stable isotope compositions of waters in the great basin united states 3 comparison of groundwaters with modern precipitation 1 groundwater samples from wells and springs scattered over most of the great basin province were collected and analyzed for their isotopic makeup they were augmented by previously published isotopic data on groundwaters from southeast california and by several hundred unpublished isotopic analyses the ratio of 2h deuterium d to 1h in water samples from valleys in parts of california idaho nevada oregon and utah are here compared with the winter summer and annual isotopic compositions of precipitation falling in or near the sampled areas the main goal of this study was to identify basins where the groundwaters have isotopic compositions that are lighter depleted in the heavier isotope d relative to modern winter precipitation where these basins do not adjoin substantially higher terrain we consider those light groundwaters to be of pleistocene age and thus more than 10 000 years old where the groundwater is 10 to 19 lighter than local winter precipitation we consider it to be possibly an indication of pleistocene water where the d makeup is 20 lighter we consider it to be probably pleistocene water more than 80 sites underlain by waters of possible or probable pleistocene age were identified

potential for travertine formation fossil creek arizona abstract chemical analyses of water emanating from fossil springs in central arizona were conducted to predict changes in travertine deposition related to changes in stream discharge caused by diversion for hydroelectric power generation during spring of 1996 water was sampled at 15 locations during normal seepage flow in a 6 7 km reach below fossil springs and at full baseflow during turbine maintenance analyses resulted in a rate of 11 923 kg d 1 of caco3 precipitated from 1218 l s 1 of water emanating from the springs while flows of 5 6 l s 1 that seep past a diversion dam produced 46 kg d 1 of caco3 precipitation active travertine dams currently occur predominantly below the irving hydroelectric powerplant with partial return of diverted flow back into the natural channel the lower reach resulted in 519 kg d 1 of caco3 precipitated from the return of 56 6 l s 1 with a reduced rate of precipitation during surface runoff conditions due to a dilution effect artificial substrates were located at sites in the lower reach for comparison with mass transfer rates derived from changes in water chemistry comparison between actual precipitation rates and overall mass transfer rates suggested preferential deposition was occurring at dam locations rates of mass transfer for high flow were greater than lower flow overall mass transfer rates for the upper reach were proportional to the flow velocities with total mass transfers for both flows being approximately equal

land surface feedbacks and palaeomonsoons in northern africa we ran a sequence of climate model experiments for 6000 years ago with land surface conditions based on a realistic map of palaeovegetation lakes and wetlands to quantify the effects of land surface feedbacks in the saharan region vegetation induced albedo and moisture flux changes produced year round warming forced the monsoon to 17 25 n two months earlier and shifted the precipitation belt 300 km northwards compared to the effects of orbital forcing alone the addition of lakes and wetlands produced localised changes in evaporation and precipitation but caused no further extension of the monsoon belt diagnostic analyses with biome and continental hydrology models showed that the combined land surface feedbacks although substantial could neither maintain grassland as far north as observed 26 n nor maintain lake megachad 330 000 km

true amplitude imaging and dip moveout true amplitude seismic imaging produces a three dimensional 3 d migrated section in which the peak amplitude of each migrated event is proportional to the reflectivity for a constant velocity medium the standard imaging sequence consisting of spherical divergence correction normal moveout nmo dip moveout dmo and zero offset migration produces a true amplitude image if the dmo step is done correctly there are two equivalent ways to derive the correct amplitude preserving dmo the first is to improve upon hale s derivation of f k dmo by taking the reflection point smear properly into account this yields a new jacobian that simply replaces the jacobian in hale s method the second way is to calibrate the filter that appears in integral dmo so as to preserve the amplitude of an arbitrary 3 d dipping reflector this latter method is based upon the 3 d acoustic wave equation with constant velocity the resulting filter amounts to a simple modification of existing integral algorithms the new f k an

