source spectra and spectral attenuation of seismic waves from mexican earthquakes and evidence of amplification in the hill zone of mexico city it is well known that seismic waves are enormously amplified at lake bed sites with respect to hill zone sites in mexico city it has been suggested that even hill zone sites suffer amplification in this paper we use eight recent well recorded earthquakes to determine spectral attenuation of seismic waves and to quantify the suggested amplification in the hill zone results show that indeed spectral amplitudes at these sites are roughly 10 times greater than expected for sites outside mexico city at similar hypocentral distances this implies that lake bed sites are amplified by a factor of 100 to 500 at some frequencies between 0 2 to 0 7 hz the source spectra retrieved from the regression analysis agree well with those obtained from epicentral recordings alone for two of the largest earthquakes ms 8 1 and 7 6 whose seismic moments are well determined and whose source spectra can be patched in a very broad band of frequencies 0 015 to 20 hz we observe clear deviation from an 2 model the spectra at intermediate frequencies from about 0 05 to 2 hz show a sag whether the observed spectra deviate from an 2 model for events with ms 6 depends on the reliability of the reported long period seismic moments if they are considered reliable then these events also deviate from the model if however the reported seismic moments of smaller events are ignored then the observed spectra can be fitted well with the 2 model

acceleration source spectra anticipated for large earthquakes in northeastern north america a set of 16 large intraplate earthquakes are considered on the basis of their tectonic settings compressive focal mechanisms and shallow focal depths as possible analogs for large earthquakes that may be anticipated to occur in northeastern north america the earthquakes range in seismic moment from 5 1023 to 3 1026 dyne cm and include the mainshocks and largest aftershocks of the 1982 miramichi canada the 1985 to 1988 nahanni canada and the 1988 tennant creek australia earthquake sequences the teleseismic recordings of these earthquakes are spectrally analyzed by correcting the logarithmically averaged acceleration spectra for the frequency dependent effects of attenuation and interference of the depth and direct phases the resulting acceleration source spectra are generally flat at high frequencies the mainshock spectra exhibit intermediate spectral falloffs that is u where 1 over frequencies that characteristically extend from 0 1 to 0 5 hz in contrast the largest aftershocks of the three sequences named above exhibit 2 spectral shapes the high frequency source spectral levels increase with seismic moment approximately as ru m 1 3 the 1988 saguenay canada earthquake which occurred significantly deeper than the other earthquakes exhibits an anomalous spectral shape combining the teleseismic data with strong motion recordings indicates that the intermediate spectral falloff for this earthquake extends from 0 2 to 3 hz the spectral amplitude at 3 hz is a factor of 3 higher than the amplitude anticipated for an earthquake of this size from the other 15 earthquakes

gas hydrates off eastern canada velocity models from wide angle seismic profiles on the scotian slope abstract coincident single channel reflection and wide angle ocean bottom seismometer obs reflection refraction profiles were taken on the central continental slope off nova scotia in a region of possible gas hydrate occurrence as evidenced by recent identification of bottom simulating reflectors bsrs joint travel time inversions of wide angle reflections and refractions in the obs data result in three similar velocity models which all display anomalies consistent with the presence of gas hydrate and underlying free gas a high velocity zone hvz 60 120 m thick has velocity increases of 70 130 m s above a low velocity zone lvz 150 260 m thick where velocities decrease by 50 75 m s sensitivity analysis indicates that the refracted phase from the hvz offers better velocity resolution than the reflected phase from the top of the lvz comparison of synthetic seismograms for alternate models can further help to resolve details in the structure of the hvz and lvz layers depth migrations of the reflection profiles using the velocity models show that the depths of the velocity inversions and bsrs are consistent preliminary predictions of 2 6 bulk gas hydrate and

gas hydrate reservoirs and gas migration mechanisms in the terrebonne basin gulf of mexico highlights identify new fine grained hydrate filled fracture units in the terrebonne basin identify new hydrate bearing thin sands mostly within fractured muds present detailed seismic amplitude maps of the new hydrate bearing units discuss methane migration mechanisms and hydrate formation in thin sands identify and discuss source reservoir relationships between thick muds and thin sands abstract the interactions of microbial methane generation in fine grained clay rich sediments methane migration and gas hydrate accumulation in coarse grained sand rich sediments are not yet fully understood the terrebonne basin in the northern gulf of mexico provides an ideal setting to investigate the migration of methane resulting in the formation of hydrate in thin sand units interbedded with fractured muds using 3d seismic and well log data we have identified several previously unidentified hydrate bearing units in the terrebonne basin two units are 100 m thick fine grained clay rich units where gas hydrate occurs in near vertical fractures in some locations these fine grained units lack fracture features and they contain 1 4 m thick hydrate bearing sands in addition several other thin sand units were identified that contain gas hydrate including one sand that was intersected by a well at the location of a discontinuous bottom simulating reflector using correlation of well log data to seismic data we have mapped and described these new units in detail across the extent of the available data allowing us to determine the variation of seismic amplitudes and investigate the distribution of free gas and or hydrate we present several potential source reservoir scenarios between the thick fractured mud units and thin hydrate bearing sands we observe that hydrate preferentially forms within thin sand layers rather than fractures when sands are present in larger marine mud units based on regional mapping showing the patchy lateral extent of the thin sand layers we propose that diffusive methane migration or short migration of microbially generated methane from the marine mud units led to the formation of hydrate in these thin sands as discontinuous sands would not be conducive to long range migration of methane from deeper reservoirs

solubility of silica polymorphs in electrolyte solutions ii activity of aqueous silica and solid silica polymorphs in deep solutions from the sedimentary paris basin abstract activity coefficient for aqueous silica in saline waters and brines from the paris basin was calculated using pitzer s specific interaction model quartz and chalcedony are the only reported authigenic silica minerals in the dogger aquifer of the paris basin france however the measured silica concentrations fall between those of these two phases the silica concentrations measured in dogger fluids seem to be controlled by a microcrystalline quartz phase with a grain size computed to be about 20 nm studies have shown that pressure can preserve small grain size for a long time at the geological scale the effective mechanism of pressure action is probably linked to the fact that pressure simultaneously favours dissolution at the grain contact inducing a quartz supersaturation and prohibits the increase in size of reprecipitated microcrystalline quartz grains this hypothesis is supported by other studies reported in the literature the proposed model which incorporates silica mineralogy and a precise calculation of aqueous silica activity allows us to explain measured silica concentrations in the deep sedimentary solutions of the dogger aquifers in the keuper brines silica solubility can in most cases be explained by an equilibrium with either chalcedony or quartz another application of the present work is shown by an example where we examined the importance of precisely evaluating the activity coefficient in basin characterisation as the goal of reservoir characterisation is to describe the spatial distribution of petrophysical parameters such as porosity permeability and saturations

method of estimating the amount of in situ gas hydrates in deep marine sediments abstract the bulk volume of gas hydrates in marine sediments can be estimated by measuring interval velocities and amplitude blanking of hydrated zones from true amplitude processed multichannel seismic reflection data in general neither velocity nor amplitude information is adequate to independently estimate hydrate concentration a method is proposed that uses amplitude blanking calibrated by interval velocity information to quantify hydrate concentrations in the blake ridge area of the us atlantic continental margin on the blake ridge blanking occurs in conjunction with relatively low interval velocities the model that best explains this relation linearly mixes two end member sediments hydrated and unhydrated sediment hydrate concentration in the hydrate end member can be calculated from a weighted equation that uses velocity estimated from the seismic data known properties of the pure hydrate and porosity inferred from a velocity porosity relationship amplitude blanking can be predicted as the proportions of hydrated and unhydrated sediment change across a reflection boundary our analysis of a small area near dsdp 533 indicates that the amount of gas hydrates is about 6 in total volume when the interval velocity is used as a criterion and about 9 5 when amplitude information is used this compares with a calculated value of about 8 derived from the only available measurement in dsdp 533

testing the reliability of paper seismic record to segy conversion on the surface and shallow sub surface geology of the barra fan ne atlantic ocean marine seismic reflection data have been collected for decades and since the mid to late 1980s much of this data is positioned relatively accurately this older data provides a valuable archive however it is mainly stored on paper records that do not allow easy integration with other datasets a method for converting these records to digital segy format has been reported previously although the accuracy and reliability has not been documented using 3 5 khz pinger bathymetric and side scan sonar data we test the accuracy and data resolution of converted paper seismic records on the surface and shallow subsurface of the barra fan ne atlantic ocean frequencies of navigational updates and printed fix marks are vital in determining positional accuracy as interpolation between fixes which may be 30 minutes apart is a significant source of potential error vertical data resolution of the converted records is primarily governed by the dimensions of the printed record with order of magnitude differences noted between the scanned bgs and noc pinger records positional accuracy of pinger data is observed to be within the resolution of the bathymetric grid bgs 1985 pinger data is located within 40 m of bathymetric constrained locations at 635 m below mean sea level and noc 1998 pinger data within 25 m of bathymetric locations at 1180 m below mean sea level this paper to digital conversion is not a replacement for modern data acquisition but it does provide a valuable complement to such data where appropriate archival paper data is available for conversion it could be used to assist efficiency of forthcoming surveys and may be used to provide time series evidence of seabed processes

ground motion relations for eastern north america predictive relations are developed for ground motions from eastern north american earthquakes of 4 0 m 7 25 at distances of 10 r 500 km the predicted parameters are response spectra at frequencies of 0 5 to 20 hz and peak ground acceleration and velocity the predictions are derived from an empirically based stochastic ground motion model the relations differ from previous work in the improved empirical definition of input parameters and em pirical validation of results the relations are in demonstrable agreement with ground motions from earthquakes of m 4 to 5 there are insufficient data to adequately judge the relations at larger magnitudes although they are consistent with data from the saguenay m 5 8 and nahanni m 6 8 earthquakes the underlying model parameters are constrained by empirical data for events as large as m 6 8

ground motion simulation by the empirical green s function method with a source defined by two corner frequencies and a two stage summation schemeground motion simulation by the egf method

