can be interpreted as due to a tilt of the ground of 2 5 10 8 radians to the northwest a slowly propagating pressure wave with an amplitude of about 15 mbars could be the cause of the tilt however there were no reports suggesting such pressure changes a more likely cause is a slow tectonic event near pasadena the required magnitude of such a slow event is m w 0 2 and 4 for a distance of 0 1 1 and 10 km respectively this event could be part of a tectonic episode associated with the larger earthquakes which occurred in southern california around this time especially the december 3 1988 pasadena earthquake m l 4 9 which occurred six months later within 4 km of the pasadena station

assessing the ability of the 14c projection age method to constrain the circulation of the past in a 3 d ocean model radiocarbon differences between benthic and planktonic foraminifera b p ages and radiocarbon projection ages are both used to determine changes of the past ocean circulation rate a global 3 d ocean circulation model with a constant modern ocean circulation is used to study which method is less influenced by atmospheric 14c variations three factors cause uncertainties first the long equilibration time of the ocean after atmospheric 14c changes second different mixing processes in the ocean which cause an ocean response of smaller amplitude than the atmospheric forcing and third the unknown source region and corresponding initial surface 14c reservoir age of subsurface waters the model suggests that b p ages and projection ages have lower uncertainties the closer they are to deepwater formation zones in the north atlantic the b p age method is less influenced by atmospheric 14c variations than the projection age method projections ages vary less in the pacific as long as atmospheric 14c decreases linearly a more irregular atmospheric 14c evolution leads to age variations of similar magnitude with both methods on the basis of the model experiment we suggest a potential improvement of the projection age method

transit time distributions in lake issyk kul 1 measurements of sulfur hexafluoride sf6 and chlorofluorocarbons cfcs are used to constrain the timescales for deep water renewal in lake issyk kul as these tracers have different tropospheric histories their combination provides more transport information than one tracer alone in particular from these measurements the mean and standard deviation of the distributions of transit times since water made last contact with the surface can be tightly constrained is older than the age determined from sf6 and younger than the ages from the cfcs and increases from around 4 yrs at 200 m to around 10 5 yrs at the deepest location 655 m also increases with depth and equals around 0 7 to 0 8 which corresponds to large ranges of transit times and implies mixing processes play a major role in the transport the approach used can also be applied to similar tracer measurements in the oceans and groundwaters to constrain transport in these geophysical systems

shear wave attenuation and site response in guerrero mexico spectra from moderate earthquakes are used to investigate shear wave attenuation and site response for accelerographs located on rock outcrops in guerrero mexico a least squares method is used to model the strong motion spectra assuming an 2 source and an exponential decay term e f to describe the high frequency spectral shape the spectral decay variable associated with attenuation is parameterized as the sum of a distance dependent component r and a site term s which takes a different value for each site s frequency dependent site response is determined by comparing observed spectra to model event spectra corrected for and geometrical spreading averaging these residuals over several earthquakes results in an estimate of site response that is generally free of source and path effects the distance dependence of the spectral decay parameter as expressed by r is very weak in the guerrero region in contrast to that in southern california anderson 1991b this difference may be due to greater shear strength of the top 20 km of crust in the guerrero subduction zone near site attenuation represented by s is greater in guerrero compared to the stations in the anza array of southern california no obvious correlation with site geology was found a combination of a deeper weathered layer of crystalline rock and varying subsurface lithology may be responsible for the greater mean values of s in guerrero the frequency dependent site functions show significant amplification and deamplification effects for the hard rock sites no direct correlation with local topography is observed to explain these effects the site functions obtained here by constraining the source shape and geometrical spreading closely resemble those derived for guerrero stations by castro et al 1990 using much less restrictive assumptions about the source and propagation effects

millennial scale glacial meltwater pulses and their effect on the spatiotemporal benthicδ18o variability 1 ratios of oxygen isotope values obtained from foraminiferal calcite are one of the most established paleoceanographic proxies they are used in the context of estimating variations in ice volume ocean temperature and salinity and provide a means to date marine sediment cores across different ocean basins our study addresses the question how the 18o of the deglacial meltwater signal propagates into the interior ocean when large scale millennial scale reorganizations of the atlantic meridional overturning circulation amoc are present analyzing a series of idealized tracer injection experiments conducted with an earth system model we find that a substantial weakening of the amoc leads to a massive delay in the export of the glacioeustatic oxygen isotope signal into the deep ocean whereas the atlantic pacific lag in benthic oxygen isotope signals is not increased furthermore it is shown that an amoc cessation causes a decoupling of 18o propagation time and water mass age in particular in the deep pacific our results lend further support to the notion that benthic oxygen isotope records obtained from stacks are not a useful global chronostratigraphic tool during periods of millennial scale global ocean circulation changes the regionally varying delay effect of the deglacial sea level signal studied here adds onto existing uncertainties in the interpretation and decomposition of benthic oxygen isotope in terms of sea level temperature and hydrographic variations