effective pressure or what is the effect of pressure using time lapse seismics as a reservoir monitoring tool geophysics can help distinguish different reservoir production scenarios for example eiken et al 2000 successfully detected fluid saturation changes after co2 injection using time lapse seismics at sleipner field over the cycle of a reservoir life oil saturation usually decreases reservoir pressure declines and gas breakout may occur these changes cause rock property changes that are detectible in time lapse seismics therefore it is important to understand the effects of pressure and saturation changes on rock properties while the effects of saturation changes are often well described by gassmann 1951 brown and korringa 1975 and mavko 1975 the effects of pressure changes are less understood here we focus on understanding the effects of fluid pressure on velocities

temporal and environmental significance of microbial lamination insights from recent fluvial stromatolites in the river piedra spain despite extensive research the environmental and temporal significance of microbial lamination is still ambiguous because of the complexity of the parameters that control its development a 13 year monitored record of modern fast accreting calcite stromatolites mean 14 mm year 1 from artificial substrates installed in rapid flow in the river piedra north east spain allows comparison of the sedimentological attributes of successive six month depositional packages with the known climatic hydrophysical and hydrochemical parameters of the depositional system the stromatolites are formed of dense porous and macrocrystalline composite laminae the dense and porous composite laminae which are composed of two to eight laminae consisting largely of calcified cyanobacteria are characterized by i dense composite laminae up to 15 mm thick mostly with successive dense laminae and minor alternating dense and porous laminae and ii porous composite laminae up to 12 mm thick consisting mainly of porous laminae alternating with thinner dense laminae most of the dense composite laminae formed during the warm periods april to september whereas most of the porous composite laminae developed in the cool periods october to march each dense and porous composite lamina represents up to or slightly longer than six months the alternation of these two types of composite laminae parallels seasonal changes in temperature the dense and porous laminae result from shorter for example intraseasonal variations in temperature insolation and hydrological conditions the macrocrystalline laminae with crystals 100 m long occur isolated and grouped into composite laminae up to 1 7 mm thick their occurrence suggests the absence or poor development of microbial mats over periods of weeks to several months thus stromatolite lamination can record different order periodic and non periodic changes in the magnitude of environmental parameters over a single year these results hold important implications for the temporal and environmental interpretation of lamination in microbial structures

2 d p sv and sh spectral element modelling of seismic wave propagation in non linear media with pore pressure effects it has long been recognized that the effects of superficial geological layers or site effects can play a major role on the seismic ground motion at the free surface in this study we compute wave propagation in a 2 d asymmetrical basin considering both soil non linearity and pore pressure effects equations of elastodynamics of wave propagation are solved using the spectral element method sem the geometry of the basin gives rise to basin edge generated waves that are different for in plane p sv and out of plane sh wave propagation and resulting in different non linear response moreover the excess pore pressure development in superficial liquefiable layers effective stress analysis brings larger deformation and loss of strength than the analysis without pore pressure effects total stress analysis the coupling of vertically propagating waves and the waves specifically generated in 1 d model leads to waves whose amplitude and duration are higher than the 1 d case this multidimensional effect increases material non linearity such complex wavefield provokes larger deformation and higher pore pressure rise that cannot be predicted by 1 d modelling therefore our paper suggests the use of multidimensional modelling while studying seismic wave propagation in both linear and non linear complex media

fault whispers transmission distortions on prestack seismic reflection data transmission distortions are observed on prestack seismic data at two locations in the gulf of mexico these distortions produce anomalous amplitude versus offset avo signatures the locations of the distortion zones are determined using acquisition geometry and ray tracing no obvious reflection events such as shallow gas zones are observed at the predicted locations of the distortion zones instead the distortion zones correlate with buried faults and unconformities it is postulated that the distortions are produced by velocity changes across buried faults and unconformities the distortions result from an interference pattern resulting from seismic waves arriving from different sides of the faults a simple model is developed to explain many of the characteristics of the distortion pattern