2d waveform tomography applied to long streamer mcs data from the scotian slope detailed velocity models of the earth s subsurface can be obtained through waveform tomography the accuracy of the long wavelength component of such velocity models which is the background velocity field is particularly sensitive to modeling low frequency refracted waves that have long paths through target structures thus field examples primarily have focused on the analysis of long offset wide angle data sets collected using autonomous receivers in which refractions arrive at significantly earlier times than reflections modern marine acquisition with long streamers now offers the ability to record refracted waves with high spatial density and uniform source both in shallow and deep water we used 2d multichannel seismic mcs data acquired with a 9 km long streamer over the scotian slope in water depths of 1600 m the refracted arrivals although mostly restricted to far offset receivers provided sufficient information to successfully invert for a high resolution background velocity field usin

elasticity of marine sediments rock physics modeling we offer an effective medium model for the elastic moduli of high porosity ocean bottom sediments the elastic constants of the dry sediment frame depend on porosity elastic moduli of the solid phase and effective pressure the model connects two end points in the elastic modulus porosity plane the hertz mindlin modulus of a dense elastic sphere pack at critical porosity and zero at 100 porosity the elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation unlike the suspension model our model assigns non zero elastic constants to the dry sediment frame and can predict the shear wave velocity unlike various modifications of the travel time average equation it is first principle based and contains only physical parameters we justify this model by matching sonic data in shallow marine sediments and in an odp well

authigenic k feldspar in recent submarine gypsum concretions from denmark abstract authigenic k feldspar is a minor constituent in recent submarine gypsum concretions from kattegat denmark associated with authigenic quartz the k feldspar is adularia a pure end member of potassium feldspar the k feldspar occurs as floating euhedral monoclinic crystals enclosed in coarsely crystalline gypsum formation of the adularia containing gypsum is apparently related to the introduction of so 4 2 enriched formation water from quaternary gas reservoirs

characterization of a stratigraphically constrained gas hydrate system along the western continental margin of svalbard from ocean bottom seismometer data the ongoing warming of bottom water in the arctic region is anticipated to destabilize some of the gas hydrate present in shallow seafloor sediment potentially causing the release of methane from dissociating hydrate into the ocean and the atmosphere ocean bottom seismometer obs experiments were conducted along the continental margin of western svalbard to quantify the amount of methane present as hydrate or gas beneath the seabed p and s wave velocities were modeled for five sites along the continental margin using ray trace forward modeling two southern sites were located in the vicinity of a 30 km long zone where methane gas bubbles escaping from the seafloor were observed during the cruise the three remaining sites were located along an e w orientated line in the north of the margin at the deepest northern site vp anomalies indicate the presence of hydrate in the sediment immediately overlying a zone containing free gas up to 100 m thick the acoustic impedance contrast between the two zones forms a bottom simulating reflector bsr at approximately 195 m below the seabed the two other sites within the gas hydrate stability zone ghsz do not show the clear presence of a bsr or of gas hydrate however anomalously low vp indicating the presence of free gas was modeled for both sites the hydrate content was estimated from vp and vs using effective medium theory at the deepest northern site modeling suggests a pore space hydrate concentration of 7 12 if hydrate forms as part of a connected framework and about 22 if it is pore filling at the two other northern sites located between the deepest site and the landward limit of the ghsz we suggest that hydrate is present in the sediment as inclusions hydrate may be present in small quantities at these two sites 4 5 of the pore space the variation in lithology for the three sites indicated by high resolution seismic profiles may control the distribution concentration and formation of hydrate and free gas

mineral equilibria in the six component seawater system na k mg ca so4 cl h2o at 25 c ii compositions of the saturated solutions abstract using the solubility model developed by harvie and weare 1980 the stable mineral solution assemblages for the six component system na k mg ca so 4 cl h 2 o and its constituent 5 4 and 3 component systems at 25 c have been defined invariant point maps have been constructed showing the connections by univariant lines the solubility volumes for all 20 minerals considered are also illustrated of the 37 invariant points only 3 have solutions which are ca rich the remaining 34 can be plotted in the reciprocal system na k mg so 4 cl which is similar to the seawater system except that the restriction of halite saturation has been removed application of these results and implications for the evolution of major brine types are briefly discussed

fluid history during deep burial and exhumation of oil bearing volcanics hercynian belt of southern brittany france in southern brittany france the porphyroid nappe a variscan tectonic unit made of volcanic pyroclastic deposits that was involved in a low temperature high pressure prograde metamorphic path followed by exhumation and extension displays evidence for very large and distributed strains achieved through dissolution crystallization processes such distributed strains at low temperature are rather unusual in felsic lithologies recent studies have documented that these rocks deformed within a system closed to fluids this paper presents results of detailed analyses of fluid inclusions and stable isotopes within the deformed metavolcanics and their undeformed counterparts in order to document the fluid history of the tectonic unit results show that its initial thermal evolution likely occurring in a context of basin inversion involved important open system alteration with k metasomatism adularization oil impregnation and hydration during the subsequent tectonic evolution the fluids maintained a constant composition in the c h o n system within the porphyroid nappe and its para autochthon being water rich and containing hydrocarbon components likely related to oil pyrolysis oxidation atoll like fluid inclusion morphology and rare undisturbed fluid inclusions with steep isochore slopes are consistent 750 mpa with pressures estimated for the peak of metamorphism using mineral solid solution thermodynamics fluid inclusions indicate a sub isothermal retrograde path between 420 and 300 c followed by a high thermal gradient at the end of the retrograde path within the underlying para autochthonous unit only the late high temperature gradient is recorded by fluid inclusions the sub isothermal low grade path suggests fast exhumation of the unit without thermal relaxation the high geotherm observed at the end of the p t path is consistent with the post thickening extensional collapse and associated intrusion of granites documented in the area hydration and heterogeneous oil water distribution achieved in the early metasomatic evolution of the porous pyroclastic deposits are responsible for subsequent fluid confining in the porphyroid nappe and for its strongly distributed strain pattern

mudstones and embedded concretions show differences in lithology related but not source related biomarker distributions abstract the mudstones of the western australian gogo formation harbour numerous carbonate concretions which often contain preserved fossils of lagerstatte like quality these are especially notable in places where the mudstone has eroded giving way to nodule fields which afford valuable paleobiological records it is however a challenge to determine their paleoenvironmental context here we analysed concretions from a core drilled in the canning basin gogo formation at two different depths concretions were compared to the surrounding mudstone found at the same depth electron microscopy and x ray spectroscopy showed that the concretions were carbonate rich and contained detrital fragments biomarker data showed that mudstone and concretions had very similar distributions and presented marine biosignatures including indicators of anoxic depositional conditions a stratified water column and photic zone euxinia the concretions contained higher amounts of c 27 steranes indicating that more labile organic matter such as animal remains could have triggered concretion formation statistical analyses of the results showed that concretions and shales differed p p 31 homohopanes and hopane sterane ratios p

gas hydrates at the storegga slide constraints from an analysis of multicomponent wide angle seismic data geophysical evidence for gas hydrates is widespread along the northern flank of the storegga slide on the mid norwegian margin bottom simulating reflectors bsr at the base of the gas hydrate stability zone cover an area of approximately 4000 km 2 outside but also inside the storegga slide scar area traveltime inversion and forward modeling of multicomponent wide angle seismic data result in detailed p and s wave velocities of hydrate and gas bearing sediment layers the relationship between the velocities constrains the background velocity model for a hydrate free gas free case the seismic velocities indicate that hydrate concentrations in the pore space of sediments range between 3 and 6 in a zone that is as much as 50 m thick overlying the bsr hydrates are most likely disseminated neither cementing the sediment matrix nor affecting the stiffness of the matrix noticeably average free gas concentrations beneath the hydrate stability zone are approximately 0 4 to 0 8 of the pore volume assuming a homogeneous gas distribution the free gas zone underneath the bsr is about 80 m thick amplitude and reflectivity analyses suggest a rather complex distribution of gas along specific sedimentary strata rather than along the base of the gas hydrate stability zone bghs this gives rise to enhanced reflections that terminate at the bghs the stratigraphic control on gas distribution forces the gas concentration to increase slightly with depth at certain locations gas bearing layers can be as thin as 2 m