oxygen isotope analyses of fine grained minerals and rocks using the laser extraction technique abstract the 18o values of fine grained materials made with the laser extraction technique are often lower than those obtained conventionally we propose a simple method to overcome this problem that involves the mixing of samples with a lif binder and pressing the powder into pellets prior to evacuation and analysis the presence of the lif allows for rapid fluorination of all minerals or whole rock samples to be made thus eliminating potential problems related to loss of partially reacted ejecta slow laser heating and incidental fluorination accurate isotope determinations of fine grained materials can be made with this method

geophysics a moving fluid pulse in a fault zone evidence from the south eugene island block 330 oil and gas field in the gulf of mexico off louisiana suggests that the fault zones there have acted as conduits for fluids rising from deep in the earth s crust and now one of these fault burps has been observed directly seismic reflection imaging revealed the rapid ascent of a pulse of fluid rising vertically up the fault by 1 km between 1985 and 1992 burps like this may be important factors in subsurface hydrocarbon migration in the gulf of mexico fault zones are linked with a complex and dynamic system of plumbing in the earth s subsurface here we use time lapse seismic reflection imaging to reveal a pulse of fluid ascending rapidly inside one of these fault zones such intermittent fault burping is likely to be an important factor in the migration of subsurface hydrocarbons

late quaternary zonation of vegetation in the eastern grand canyon fossil assemblages from 53 packrat middens indicate which plant species were dominant during the last 24 000 years in the eastern grand canyon past vegetational patterns show associations that cannot be attributed to simple elevational displacement of the modern zones a model emphasizing a latitudinal shift of climatic values is proposed

cyanobacterial tufa calcification in two freshwater streams ambient environment chemical thresholds and biological processes abstract calcareous freshwater streams near bad urach southwest germany were studied to determine the environmental limits to cyanobacterial calcification daily variations in temperature ph calcium concentration and alkalinity were measured at seasonal intervals from september 1993 to january 1995 in two small woodland streams with lengths of 1 0 and 1 5 km the principal cause of supersaturation in these fast flowing streams is inorganic carbon dioxide outgassing from resurging groundwater locally enhanced by turbulence at waterfalls and cascades photosynthetic uptake of carbon dioxide and temperature effects are negligible nonetheless organic substrates particularly cyanobacteria dominated microbial mats and biofilm significantly localize precipitation by providing suitable nucleation sites precipitation rates on artificial substrates up to 2 2 mm yr on limestone correlate with high saturation index values copper substrates inhibited microbial colonization and received negligible encrustation tufa formation and external calcium carbonate encrustation of cyanobacteria are conspicuous where the annual wateq4f saturation index exceeds 0 8 and are slightly below 0 7 calcium carbonate impregnation of cyanobacterial sheaths has not been observed we infer that in these fast flowing streams cyanobacteria utilize co2 in photosynthesis whereas elsewhere in sluggish freshwater cyanobacteria utilize hco3 leading to sheath impregnation by calcium carbonate even where saturation index is only 0 2 0 3 thus photosynthetic influence on cyanobacterial calcification appears to be negligible in fast flowing co2 rich streams and cyanobacterial calcification takes the form of external encrustation in slow flowing co2 poor streams and lakes cyanobacteria preferentially utilize bicarbonate and sheath impregnation can result modern tropical marine carbonate shelves have saturation indices commonly in the range 0 62 0 82 and cyanobacterial calcification is negligible extrapolation suggests that cyanobacterial calcification could occur in present day seas if saturation index exceeded 0 8 this level corresponding with heavy calcification in the freshwater streams studied may have been widespread in marine environments in the geological past at times when calcified cyanobacteria and environmentally influenced non skeletal calcium carbonate precipitates were abundant

effects of non welded interfaces on guided sh waves the effects of non welded interfaces on guided sh waves are examined analytically and numerically using the displacement discontinuity model to describe the mechanical shear stiffnesses of the interfaces a modal solution for guided sh waves is developed for a layer in non welded contact with underlying and overlying half spaces for low interface stiffnesses the modal solution degenerates to the sh plate wave solution and for high interface stiffnesses to the love channel wave solution it is demonstrated that a single non welded interface separating two half spaces is incapable of supporting an sh interface wave the effects of interface stiffness on the phase velocities and displacement profiles of guided sh waves are examined for a low velocity layer in non welded contact with bounding half spaces using the modal solution and a numerical boundary integral equation code the results of this analysis demonstrate that the magnitude of the interface stiffness can have a first order effect on the velocities amplitudes and displacement profiles of guided sh waves in a low velocity layer which should make these waves highly detectable in crosshole seismic surveys

seasonal temperatures from δ18o in recent spanish tufa stromatolites equilibrium redux this study focuses on recent debate over the value of stable isotope based environmental proxies recorded in riverine tufa stromatolites we recovered a 12 year record 1999 2012 of river bed tufa stromatolites in the river piedra ne spain along with a partly overlapping 15 year record 1994 to 2009 of accumulations in a drainage pipe both deposits formed in water with near identical physico chemical parameters measured water temperature tw data and near constant 18owater composition allowed selection of an equilibrium palaeotemperature equation that best replicated actual temperatures we found as other have that both the epstein et al 1951 and kim o neil 1997 formulas for tw calculation from equilibrium calcite 18o compositions were appropriate for the river piedra where tufa deposition rates are high means between 5 6 and 10 8 mm in six months 18ocalcite in both the river and the pipe deposits record essentially the full actual seasonal tw range only the coldest times tw 10oc when calcite precipitation mass decreased to minimum are likely to be unrepresented an effect most noticeable in the pipe where depositional masses are smaller and below sample resolution while we cannot rule out kinetic effects on 18ocalcite based calculated tw the good fit between measured tw and 18ocalcite calculated tw indicates that temperature is the principal control textural and deposition rate variability between the river and pipe settings are caused by differences in flow velocity and illumination in the river calcification of growing cyanobacterial mat occurred throughout the year producing composite dense and porous laminae whereas in the pipe discontinuous cyanobacterial growth in winter promoted more abiogenic calcification high resolution 18ocalcite data from synchronous pipe and river laminae show that reversals in tw occur within laminae not at lamina boundaries a pattern consistent with progressive increase in calcite precipitation rate as cyanobacterial growth re established in spring

effect of in stream physicochemical processes on the seasonal variations in δ13c and δ18o values in laminated travertine deposits in a mountain stream channel abstract travertines are potential archives of continental paleoclimate records of stable carbon and oxygen isotopic composition 13 c and 18 o in laminated travertine deposits from endogene spring waters show regular cyclic patterns which may be due to seasonal change in climate determinants such as temperature and rainfall in this study 13 c and 18 o measurements of three travertine specimens that grew naturally over the eight years 2004 2011 at upstream middle and downstream sites in a canal at baishuitai sw china are presented they exhibit clear seasonal variations that generally correlate with biannual laminations specifically 13 c and 18 o values show significant positive correlation with each other for the three travertine specimens with the correlation coefficients increasing downstream along the canal to reveal the factors governing the seasonal and spatial variations in 13 c and 18 o values newly formed travertines precipitated on plexiglas substrates are also examined both 13 c and 18 o of the substrate travertines are low in the summer rainy season and high in the winter dry season showing a great consistency with the patterns in the natural travertines spatially isotope values increase downstream in both seasons with higher increase rates in winter that are related to removal of larger fractions of dissolved inorganic carbon dic from the solution and stronger kinetic isotopic fractionation in winter due to in stream physicochemical processes including caco 3 precipitation and the associated degassing of co 2 seasonal changes in 13 c and 18 o in the travertines are amplified by two times between the upstream and downstream sites this is opposite to trends for epigene meteogene tufas whose seasonal changes in stable isotope compositions are reduced downstream we suggest in stream physicochemical processes are a potential reason for underestimation of annual temperature ranges that are inferred from epigene tufa 18 o data