methane derived carbonate cementation of marine sediments from the kattegat denmark geochemical and geological evidence abstract methane derived carbonate cementation of holocene marine sediment occurs at several locations in the kattegat denmark generally the carbonate cemented sandstones occur at the seafloor as individual slabs or widely distributed thinly lithified pavements in some areas the lithified sandstones take the form of large and spectacular vertical pillar and mushroom like bodies standing up from the seafloor the carbonate cement consists of aragonite high mg calcite and dolomite the high mg calcite is the most widespread carbonate solid phase found aragonite is primarily associated with shell accumulations in the sediments so far dolomite has only been recorded as cement in the large vertical lithified sandstone bodies the carbonates are remarkably depleted in 13c i e generally in the range of 55 to 45 pdb and are therefore interpreted as being the result of methane oxidation by sulphate reducing bacteria in the anaerobic diagenetic zone of the sediments however the oxygen isotopic composition and the concentrations of minor elements indicate that carbonate precipitation took place under relatively constant temperature and salinity conditions in normal marine pore fluids the habit and mineralogy of the carbonate cemented sandstone observed most likely are controlled by the effect of bacterial activity and upward gas migration on the interstitial geochemical microenvironment the lithified sediments were subsequently exposed as spectacular sandstone formations on the seafloor by submarine erosion data from onshore exploration wells reveal that methane is encountered in eemian early weichselian late quaternary marine deposits the chemical composition and the isotopic signatures of the methane suggest a gas of microbial origin and it is thought that the gas is generated within the host marine quaternary sediment the locations of submarine gas seeps along with those of the carbonate cemented sandstone formations and the onshore gas seeps show a linear distribution which largely coincides with the extension of the fennoscandian border zone thus the seeps may be controlled by the structural framework in the northeastern part of the danish subbasin

controls on the formation and stability of gas hydrate related bottom simulating reflectors bsrs a case study from the west svalbard continental slope 1 the growth and stability of the free gas zone fgz beneath gas hydrate related bottom simulating seismic reflectors bsrs is investigated using analytical and numerical analyses to understand the factors controlling the formation and depletion of free gas for a model based on the continental slope west of svalbard a continental margin of north atlantic type we find that the fgz is inherently unstable under a wide range of conditions because upward flow of under saturated liquid depletes free gas faster than it is produced by hydrate recycling in these scenarios the 150 m thick fgz that presently exists there would deplete within 105 106 years we suggest the fgz is in a stable state however that is formed by a diffusion dominated mechanism that produces low concentrations of gas in a fgz of steady state thickness gas forms across a thick zone because the upward fluid flux is relatively low and because the gas water solubility decreases to a minimum several hundred meters below the seabed this newly understood solubility curvature effect is complementary to hydrate recycling but becomes the most important factor controlling the presence and properties of the bsr in environments where the rate of upward fluid flow and the rate of hydrate recycling are both relatively low i e rifted continental margins if the present day fgz is in steady state we estimate that the upward fluid flux in the west svalbard site must be less than 0 15 mm a 1

applying the squeezing technique to highly consolidated clayrocks for pore water characterisation lessons learned from experiments at the mont terri rock laboratory abstract knowledge of the pore water chemistry in clayrock formations plays an important role in determining radionuclide chemical speciation and migration in the context of nuclear waste disposal among the different in situ and ex situ techniques for pore water sampling in clay sediments and soils the squeezing technique dates back 115 years although different studies have been conducted on the reliability and representativeness of squeezed pore waters most of these involved high porosity high water content and unconsolidated clay sediments very few of them analysed squeezed pore water from low porosity low water content and highly consolidated clayrocks in this paper a specially designed and fabricated one dimensional compression two directional fluid flow cell was used to extract and analyse the pore water composition of opalinus clay core samples from mont terri switzerland with water contents between 6 2 and 7 8 wt a study of the influence of the squeezing pressure on the chemistry of the pore water was performed at pressures of up to 200 mpa to validate and demonstrate the validity of the squeezing technique for obtaining reliable pore water different tests were performed at various pressures and the squeezed waters were compared with those obtained by other methods such as in situ collected borehole waters the reproducibility and quality of the squeezing method was also checked as well as different artifacts which could affect the reliability of the pore water data the results show that the reproducibility of the technique is good and no membrane effect ionic ultrafiltration chemical fractionation anion exclusion was found in opalinus clay at mont terri in the range of pressures analysed 70 200 mpa pore waters extracted in this range of pressures do not decrease in concentration as a function of pressure which indicates that a dilution of water by mixing of the free pore water and the outer layers of diffuse layer water does not occur and a threshold safety squeezing pressure of 175 mpa could be established to avoid membrane effects furthermore a direct comparison against in situ collected borehole waters shows that the pore waters extracted at these pressures are representative of the opalinus clay formation at mont terri extraction artifacts such as temperature effect oxidation and degassing can be avoided or minimised by taking special precautions when preparing and handling the core sample and extracting the pore water finally we show that it is possible to obtain insightful information on anion accessible porosity by combining squeezing data with aqueous leaching data to characterise the pore water composition and solute distribution for different rock porosities

is there truly a hard rock site in the valley of mexico to understand the cause of observed amplification of seismic waves even at hill zone sites in the valley of mexico digital accelerographs have been installed at three especially chosen sites two of these sites madi and texc located on hard pleistocene lava a few meters in thickness overlying oligocene andesites were expected to be free of site effects analysis of the data recorded by these and other accelerographs during three moderate shallow subduction zone events however shows significant amplification at madi and texc between 0 2 and 0 6 hz with respect to established attenuation relations the cause of the amplification at hill zone sites in the valley of mexico including madi and texc may be pervasive low s wave velocity in and complex structure of the upper layers of the volcanic rocks if so then there may not be a truly hard rock site in the valley

physical and chemical controls on the composition of waters in sedimentary basins abstract the composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion the south west louisiana gulf coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section the simultaneous systematic variation in dissolved na k mg ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate carbonate mineral assemblages is a first order control on fluid compositions the chemical potential of chloride or alternatively total anionic charge appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region this variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper mudstone dominated sediments where the composition of the fluid is largely rock buffered the ultimate origin of the fluid and its pathway of chemical evolution may be obscured at least in terms of major solute composition non buffered components such as cl and br or isotopic compositions are more likely to retain information on original end member fluid compositions and reaction pathways

etude geochimique de la nappe du dogger du bassin parisien abstract waters from the dogger nappe of the paris basin which are utilized for heating have a chlorinity in the range 0 07 0 6 mole litre and a temperature in the range 48 80 c we present here a geochemical study of waters sampled in about 25 boreholes except chloride and sulfates elements seem to be controlled by equilibrium with minerals commonly encountered in hydrothermal systems calcite disordered dolomite chalcedony ca montmorillonite disordered kaolinite albite and adularia a chalcedony geothermometer indicates temperatures close to the observed temperatures whereas the na k geothermometer gives a mean value close to the mean observed temperature but na k variations do not follow local variations of temperature the cl br ratio 420 suggests that the chlorinity of dogger water is related to sea water dogger water can be the result of interaction between a mixture of evolved sea water and fresh water and minerals of the reservoir

stable isotope conservation and origin of saline waters from the middle jurassic aquifer of the paris basin france abstract seventy nine samples of formation water from geothermal boreholes oil field and ground water wells tapping the dogger middle jurassic limestone reservoir of the paris basin have been analyzed for their 2h 18o tds bromide and chloride contents geostatistical mapping of 2h and of 18o contents and their variations with chloride content indicate that both isotopes have a common origin and a similar behaviour within the aquifer the 2h and 18o values increase progressively from the outcrops toward a zone where temperature cl and tds values are high the 2h and 18o values are very uniform in this zone and do not show any relation with formation temperature in contrast to what is generally observed in other sedimentary basins a discussion of isotopes exchange with the carbonate matrix for oxygen and with h2s for deuterium shows that such processes are not controlling the heavy isotope contents of groundwaters this is attributed to the relatively short residence time of groundwaters in the aquifer both 2h and 18o contents of groundwater may thus be considered as conservative tracers in the dogger aquifer of the paris basin and their initial contents are modified only by fluid mixings complex mixing processes account for stable isotope composition tds cl and br values of the dogger fluids all mixtures include a brine component meteoric water percolated and dissolved halite in the triassic aquifer secondary brine this saline solution mixed with some residual primary brine then migrated via vertical faults into the dogger aquifer such migration is found to be in agreement with data from fluid inclusions in late carbonate cements from the liassic and dogger strata the requirement that these units were exposed implies that meteoric invasion could not have occurred before the tertiary successive mixings of the brine with several types of meteoric waters depleted in heavy isotopes account for the present isotope and chemical compositions of the dogger fluids

modes of covariability between sea surface temperature and wind stress intraseasonal anomalies along the coast of peru from satellite observations 2000 2008 1 the tropical rainfall measuring mission microwave imager sea surface temperature sst and quikscat wind stress satellite data are used to investigate the intraseasonal upwelling variability along the coat of peru over the period 2000 2008 two regions of peak variance correspond to the central peru region pisco region 15 s and the northern peru region piura region 5 s a covariance analysis reveals a significant coherency between winds and sst anomalies off pisco consistent with ekman pumping and transport dynamics the upwelling cell consists in a meridionally extended fringe of colder warmer water extending as far as 250 km from the coast at 15 s in the piura region the intraseasonal covariability pattern is represented by two modes one relevant to the direct ekman dynamics and the other one associated with the remote forcing of intraseasonal oceanic kelvin wave two regimes of variability are evidenced a low period regime 10 25 days is the signature of ekman transport pumping dynamics and is remotely forced by the migratory atmospheric disturbances across the southeastern pacific anticyclone a high period regime 35 60 day band is associated with the combined forcing of oceanic equatorial kelvin waves and migratory atmospheric disturbances in the midlatitudes in particular the modes of covariability exhibit a prominent 50 day period energy peak it is shown that this period arises from the impact of the first two baroclinic modes kelvin wave with the second baroclinic mode kelvin wave being more influential on the piura region