the echo of a fault or fracture the seismic response of single faults joints or fractures of large planar extent is analyzed these are modeled as nonwelded interfaces in spite of the large range of scale all are assumed to behave according to linear slip theory such a model has been considered theoretically and experimentally before the aim of this paper is to give a physical interpretation to such a linear slip interface to provide simple analytical formulae for the scattering response even when the fracture is embedded in an anisotropic background medium and to relate the properties of this scattering response in the isotropic case to the physical features of the fracture the analysis shows that the reflectivity and transmissivity of the fracture depend on slowness along the fracture and on frequency the frequency dependence arises from the fact that even though the fracture is assumed to be an interface of zero thickness it still has at least two characteristic widths that provide the length scales necessary for scatt

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

high resolution records of rainfall events from clay bands in tufa individual rainfall events are recorded as thin clay bands in annually laminated freshwater carbonate tufa runoff from rainfall events entrains clay that then adheres to the tufa surface a specimen collected from southwest japan exhibits 100 clay bands formed during a 15 yr period from 1988 to 2002 the chronology of the clay bands was determined by using the temperature dependent cycle of oxygen isotope ratios in the carbonate and the clay content was quantified by electron probe microanalysis si x ray strength the obtained si x ray strength curve and actual rainfall record follow a nearly identical pattern the clay content of each band correlates with rainfall amount rainfall of 50 mm week produced sufficient amounts of suspended cave clay to produce clay bands in the tufa because the clay bands survive postdepositional alteration this new method can be applied to ancient tufa deposits and has great potential as a high resolution rainfall record for use in climate studies

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

laminated carbonate deposits in roman aqueducts origin processes and implications carbonate deposits in roman aqueducts of patara and aspendos southern turkey were studied to analyse the nature of their regular layering optical microscopy and electron backscattered diffraction results show an alternation of dense coarsely crystalline translucent laminae composed of bundles and fans of elongate calcite crystals with their c axes parallel to the long axis and porous fine grained laminae with crystals at near random orientation the 18o and 13c data show a strong cyclicity and anti correlation whereby high and low 18o values correspond to dense columnar and porous fine grained laminae respectively geochemical analyses show similar cyclic changes in carbonate composition electron microprobe and laser ablation inductively coupled mass spectrometry analyses show that porous fine grained laminae are enriched in elements associated with detrital material fe mg k al and si whereas the dense columnar laminae are nearly pure calcite the gradient in major and trace element distribution regular changes in crystal type and in oxygen and carbon isotope composition from porous fine grained to dense columnar laminae reflect changes in water chemistry discharge temperature and biological activity because of the strong bimodal cyclicity of the mediterranean climate in southern turkey the observed laminae can be attributed to calcite deposition during the dry porous fine grained and wet season dense columnar respectively this observation implies that with proper geochemical and microstructural control lamination in carbonate deposits in roman aqueducts can be used for relative dating of aqueduct construction and maintenance and to obtain data on external factors that influenced the aqueducts such as palaeoclimate and natural hazards carbonate deposits in roman aqueducts show properties of both flowstone speleothems and riverine fresh water tufa as many aqueducts of nearly identical channel geometry are present in different climate zones and with different source water characteristics they can be used as natural experiment setups to test and improve existing models of how laminated fresh water carbonates record climate on time scales ranging from seasonal to millennial

the role of gypsum and or dolomite dissolution in tufa precipitation lessons from the hydrochemistry of a carbonate sulphate karst system the precipitation of freshwater carbonates tufa along karstic rivers is enhanced by degassing of carbon dioxide co2 downstream of karstic springs however in most karstic springs co2 degassing is not enough to force the precipitation of tufa sediments little is known about the role of dissolution of gypsum or dolomite in the hydrochemistry of these systems and how this affects the formation of tufa deposits here we present a monitoring study conducted over a year in trabaque river spain the river has typical karst hydrological dynamics with water sinking upstream and re emerging downstream of the canyon mixing of calcium magnesium bicarbonate and calcium sulphate waters downstream of the sink enhances the dissolution of carbonates and potentially plays a positive role in the formation of tufa sediments however due to the common ion effect dissolution of dolomite and or gypsum causes precipitation of underground calcite cements as part of the incongruent dissolution of dolomite dedolomitization process which limits the precipitation of tufa sediments current precipitation of tufa is scant compared to previous holocene tufa deposits which likely precipitated from solutions with higher saturation indexes of calcite sicc values than nowadays limited incongruent dissolution of dolomite dedolomitization favours higher sicc values this circumstance occurs when waters with relatively high supersaturation of dolomite and low so42 composition sink in the upper sector of the canyon in such a scenario the process of mixing waters enhances the exclusive dissolution of limestones preventing the precipitation of calcite within the aquifer and favouring the increase of sicc values downstream of the springs such conditions were recorded during periods of high water level of the aquifers and during floods this research shows that the common ion effect caused by the dissolution of gypsum and or dolomite rocks can limit or favour the precipitation of tufa sediments depending on the occurrence or not of incongruent dissolution of dolomite dedolomitization copyright 2016 john wiley sons ltd

mid holocene climates of the americas a dynamical response to changed seasonality simulations of the climatic response to mid holocene 6 ka bp orbital forcing with two coupled ocean atmosphere models foam and csm show enhancement of monsoonal precipitation in parts of the american southwest central america and northernmost south america during northern hemisphere summer the enhanced onshore flow that brings precipitation into central america is caused by a northward displacement of the inter tropical convergence zone driven by cooling of the equatorial and warming of the northern subtropical and mid latitude ocean ocean feedbacks also enhance precipitation over the american southwest although the increase in monsoon precipitation there is largely driven by increases in land surface temperature the northward shift in the equatorial precipitation band that causes enhanced precipitation in central america and the american southwest has a negative feedback effect on monsoonal precipitation in northern south america the simulations demonstrate that mid holocene aridity in the mid continent of north america is dynamically linked to the orbitally induced enhancement of the summer monsoon in the american southwest with a spatial structure wet in the southwest and dry in the mid continent similar to that found in strong monsoon years today changes in winter precipitation along the west coast of north america in central america and along the gulf coast caused by southward displacement of the westerly storm tracks indicate that changes in the northern hemisphere winter monsoon also play a role in regional climate changes during the mid holocene although the simulations with foam and csm differ in detail the general mechanisms and patterns are common to both the model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation lake status and aeolian data from the americas

wet dry seasonal and spatial variations in the δ13c and δ18o values of the modern endogenic travertine at baishuitai yunnan sw china and their paleoclimatic and paleoenvironmental implications abstract seasonal and spatial variations in the 13 c and 18 o values of the modern endogenic thermogene travertine deposited in a calcite depositing canal at baishuitai yunnan sw china were examined to understand their potential for paleoclimatic and paleoenvironmental implications the sampling sites were set in the upstream middle reach and downstream of the canal and the modern endogenic travertine samples were collected semimonthly to measure their 13 c and 18 o values it was found that both 13 c and 18 o values of the endogenic travertine were low in the warm rainy season and high in the cold dry season and correlated with each other the low 18 o values in warm rainy season were mainly related to the higher water temperature and the lower 18 o values of rainwater and the low 13 c values are caused by the dilution effect of overland flow with low 13 c values in the warm rainy season and the reduced co 2 degassing of canal water caused by the dilution effect of the overland flow the linear negative correlation between the travertine 18 o or 13 c values and rainfall amount may be used for paleo rainfall reconstruction if one knows the 18 o or 13 c values of the fossil endogenic travertine at baishuitai though the reconstruction was not straightforward it was also found that there was a progressive downstream increase of the 18 o and 13 c values of the travertine along the canal the former being mainly due to the preferential evaporation of h 2 16 o to the atmosphere and the latter to the preferential release of 12 co 2 to the atmosphere during co 2 degassing however the downstream increase of the travertine 18 o and 13 c values was less intensive in rainy season because of the reduced evaporation and co 2 degassing during the rainy season to conclude the downstream travertine sites could be more favorable for the paleo rainfall reconstruction while the upstream travertine sites are more favorable for the paleo temperature reconstruction so this study demonstrates that endogenic travertine like epigenic meteogene tufa could also be a good candidate for high resolution paleoclimatic and paleoenvironmental reconstruction