gas hydrate fluid flow and free gas formation of the bottom simulating reflector abstract gas hydrate in continental margins is commonly indicated by a prominent bottom simulating seismic reflector bsr that occurs a few hundred metres below the seabed the bsr marks the boundary between sediments containing gas hydrate above and free gas below most of the reflection amplitude is caused by the underlying free gas gas hydrate can occur without a bsr however and the controls on its formation are not well understood here we describe two complementary mechanisms for free gas accumulation beneath the gas hydrate stability zone ghsz the first is the well recognised hydrate recycling mechanism that generates gas from dissociating hydrate when the base of the ghsz moves upward relative to hydrate bearing sediment the second is a recently identified mechanism in which the relationship between the advection and diffusion of dissolved gas with the local solubility curve allows the liquid phase to become saturated in a thick layer beneath the ghsz when hydrate is present near its base this mechanism for gas production called the solubility curvature mechanism is possible in systems where the influence of diffusion becomes important relative to the influence of advection and where the gas water solubility decreases to a minimum several hundred metres below the ghsz we investigate a number of areas in which gas hydrate occurs to determine where gas formation is dominated by the solubility curvature mechanism and where it is dominated by hydrate recycling we show that the former is dominant in areas with low rates of upward fluid flow such as old rifted continental margins low rates of seafloor uplift and high geothermal gradient and or pressure conversely free gas formation is dominated by hydrate recycling where there are rapid rates of upward fluid flow and seabed uplift such as in subduction zone accretionary wedges using these two mechanisms to investigate the formation of free gas beneath gas hydrate in continental margins we are able to resolve a number of problems these include why sub bsr free gas zones are generally thin in convergent margins and thick in passive margins and why areas such as the gulf of mexico and mackenzie delta do not have regional bsrs finally we use our understanding of how bsrs are formed to show that regional gas hydrate is unlikely to exist in substantial quantities in passive continental margins outside areas with an observable bsr

frontal ridge slope failure at the northern cascadia margin margin normal fault and gas hydrate control on the northern cascadia accretionary margin off vancouver island canada there are numerous sedimentary slide features near the base of the slope and significant amounts of gas hydrate found beneath the ridge suggest a possible connection a 2 km wide collapse structure with a 300 m high headwall has been studied by multibeam bathymetry and a single channel seismic reflection grid migrated seismic reflection lines on the frontal ridge image multiple 15 75 m high seafloor scarps perpendicular to the margin that are the seafloor expressions of normal faults that cut deeply into the sediment section two of the largest faults are aligned with the sidewalls of the slide indicating that the lateral extent of the collapse is fault controlled the presence of marine gas hydrate beneath the ridge is based on a widespread hydrate bottom simulating reflector bsr high velocities determined by ocean bottom seismograph data and sediment core samples and downhole logs collected by the integrated ocean drilling program at site u1326 the depth of the bsr at 255 15 m coincides closely with the estimated depth of the glide plane beneath the slide this suggests that the base of the slope failure is related to the contrast between strong hydrate cemented sediments above the bsr and underlying weak sediments containing free gas strong earthquake shaking on this convergent margin likely provides the trigger for the slide

quantitative study of geochemical processes in the dogger aquifer paris basin france the quantification of geochemical reactions in hydrothermal aquifers requires an exhaustive approach because of their interdependence all chemical elements likely to have a quantitative influence on dissolution or precipitation reactions have to be taken into account geochemical constraints the number of which equals the number of the chemical elements were determined with the help of chemical analyses of the solution in the case study of the dogger aquifer paris basin france 12 elements are taken into account al c cl ca f h k mg na o s si three constraints apply to water activity neutrality and conservation of chloride in aqueous solutions the determination of the remaining constraints was based on saturation indices stability diagrams of minerals and partial pressure of carbon dioxide pco2 the present day solutions are at equilibrium with respect to nine minerals albite anhydrite chalcedony calcite dolomite fluorite kaolinite k feldspar and illite or chlorite thus it was demonstrated that the composition of these solutions including computed pco2 is only a function of temperature and chloride content moreover it was possible to test the validity of the geochemical system by computing its speciation and comparing these theoretical results with actual chemical analyses ph and concentrations finally geochemical simulations were used in predicting what quantities would be dissolved or precipitated as either the temperature or the chlorinity varied although the rock is predominantly calcareous these quantities could not be determined if the influence of the aluminosilicates were neglected this chemical component approach with which one can pose and solve rigorously the chemical equilibrium problem constitutes a prerequisite for the quantitative study of geochemical processes related to fluid flow

geochemistry of brines of the chott el jerid in southern tunesia application of pitzer s equations abstract in the chott el jerid in southern tunisia waters are strongly concentrated by evaporation under an arid climate the diluted waters are of the ca 1bmg 1bso4 type whereas the brines are of the na 1bcl type relatively concentrated in k and mg in concentrated waters ca and silica concentrations remain at a very low level the use of pitzer s equations for calculating the ion activities has permitted accurate saturation tests and a fairly good evaluation of the mineral sequences controlling the geochemistry of the basin

earthquake source spectra in eastern north america source spectra for earthquakes of 3 m 7 in eastern north america ena are determined from regional seismograph recordings strong motion records teleseismic data and modified mercalli intensity data the inclusion of mmi data is made possible by the finding that the felt area of an earthquake is an accurate measure of its high frequency spectral level at the source current ground motion relations for ena are based on a brune source model with a stress parameter of 100 bars this model is a poor fit to ena source spectra for events of m 4 at high frequencies 5 to 10 hz observed spectral amplitudes are more nearly matched by a brune stress parameter of 200 bars intermediate frequency amplitudes near 1 hz are matched by a stress parameter of approximately 50 bars an empirical two corner source model is proposed to describe ena source spectra for events of 4 m 7

a margin wide bsr gas hydrate assessment canada s atlantic margin abstract it is the intent of this paper to explore a significant extent of an entire passive continental margin for hydrate occurrence to understand hydrate modes of occurrence preferred geologic settings and estimate potential volumes of methane the presence of gas hydrates offshore of eastern canada has long been inferred from estimated stability zone calculations but little physical evidence has been offered an extensive set of 2 d and 3 d single and multi channel seismic reflection data comprising in excess of 140 000 line km was analyzed bottom simulating reflections bsr were unequivocally identified at seven sites ranging between 250 and 445 m below the seafloor and in water depths of 620 2850 m the combined area of the bsrs is 9311 km 2 which comprises a small proportion of the entire theoretical stability zone along the canadian atlantic margin 715 165 km 2 the bsr within at least six of these sites lies in a sedimentary drift deposit or sediment wave field indicating the likelihood of grain sorting and potential porosity and permeability reservoir development although there are a variety of conditions required to generate and recognize a bsr one might assume that these sites offer the most potential for highest hydrate concentration and exploitation total hydrate in formation at the sites of recognized bsr s is estimated at 17 to 190 10 9 m 3 or 0 28 to 3 12 10 13 m 3 of methane gas at stp although it has been shown that hydrate can exist without a bsr the results from this regional study argue that conservative estimates of the global reserve of hydrate along continental margins are necessary

evaluation of models for earthquake source spectra in eastern north america there have been several relations proposed in the last few years to describe the amplitudes of ground motion in eastern north america ena these relations differ significantly in their assumptions concerning the amplitude and shape of the spectrum of energy radiated from the earthquake source in this article we compare ground motions predicted for these source models against the sparse ena ground motion database the source models evaluated include the two comer models of boatwright and choy 1992 atkinson 1993a haddon 1996 and joyner 1997a b and the one comer model of brune as independently implemented by frankel et al 1996 and by toro et al 1997 the database includes data from ena mainshocks of m 4 and historical ena earthquakes of m 5 5 for a total of 110 records from 11 events of 4 m 7 3 all recorded on rock we also include 24 available rock records from 4 large earthquakes in other intraplate regions con clusions are checked to determine whether they are sensitive to the addition of these non ena data the atkinson source model as implemented in the ground motion relations of atkinson and boore 1995 is the only model that provides unbiased ground motion predictions over the entire period band of interest from 0 1 to 10 sec the source models of frankel et al 1996 toro et al 1997 and joyner 1997a b all provide unbiased ground motion estimates in the period range from 0 1 to 0 5 sec but over estimate motions at periods of 1 to 10 sec the haddon 1996 source model over predicts motions at all periods by factors of 2 to 10 these conclusions do not change significantly if data from non ena intraplate regions are excluded although the ten dency of all models toward overprediction of long period amplitudes becomes more pronounced the tendency of most proposed ena source models to overestimate long period motions is further confirmed by an evaluation of the relationship between ms a measure of the spectrum at 20 sec period and moment magnitude a worldwide catalog of shallow continental earthquakes triep and sykes 1996 is compared to the ms m relations implied by each of the source models the atldnson source model is consistent with these data while other proposed ena models overpredict the average ms for a given m the implications of mmi data from historical earthquakes are also addressed by exploiting the correlation between felt area and high frequency source spectral level high frequency spectral amplitudes as specified by the atkinson and boore 1995 frankel et al 1996 toro et al 1997 and joyner 1997a b source models equal or exceed the levels inferred from the felt areas of most of the large ena events with the noteable exception of the saguenay earthquake by contrast high frequency spectral amplitudes specified by the haddon 1996 source model agree with the felt area of the saguenay earthquake but overpredict the felt areas of nearly all other large events in general models that fit the saguenay data be it intensity data strong ground motion data regional seismographic data or teleseismic data will not fit the data from the remaining earthquakes a source model derived from the california database suitably modified for re gional differences in crustal properties is also evaluated this model is not signifi

new perspectives on self similarity for shallow thrust earthquakes scaling of dynamic rupture processes from small to large earthquakes is critical to seismic hazard assessment large subduction earthquakes are typically remote and we mostly rely on teleseismic body waves to extract information on their slip rate functions we estimate the p wave source spectra of 942 thrust earthquakes of magnitude mw 5 5 and above by carefully removing wave propagation effects geometrical spreading attenuation and free surface effects the conventional spectral model of a single corner frequency and high frequency falloff rate does not explain our data and we instead introduce a double corner frequency model modified from the haskell propagating source model with an intermediate falloff of f 1 the first corner frequency f1 relates closely to the source duration t1 its scaling follows m0 t13 for mw 7 5 and changes to m0 t12 for larger earthquakes an elliptical rupture geometry better explains the observed scaling than circular crack models the second time scale t2 varies more weakly with moment m0 t25 varies weakly with depth and can be interpreted either as expressions of starting and stopping phases as a pulse like rupture or a dynamic weakening process estimated stress drops and scaled energy ratio of radiated energy over seismic moment are both invariant with seismic moment however the observed earthquakes are not self similar because their source geometry and spectral shapes vary with earthquake size we find and map global variations of these source parameters

a neoproterozoic multi phase rift sequence the grampian and appin groups of the southwestern monadhliath mountains of scotland the grampian and appin groups of the southwestern monadhliath mountains form the earliest known syn rift sequences of the scottish central highlands they were likely to have formed in an intracontinental setting and represent deposition of mixed clastic and carbonate shallow and deep marine strata the grampian group of the southern monadhliath mountains was deposited during a period of initial basin rifting nw se extension followed by a phase of thermal subsidence syn rift sediments comprise a 2 5 6 km thick turbidite system thermal subsidence brought about the basinward progradation of shallow marine shelf sediments resulting in the infilling of pre existing basin topography the overlying appin group commenced with deposition of a shallow marine sequence alternating between nearshore tidal sand and offshore mud deposition this formed in response to renewed rifting and concomitant subsidence accelerated rifting resulted in localized footwall uplift and erosion while sedimentation continued in the hanging wall areas resultant subsidence perhaps partly thermally driven caused gradual basin widening and produced an onlapping marine sequence there followed a period of progressive clastic deprivation when carbonates were precipitated and at the onset of anoxic conditions deposition of organic muds the fundamental structural elements responsible for the formation of the grampian and appin group basins were also influential in the orogenic evolution of the basin fill half graben fills were deformed to produce regionally extensive folds such as the stob ban craig a chail synform

seismic velocities on the nova scotian margin to estimate gas hydrate and free gas concentrations abstract this article provides new constraints on gas hydrate and free gas concentrations in the sediments at the margin off nova scotia two dimensional 2 d velocity models were constructed through simultaneous travel time inversion of ocean bottom seismometer obs data and 2 d single channel seismic scs data acquired in two surveys in 2004 and 2006 the surveys separated by 5 km were carried out in regions where the bottom simulating reflection bsr was identified in seismic reflection datasets from earlier studies and address the question of whether the bsr is a good indicator of significant gas hydrate on the scotian margin for both datasets velocity increases by 200 300 m s at a depth of approximately 220 m below seafloor mbsf but the results of the 2006 survey show a smaller velocity decrease 50 80 m s at the base of this high velocity layer 310 330 mbsf than the results of the 2004 survey 130 m s when converted to gas hydrate concentrations using effective medium theory the 2 d velocity models for both datasets show a gas hydrate layer of 100 m thickness above the identified bsr gas hydrate concentrations are estimated at approximately 2 10 for the 2006 data and 8 18 for the 2004 survey the reduction in gas hydrate concentration relative to the distance from the mohican channel structure is most likely related to the low porosity within the mud dominant sediment at the depth of the bsr free gas concentrations were calculated to be 1 2 of the sediment pore space for both datasets

major ion geochemistry and mineralogy of the salt lake tuz golu basin turkey in the salt lake basin of turkey chemical composition of inflow surface waters defines a continuous trend from ca hco3 rich spring waters to na so4 cl rich brines considerable compositional variation exists among the surface waters water rock interaction governs compositional variations in springs streams and rivers and is enhanced by evaporation and precipitation of calcite and protodolomite solute concentrations of the streams and the rivers are partially controlled by the mineralogy of the playa deposits the concentration increase from inflow surface waters to a na cl type lake surface brine is not accessible to direct observation the principal cause of the evolution from so4 rich brine to cl rich brine in the lake is interpreted as the recycling of solutes through the differential dissolution of efflorescent crusts in the lake major ion concentrations generally exhibit an evaporation dependent evolution trend that is further modified by precipitating halite gypsum aragonite and calcite sediments of the lake are predominantly composed of gypsum dolomite huntite magnesite and subordinately of polyhalite minerals magnesite and huntite are mostly early diagenetic minerals that have been formed by the transformation of dolomite in the presence of pore fluids with a high mg ca ratio

global comparisons of earthquake source spectra a key question in earthquake seismology is whether earthquake sources in different tectonic regions are functionally equivalent in terms of radiated ground motion this article compares the apparent earthquake source radiation for six different regions japan mexico turkey california british columbia b c western canada and eastern north america ena the source path and site effects were consistently separated to the extent possible to allow interregional comparisons of apparent source radiation large fourier spectra databases were created for both horizontal and vertical components for earthquakes with magnitude between 2 5 and 7 5 regional parameters characterizing source path and site properties including crustal and near surface site amplifications were investigated to isolate apparent source spectra based on the vertical component spectra at a reference distance of 1 km a new magnitude scale m 1 was defined from the spectral level at 1 hz the magnitude scale m 1 is a natural complement to existing magnitude scales that measure the low frequency and high frequency spectral amplitudes moment magnitude m and high frequency m hf respectively in all regions the apparent source spectra for small to moderate earthquakes m 6 show good agreement with the single corner brune 1970 1971 point source model 2 spectrum suggesting a stress parameter of about 100 bars 10 mpa a two corner source model better matches the spectra for large magnitude earthquakes m 6 especially at intermediate frequencies for small to moderate events the two source models are nearly equal apparent source spectra from small to moderate earthquakes e g m 1 3 5 6 5 suggest a general similarity between the different tectonic regions studied although minor discrepancies appear in some cases there are no noticeable regional characteristics no evident depth effects were found on the apparent source spectra manuscript received 1 april 2001

determination of the solubility products of sodium carbonate minerals and an application to trona deposition in lake magadi kenya the ion interaction model of pitzer 1973 is very effective in deriving stability relationships at high concentrations for the system na cl hco3 co3 oh h2o the solubility products of the main sodium carbonates have been calculated from solubility data between 5 and 50 c the stability diagram in log pco2 temperature coordinates and the invariant points deduced from the newly determined data are in good agreement with the most recent measurements these results are used to calculate the activities of the major dissolved species in lake magadi brines kenya the thermodynamic treatment confirms the main conclusions reached earlier by eugster 1970 1980 mainly from field observations trona precipitation occurs at equilibrium while natron is likely to form when the temperature decreases below 25 c after the salt deposition the co2 supply from the atmosphere is too slow to allow equilibrium between the atmosphere and the brines in the next stages of evaporative concentration thermonatrite and halite precipitate the deposition of the latter salts along with the observed hco 3 depletion suggest that fractional crystallization is likely to control trona deposition