stable isotopic and elemental characteristics of recent tufa from a karstic krka river south east slovenia useful environmental proxies tufa samples from 16 consecutive barrages along a 13 km section of a groundwater fed krka river slovenia were analysed for their petrographical mineralogical elemental and stable carbon 13c and oxygen 18o isotope composition to establish their relation to current climatic and hydrological conditions waters constantly oversaturated with calcite and the steep morphology of the krka riverbed stimulate rapid co2 degassing and subsequent tufa precipitation the carbon isotope fractionation 13c between dic and tufa in the krka river evolves toward isotopic equilibrium being controlled by continuous co2 degassing and tufa precipitation rate downstream the 13c increased from 1 9 to 2 5 vienna peedee belemnite however since tufa precipitation rates remain similar downstream the major controlling factor of carbon isotope exchange is most probably related to the continuous 12co2 degassing downstream leaving the carbon pool enriched in 13c in the case of oxygen the isotope fractionation was found to be from 1 0 to 2 3 smaller than reported in the literature the observed discrepancies are due to different precipitation rates of calcite deposits as krka tufas on cascades grow relatively faster compared to slowly precipitated calcite deposits in cave or stream pools due to non equilibrium oxygen isotope exchange between krka tufa and water the 18o proxy showed from 1 2 to 8 2 c higher calculated water temperatures compared to measured water temperatures demonstrating that 18o proxy based temperature equations are not reliable for water temperature calculations of fast growing tufa on cascades because mg is bound to the terrigenous dolomite fraction in the krka tufa samples the mg ca was also found to be an unreliable temperature proxy yielding over up to 20 c higher calculated water temperatures this article is protected by copyright all rights reserved

oxygen isotope composition of annually banded modern and mid holocene travertine and evidence of paleomonsoon floods grand canyon arizona usa abstract holocene and modern travertine formed in spring fed havasu creek of the grand canyon arizona was studied to determine the factors governing its oxygen isotope composition analysis of substrate grown travertine indicates that calculated calcite formation temperatures compare favorably with measured water temperatures and include silt rich laminae deposited by monsoon driven floods ancient spring pool travertine is dated by u series at 7380 110 yr and consists of 14 travertine silt couplets of probable annual deposition one hundred eighty high resolution 18o analyses of this mid holocene sample average 11 0 pdb the average value for modern travertine is 0 5 lower perhaps because mid holocene temperature was higher or there was proportionally greater summer recharge 18o cyclicity in the mid holocene travertine has average amplitude of 1 9 0 5 pdb slightly less than the inferred modern day annual temperature range of havasu creek the annual temperature range might have been reduced during the 14 yr interval compared to present although other non temperature factors could account for the muted annual variation silt rich laminae within isotopically lower calcite in the modern and mid holocene travertine verifies the seasonal resolution of both samples and suggests that similar temperature precipitation conditions as well as monsoon generated summer floods prevailed in the mid holocene as they do throughout the grand canyon region today