geochemistry and origin of formation brines from the paris basin france 2 saline solutions associated with oil fields abstract oil fields from the paris basin are located in three geological levels dogger middle jurassic rhaetian and keuper upper triassic the origin of water and dissolved salts in saline solutions associated with oil is investigated using the hypothesis that cl and br are conservative in solution in a diagram of cl br ratios vs cl the points of the dogger and the keuper aquifers lie in the zone of primary brines whereas those of the rhaetian are located in the zone of secondary brines the na cl ratio is very high in all of the saline solutions suggesting either a relatively poorly evolved brine at the beginning of halite deposition or some halite dissolution all of the saline solutions especially those from the dogger aquifer are clearly diluted dilution by meteoric waters would not alter the cl br ratio high concentrations of lithium rubidium and boron are attributed to the presence of an extremely evolved brine of marine origin in mixtures of secondary brines formed by dissolution of halite and small amounts of sylvite and ca sulphate mass balance calculations of the excess of cl with respect to na k allows the fraction x of water derived from the primary brine to be calculated assumptions are that all na and k is derived from the secondary brine and that the cl content of the primary brine is constrained by bischofite saturation at 11 000 mmol kg 1 the typical proportion of water from the primary brine waters is 0 7 in the dogger formation and 2 4 in the keuper values of the concentration factor cf expressed as the ratio of the molal concentration of a conservative ion in the evolved brine to its concentration in seawater have been calculated for lithium solutions from the dogger and keuper aquifers have cfli values of 1 8 103 and of 7 7 103 respectively in the rhaetian aquifer the linear relationship between heavy isotope contents of the brine 2h and 18o and the cl content indicates a mixture of two water sources the first one is meteoric water as defined by the intersection of the formation water line with the global meteoric water line 2 h 65 18 o 9 3 the other end member is enriched in heavy isotopes and in cl and is attributed to seawater that has dissolved evaporites mainly halite however the presence of small amounts of an extremely evolved brine is required to account for the bromide lithium rubidium and boron contents of the formation brine from the rhaetian aquifer as this contribution is very limited in amount it cannot affect the heavy isotope content of the formation water mass balance equations are solved for ternary mixtures of meteoric water seawater and brine the same constraints as above no na and k supplied by the primary brine and clbrine 11 000 mmol kg 1 are applied it is also assumed that the seawater component has the same ionic contents as modern seawater and that the salt content of the meteoric component is negligible the fraction of primary brines is x 1 4 according to the cl balance this estimate agrees with that obtained from the lithium balance assuming that the primary brine component was the same as that of the keuper formation water mass balance calculations also indicate that the so42 content of the rhaetian formation water is exclusively due to the secondary brine component in the three saline solutions the very high ca2 contents and high ca 2 mg 2 ratios may be attributed to reaction of the very evolved brine free of so2 4 with gypsum or anhydrite the result is an exchange of mg2 for ca2 which causes a release of ca2 to the solution and a precipitation of secondary mg sulphate this process can also account for the high strontium contents of the analysed samples values of the ratio 87 sr 86 sr are very high in the keuper formation water due to close contact of the solutions with the granitic basement or with granite derived detrital deposits values in the rhaetian and dogger aquifers equal those of upper triassic marine deposits the stable isotope 34s and 18o content of the dissolved aqueous so2 4 indicates a triassic origin for the bulk of sulphate involved in the evolution of formation waters from the rhaetian and from the dogger all geochemical indicators thus show a vertical migration of triassic brines however aqueous so2 4 from the keuper has a permian isotopic signature suggesting a reworking in solution of salts from this period

elastic properties of hydrate bearing sediments using effective medium theory accurate and detailed models of the seismic velocity structure of gas hydrate bearing sediments may be determined by careful analysis of controlled source seismic data however interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical formulas and or simple effective medium theory we develop a rigorous theoretical scheme to relate the seismic properties of a clay rich hydrate bearing sediment to its porosity mineralogy microstructural features and hydrate saturation we consider separately the two possible end members for the distribution of hydrate in the pore space 1 hydrates are unconnected and located in the pore voids without appreciable grain contact and 2 connected hydrates are forming cement binding around the grains the scheme is transversely isotropic to allow for anisotropy due to alignment of clay platelets and is based on a combination of a self consistent approximation a differential effective medium theory and a method of smoothing for crystalline aggregates we have applied the scheme to lithological and seismic velocity data from ocean drilling program site 995 on the blake ridge southeastern u s continental margin to make estimates of the hydrate saturation it was found that the hydrates are probably unconnected and their volume concentration varies between approx 0 at 100 m below the seabed and approx 9 at 400 m depth just above the bottom simulating reflector if the clay platelet orientation distribution resembles the function we have used

combined vertical incidence and wide angle seismic study of a gas hydrate zone blake ridge wide angle ocean bottom seismic data and single channel seismic reflection data were collected in june 1992 over an area where gas hydrates are thought to be extensive on blake ridge offshore south carolina wide angle reflections were observed on four woods hole oceanographic institution ocean bottom hydrophones at offsets up to 15 km results from traveltime inversion show that the bottom simulating reflector bsr that marks the base of the hydrate stability field lies 400 500 m below the seafloor and is overlain by a 200 to 300 m thick layer of average velocity 1 9 km s 1 there is no evidence for significant lateral velocity variation associated with lateral changes in bsr character ray tracing calculations show that the observation of the bsr out to large offsets 7 km constrains the maximum vertical velocity gradient to about 0 5 s 1 amplitude versus offset avo analysis was performed for ranges up to about 6 km and incidence angles of more than 70 the high amplitude of the bsr in the normal incidence data can be explained by two models both of which include a strong negative impedance contrast the hydrate wedge model and the free gas layer model synthetic seismogram calculation for the wedge model using the reflectivity method shows a strong increase in the bsr amplitudes at offsets between 3 and 5 km because of postcritical reflections and diving waves within the wedge this amplitude increase is clearly not observed in the data which shows only a relatively modest increase in amplitude with offset this observation agrees better with the calculated avo for the gas layer model the lack of a distinct reflection from the base of the gas layer implies that the gas layer is not thicker than 25 m and or that the base of the gas layer is a gradational boundary

studies of quaternary saline lakes iii mineral chemical and isotopic evidence of salt solution and crystallization processes in owens lake california 1969 1971 abstract as a consequence of the 1969 1970 flooding of normally dry owens lake a 2 4 m deep lake formed and 20 of the 2 m thick salt bed dissolved in it its desiccation began august 1969 and salts started crystallizing september 1970 ending august 1971 mineralogic brine composition and stable isotope data plus field observations showed that while the evolving brine composition established the general crystallization timetable and range of primary and secondary mineral assemblages it was the daily monthly and seasonal temperature changes that controlled the details of timing and mineralogy during this depositional process deuterium analyses of lake brine interstitial brine and hydrated saline phases helped confirm the sequence of mineral crystallizations and transformations and they documented the sources and temperatures of waters involved in the reactions salts first crystallized as floating rafts on the lake surface natron and mirabilite salts whose solubilities decrease greatly with lowering temperatures crystallized late at night in winter when surface water temperatures reached their minima trona nahcolite burkeite and halite salts with solubilities less sensitive to temperature crystallized during the afternoon in summer when surface salinities reached their maxima however different temperatures were generally associated with crystallization at the surface and accumulation on the lake floor because short term temperature changes were transmitted to surface and bottom waters at different rates consequently even when solubilities were exceeded at the surface salts were preserved or not as a function of bottom water temperatures halite a nearly temperature insensitive salt was always preserved monitoring the lake brine chemistry and mineralogy of the accumulating salts shows 1 an estimated 0 9 10 6 tons of co 2 was released to the atmosphere or consumed by the lake s biomass prior to most salt crystallization 2 after deposition some salts reacted in situ to form other minerals in less than one month and all salts except halite decomposed or recrystallized at least once in response to seasons 3 warming in early 1971 caused solution of all the mirabilite and some of the natron deposited a few months earlier a deepening of the lake though the lake surface lowered and an increase in dissolved solids 4 phase and solubility index data suggest that at the close of desiccation na 2 co 3 7h 2 o never reported as a mineral could have been the next phase to crystallize

modeling of strong ground motions from the 16 september 1978 tabas iran earthquake we have examined the adequacy of the published ks3 and l1 slip distribution models developed for the m s 7 4 16 september 1978 tabas earthquake hartzell and mendoza 1991 by modeling the high frequency accelerograms recorded at dayhook boshrooyeh and tabas stations the agreement in peak ground acceleration pga and duration between data and simulated accelerograms was chosen as the criterion for the model adequacy the fault was specified with a seismogenic rupture area 95 km long and 45 km wide we used a semi empirical simulation method in which accelerograms from an aftershock from the 1979 imperial valley earthquake were used to represent the radiation pattern of p sv or sh waves for a source receiver geometry the fault surface is divided into many subfaults the contributions from these subfaults weighted by the slip amounts were lagged and summed to simulate the accelerograms of the main event we computed accelerograms assuming constant rake and variable rake on the fault plane the former was simulated using the sum slip model i e the slip model representing the vector sum of the slip components at every point on the fault and the latter was simulated by summing the accelerograms generated by the slip models of the dip slip and strike slip faults the influence of the rise time of the main event was investigated the ks3 asperity model produces results marginally better than the l1 asperity model the duration is predicted consistent with the data the peak amplitudes of the simulated ground motions remain within a factor of 1 5 and 3 of the recorded data at dayhook and boshrooyeh respectively for the variable rake and within a factor for 2 5 at both stations for the fixed rake angle ks3 model the large pga recorded on the tabas accelerograms could not be reproduced which warrants additional study a notable agreement was obtained in the response spectra at dayhook this study demonstrates that accelerograms generated by an earthquake in one tectonic region can be transported to another tectonic region so that they can be used to represent the empirical radiation pattern of the subfaults in simulating ground motions for a large event

quantitative seismic pre stack analysis of potential gas hydrate resources in the makran accretionary prism offshore iran abstract gas hydrates are classified as a major unconventional resource and assumed to be a future potential energy source high pressures low temperature stability conditions of the hydrates restrict their occurrence to the permafrost and deep sea regions from the continental slop to the abyssal in these regions where the well information is most often absent or sparse the amplitude variation with angle ava is a common method of hydrate free gas assessment in this method various linear reflection coefficient equations are fitted to a series of data points to compute some pre stack attributes these attributes have the advantage of being an interface property rather than a layer property which makes them an easy to use characterization tool in lack of well data in recent years rock physics templates nomograms have been presented to estimate the hydrate free gas saturations solely based on these interface properties ava inversion approach was applied to the seismic data in the makran accretionary prism offshore iran to estimate the bottom simulating reflector bsr vicinity hydrate and gas saturations this inversion process included four pre stack attributes of the intercept gradient normal incidence ni and poisson reflectivity pr and was limited to the bsr estimated bsr vicinity saturations were ranged from 7 8 5 for hydrate and 5 6 5 for free gas at three representative locations accuracy of each attribute was analyzed by a synthetic modeling in this modeling the effective medium theory emt was considered to calculate the physical properties of hydrate and gas bearing sediments lithological properties and the acquisition geometry were set to the corresponding values of the field data results of the synthetic ava inversion showed that if the hydrate saturation does not exceed 30 the error of the estimation for pre stack attributes would be in acceptable range the expected estimation errors for actual attributes were derived from this synthetic modeling

attenuation relations of strong ground motion in japan using site classification based on predominant period a spectral acceleration attenuation model for japan is presented in the present study the data set includes a very large number of strong ground motion records up to the 2003 off tokach main and aftershocks site class terms instead of individual site correction terms are used the site classes of recording stations are from a recent study on site classification for strong motion recording stations in japan according to a classification scheme that has been used in japanese engineering design the use of site class terms enables tectonic source type effects to be identified and accounted for in the present model the effects of a faulting mechanism for crustal earthquakes also are accounted for for crustal and interface earthquakes a simple form of an attenuation model with respect to distance is able to capture the main strong motion characteristics and achieves unbiased estimates for subduction slab events a simple distance modification factor is employed to achieve plausible and unbiased predictions the effects of source depth tectonic source type and faulting mechanism of crustal earthquakes are significant the need for magnitude squared terms is evaluated and the use of magnitude squared terms reduces the interevent error further

source spectra and rms acceleration of mexican subduction zone earthquakes strong motion sm data of six mexican subduction zone earthquakes 6 4 ms 8 1 recorded near the epicentral zone are analyzed to estimate their far field source acceleration spectra at higher frequencies f 0 3 hz apart from the usual corrections such as geometrical spreading 1 r average radiation pattern 0 6 free surface amplification a factor of 2 and equal partitioning of the energy into two orthogonal horizontal components a factor of 1 sqrt 2 the observed spectra are corrected for a frequency dependentq q 100f a site dependent filter e kf and amplification ofs waves near the surface a factor of about 2 atf 2hz we taker as the average distance from the rupture area to the site if we model the high frequency plateau f 1 hz of the source spectra by a point source 2 model and interpret them in terms of brune s model we obtain between 50 and 100 bars for all earthquakes the low frequency broadband teleseismicp wave spectra corrected witht 1 0 s agrees within a factor of two with sm source spectra near 1 hz the 2 model is inadequate to explain the observed source spectra in a broad frequency range these resemble spectra given bygusev 1983 with some differences

new source duration relationships for mexican earthquakes practical application to stochastic summation methods from the gathering of 127 source time functions stfs of seismic events in mexico relationships have been developed between the seismic moment m0 and the total source rupture duration d for normal faulting inslab interplate and strike slipe mechanisms this relationship has been explored by different authors like furomoto and nakanishi j geophys res 88 2191 2198 1983 tanioka and ruff seismol res lett 68 386 400 1997 singh et al bull seismol soc ame 90 468 482 2000 among others the latter authors present a similar general relationship for mexico considering 53 stfs and with information until 1998 in this research complementary databases have been integrated and the available functions updated base on recent research works about the scardec method are used vallee et al in geophys j int 184 338 358 2011 vallee and douet in phys earth planet interiors 257 149 157 2016 with the proposed expressions it is possible to estimate reasonably the main parameters of interest for algorithms of stochastic summation such as the corner frequencies or stress drops in order to simulate strong ground motions commonly used for source rupture studies site amplification analysis or for earthquake resistant design besides a new flexible model of stochastic summation is presented which allows considering several ways to generate more realistic motions according to the spectral behavior both for low and for intermediate frequencies under earthquakes with moderate and high magnitude this model was compared with results from a two stage alternative method considering two corner frequencies

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

helium isotope fluxes and groundwater ages in the dogger aquifer paris basin the dogger aquifer of the paris basin has been shown to be first order homogeneous in its helium concentration and its helium isotopic ratio although slight differences are noted in relation to specific chemical water types the helium isotopic composition of the dogger aquifer contains a small component of mantle helium that is associated with a magmatic input although the heat accompanying this magma is small constraints on the age of the overlying albien aquifer are used to calculate the flux of helium out of the dogger this constraint allows a calculation of the residence time of waters in the dogger aquifer of 4 myr 50 which is significantly longer than estimates derived from hydrologic models and hydrologic testing it is possible that the hydrologic ages and the helium ages can be reconciled by a vertically layered dogger aquifer with gradients in he salinity and fluid flow vertical profiles of fluid composition helium concentration and permeability are required in the paris basin to resolve this question

chemical evolution of waters during long term interaction with granitic rocks in northern switzerland abstract as part of an assessment of crystalline rock as a potential host for a nuclear waste repository water samples were collected from more than 50 locations from the crystalline basement where it is under sedimentary cover in northern switzerland and where it is at the surface in the black forest these samples describe the changing chemistry of water in an extended flow system from dilute recharge waters far from chemical equilibrium with its host rock to saline waters at temperatures of 50 to 100 c with residence times far in excess of 105 a that have reached chemical equilibrium with their host rock this unique set of samples allows an analysis of the chemical evolution of granitic waters from surficial waters far from equilibrium to almost equilibrated waters mobile ions rare gases and isotopic data are used to estimate the extent of reaction between waters and their host rock the evolution of controlled elements is interpreted as a function of this extent of reaction silica contents correspond to approximate equilibrium even in the recharge waters the relative concentrations of ca and especially mg are significantly lower in the borehole waters than in recharge waters indicating a trend towards equilibrium the na k ratios correspond to equilibrium only in the most evolved older waters which are shown to be at full equilibrium

the formation and potential importance of cemented layers in inactive sulfide mine tailings abstract investigations of inactive sulfide rich tailings impoundments at the heath steele new brunswick and waite amulet quebec minesites have revealed two distinct types of cemented layers or hardpans that at heath steele is 10 15 cm thick occurs 20 30 cm below the depth of active oxidation is continuous throughout the tailings impoundment and is characterized by cementation of tailings by gypsum and fe ii solid phases principally melanterite hardpan at the waite amulet site is only 1 5 cm thick is laterally discontinuous 10 100 cm occurs at the depth of active oxidation and is characterized by cementation of tailings by fe iii minerals principally goethite lepidocrocite ferrihydrite and jarosite at heath steele an accumulation of gas phase co2 of up to 60 of the pore gas occurs below the hardpan the calculated diffusivity of the hardpan layer is only about 1 100 that of the overlying uncemented tailings the pore water chemistry at heath steele has changed little over a 10 year period suggesting that the cemented layer restricts the movement of dissolved metals through the tailings and also acts as a zone of metal accumulation generation of a cemented layer therefore has significant environmental and economic implications it is likely that in sulfide rich tailings impoundments the addition of carbonate rich buffering material during the late stages of tailings deposition would enhance the formation of hardpan layers

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

elastic velocities of partially gas saturated unconsolidated sediments abstract fluid in sediments significantly affects elastic properties of sediments and gas in the pore space can be identified by a marked reduction of p wave velocity or a decrease of poisson s ratio the elastic properties of gas saturated sediments can be predicted by the classical biot gassmann theory bgt however parameters for the bgt such as the biot coefficient or moduli of dry frame of unconsolidated and high porosity sediments are not readily available dependence of velocities on differential pressure or porosity for partially gas saturated sediments is formulated using properties derived from velocities of water saturated sediments laboratory samples for unconsolidated and consolidated sediments and well log data acquired for unconsolidated marine sediments agree well with the predictions however because the p wave velocity depends highly on how the gas is saturated in the pore space such as uniform or patch the amounts of gas estimated from the p wave velocity contains high uncertainty the modeled vp vs ratio of partially gas saturated sediment using the patch distribution is usually greater than 1 6 whereas the ratio modeled assuming a uniform distribution is about 1 6 thus poisson s ratio or vp vs ratio may be used to differentiate patch from uniform saturation but differences between various models of patch saturation cannot be easily identified

a two stage method for ground motion simulation using stochastic summation of small earthquakes we present a two stage method to simulate the ground motions produced by an earthquake by using stochastic summation of small earthquakes in this method identical small earthquakes are multiplied by a scaling factor and summed together with time delays randomly distributed during the two stages over the source duration the summation scheme is characterized by four fundamental parameters the number of summed small earthquakes the scaling factor and both probability densities of time delays used in the first and second stages by a proper choice of these parameters this method generates a large number of synthetic time histories that on average agree exactly with the 2 model in the whole frequency band the produced time histories are sufficiently realistic and different from each other to be associated with a multitude of rupture processes that could happen during an earthquake however because the extended target fault is approximated by a point source this method does not take into account possible directivity effects and is not appropriate to simulate ground motions for near source sites we test this method on the oaxaca earthquake 1999 m w 7 5 mexico at regional distances and on the two mainshocks of the umbria marche crisis 1997 m w 5 7 and m w 6 0 italy at local distances we found that the simulated ground motions fit the observed data well both in time and in frequency domains within simulation context only specification of seismic moment and stress drop is required for the target event because the magnitude and then the seismic moment are necessarily specified the stress drop plays a major role in ground motion simulation

scardec a new technique for the rapid determination of seismic moment magnitude focal mechanism and source time functions for large earthquakes using body wave deconvolution accurate and fast magnitude determination for large shallow earthquakes is of key importance for post seismic response and tsumami alert purposes when no local real time data are available which is today the case for most subduction earthquakes the first information comes from teleseismic body waves standard body wave methods give accurate magnitudes for earthquakes up to mw 7 7 5 for larger earthquakes the analysis is more complex because of the non validity of the point source approximation and of the interaction between direct and surface reflected phases the latter effect acts as a strong high pass filter which complicates the magnitude determination we here propose an automated deconvolutive approach which does not impose any simplifying assumptions about the rupture process thus being well adapted to large earthquakes we first determine the source duration based on the length of the high frequency 1 3 hz signal content the deconvolution of synthetic double couple point source signals depending on the four earthquake parameters strike dip rake and depth from the windowed real data body wave signals including p pcp pp sh and scs waves gives the apparent source time function stf we search the optimal combination of these four parameters that respects the physical features of any stf causality positivity and stability of the seismic moment at all stations once this combination is retrieved the integration of the stfs gives directly the moment magnitude we apply this new approach referred as the scardec method to most of the major subduction earthquakes in the period 1990 2010 magnitude differences between the global centroid moment tensor cmt and the scardec method may reach 0 2 but values are found consistent if we take into account that the global cmt solutions for large shallow earthquakes suffer from a known trade off between dip and seismic moment we show by modelling long period surface waves of these events that the source parameters retrieved using the scardec method explain the observed surface waves as well as the global cmt parameters thus confirming the existing trade off for some well instrumented earthquakes our results are also supported by independent studies based on local geodetic or strong motion data this study is mainly focused on moment determination however the scardec method also informs us about the focal mechanism and source depth and can be a starting point to study systematically the complexity of the stf

seismic characterization of a gas hydrate system in the gulf of mexico using wide aperture data summary gas hydrates were discovered in a mud mound in the lease block mississippi canyon 798 gulf of mexico through piston coring subsequently a seismic experiment was carried out to investigate the dynamics behind the hydrate formation during the experiment high resolution multichannel seismic reflection data using a 24 channel 240 m long streamer and wide aperture data using six ocean bottom seismometers were collected along five lines high reflectivity zones hrzs are present in the reflection data along all lines to better constrain the interpretation of the reflection data the traveltimes from the multichannel and wide aperture data sets were jointly inverted to estimate a 2 d p wave layered velocity model for each line a minimum parameter minimum structure modelling approach yielded simple models and a comparison of the models at their intersection points shows they are consistent to within 10 m s 1 in velocity and 20 m in depth in the final p wave velocity models the hrzs are associated with a lowering of velocity in the reflection data the top of the hrzs show a polarity reversal with respect to the seafloor presence of free gas in the hrzs best explains the velocity lowering and polarity reversal it is speculated that the gas has deeper sources and migrates upwards through conduits formed by salt movement in the vicinity the upward migrating gas accumulates in the axis of a channel complex and manifests itself as hrzs in the reflection data the fluids circulating along the conduits push the base of the hydrate stability zone close to the seafloor from the channel axis the free gas migrates further upwards and close to the seafloor and as it comes within the gas hydrate stability zone it forms hydrates

detection and estimation of gas hydrates using rock physics and seismic inversion examples from the northern deepwater gulf of mexico natural gas hydrates composed primarily of water and methane are solid crystalline ice like substances found in permafrost areas and deepwater basins around the world as the search for oil and gas extends into ever deeper waters particularly within the northern gulf of mexico gas hydrates are becoming more of a focus in terms of both safety and as a potential energy resource

source duration scales with magnitude differently for earthquakes on the san andreas fault and on secondary faults in parkfield california we used a comparison of source time function pulse widths to show that a group of earthquakes on the san andreas fault near parkfield have a constant duration over a magnitude range of 1 4 3 7 earthquakes on secondary faults have an increase in duration with magnitude which is the expected relationship for the usual observation of constant stress drop the constant duration suggests that fault area is the same regardless of magnitude and that variations in stress drop are due entirely to variations in slip calculated stress drop values on secondary faults range from 0 31 to 14 mpa and stress drop values on the san andreas fault range from 0 18 to 63 mpa the observation of constant duration on the san andreas fault is consistent with a model of a locked asperity in a creeping fault the differences in durations between the events on the san andreas fault and on secondary faults suggest that earthquakes on the san andreas fault are inherently different we speculate that faults with more cumulative displacement have earthquakes that may rupture differently furthermore the differences in source properties between the two populations might be explained by differences in fault surface roughness online material station subsets used to evaluate cross fault seismic velocity contrasts

geomorphology of ona basin southwestern scotia sea antarctica decoding the spatial variability of bottom current pathways abstract ona basin the westernmost oceanic basin in the southern scotia sea is affected by the opposite flows of circumpolar deep water cdw and weddell sea deep water wsdw thus it represents a key location for exploring seafloor morphologies influenced by bottom currents the present study aims to capture the spatial arrangement of recent subsurface contourite features assuming a latitudinal influence of water masses and the interactions between along and downslope processes in order to contribute to the knowledge of regional deepwater flow pathways and to the sedimentary model of small sediment starved oceanic basins to this end the investigation combines an interpretation of multibeam bathymetry and parametric echo sounder seismic data complemented with hydrological data the distribution of morpho sedimentary features in ona basin reveals two major domains the southern margin of the basin can be regarded as a mixed hybrid system containing abundant sediment drifts with channels and contourite moats and a lateral continuity interrupted by downslope morphologies in contrast the northern abyssal setting comprises relatively homogeneous large sheeted drifts with superimposed sediment waves mounded drifts and several scattered erosive features likely reflecting the more distinct influence of deepwater contourite processes our work demonstrates that tectonic features in the southern basin control the interaction between deepwater along and downslope processes as the westward flow of the wsdw is deflected channelized and intensified along its westward route in the northern region the study indicates an overall clockwise rotation of the wsdw flow with the spatial and vertical variability of cdw and wsdw affecting the distribution of bottom current features around seamounts and or structural highs the results underscore the importance of sloping interphases in the water mass vertical structure the degree of basin confinement and the influence of local bathymetric elevations in sedimentation models of small sediment starved oceanic basins

source times and scaling relations of large earthquakes source time is a kinematic fault parameter corresponding to the duration of seismic source time functions and is accurately determined from phase spectra of long period surface waves source times are determined for 36 large earthquakes during the last three decades scaling relations among source times seismic moments and fault dimensions are derived seismic moment is proportional to the cube of source time and fault dimension is proportional to source time source times for low angle thrust earthquakes along deep sea trenches are found to be longer than those of other types such as intraplate shocks and deep shocks they are also significantly longer than the rupture times expected from a haskell model suggesting that generally there exists an introductory stage of faulting that precedes the main stage of the rupture propagation

influence of depth focal mechanism and tectonic setting on the shape and duration of earthquake source time functions source time functions of 255 moderate to great earthquakes obtained from inversions of teleseismic body waves by tanioka and ruff 1997 and coworkers were compared in a systematic way they were scaled to remove the effect of moment and to allow the direct comparison and averaging of time function shape as well as duration time function durations picked by tanioka and ruff 1997 are proportional to the cube root of seismic moment if moments from the harvard centroid moment tensor catalog are used the average duration of scaled time functions is shorter and the average shape has a more abrupt termination for deeper events than shallower ones with a distinct change occurring at 40 km depth the complexity of the time functions as quantified by the number of subevents appears to decrease below 40 km depth furthermore among events shallower than 40 km the average duration of scaled time functions is shorter and their average shape has a more abrupt termination 1 for events with strike slip focal mechanisms compared to thrust events and 2 for the few thrust events associated with an intraplate setting compared to the majority associated with an interplate subduction boundary in each of these cases events in more technically and seismically active settings have a longer duration and a more gradual termination this can be interpreted in terms of lower stress drops and or slower rupture velocities at active plate boundaries suggesting that fault rheology depends on slip rate and may evolve as total fault slip accumulates furthermore differences in average time function shape and duration associated with different subduction zones suggest that differences exist in the rheology on the plate boundaries at the various subduction zones supporting data table is available via web browser or via anonymous ftp from ftp kosmos agu org directory append username anonymous password guest subdirectories in the ftp site are arranged by paper number information on seraching and submitting electronic supplements is found at http www agu org pubs esupp about html