numerical simulation of wave propagation in media with discrete distributions of fractures effects of fracture sizes and spatial distributions summary we model seismic wave propagation in media with discrete distributions of fractures using the pseudospectral method the implementation of fractures with a vanishing width in the 2 d finite difference grids is done using an effective medium theory that is the coates and schoenberg method fractures are treated as highly compliant interfaces inside a solid rock mass for the physical representation of the fractures the concept of linear slip deformation or the displacement discontinuity method is used according to this model the effective compliance of a rock mass with one or several fracture sets can be found as the sum of the compliances of the host background rock and those of all the fractures to first order the background rock and fracture parameters can be related to the effective anisotropic coefficients which govern the influence of anisotropy on various seismic signatures we test the validity of the method and examine the accuracy of the synthetic seismograms by a comparison with theoretical ray traveltimes we present three numerical examples to show the effects of different fracture distributions the first example shows that different spatial distributions of the same fractures produce different wavefield characteristics the second example examines the effects of variation of fracture scale length size compared with the wavelength the final example examines the case of fractures with a power law fractal distribution of sizes and shows how that affects the wavefield propagation in fractured rock we conclude that characterization of fractured rock based on the concept of seismic anisotropy using effective medium theories must be used with caution scale length and the spatial distributions of fractures which are not properly treated in such theories have a strong influence on the characteristics of wave propagation

a simple derivation of the effective stress coefficient for seismic velocities in porous rocks the effect of confining stress and pore pressure on seismic velocities is important for such geophysical applications as overpressure prediction from seismic data eaton 1975 dutta 2002 huffman 2002 sayers et al 2002 and more recently for hydrocarbon production monitoring using time lapse seismic measurements tura and lumley 1999 landr 2001 the dependence of seismic velocity on pressure has been confirmed for a variety of rocks by laboratory measurements of elastic wave velocities in samples with varying pressure in pore fluids see e g wyllie et al 1958 todd and simmons 1972 eberhart phillips et al 1989 prasad and manghnani 1997 in general for a rock subjected to a given confining stress c higher pore pressures p correspond to lower compressional and shear velocities confining stress has a similar effect but with opposite sign on seismic velocities

ages of travertine deposits in eastern grand canyon national park arizona abstract travertine deposits in eastern grand canyon national park arizona are inferred to have formed under conditions of effective wetness that were greater than present uranium series dating of 17 samples indicates that the deposits formed at about 15 000 71 000 111 000 171 000 and 338 000 yr b p intervals of travertine deposition are essentially contemporaneous with dated high paleolake levels montane glacial maxima and high paleowater table levels from several great basin localities

negative polarity seismic reflections along faults of the oregon accretionary prism indicators of overpressuring either thrusting of higher over lower impedance sediment or reduction of impedance locally in the fault zone can produce negative polarity reflections along the protothrusts and the frontal thrust of the oregon accretionary prism the vertical displacement of protothrusts averages only 27 m is uncorrelated to the occurrence of polarity reversals and is insufficient to generate the impedance contrasts associated with the negative polarity reflections the negative polarity reflections from the protothrusts are probably due to localized impedance reduction in the fault zone waveform modeling of these negative polarity reflections indicates they develop due to a single interface with negative impedance contrast this seismic signal could be produced by a fault zone with a sharp upper boundary with low impedance fault material and a transition down section to sediment of higher impedance an impedance inversion due to the 1 km throw on the frontal thrust could create the observed negative polarity reflections interval velocities do not decrease across the frontal thrust but core scale velocity measurements indicate a velocity decrease in the frontal thrust therefore the negative polarity reflections are apparently produced by a localized decrease in impedance waveform modeling of frontal thrust reflections suggests the low impedance interval is about 20 m thick with discrete upper and lower boundaries moderately dipping faults are an efficient path for fluid expulsion across the turbidites with low equivalent vertical permeability fluid migration from the over pressured section up the faults can locally reduce the velocity sufficiently to cause the negative polarity reflections

wet dry seasonal variations of hydrochemistry and carbonate precipitation rates in a travertine depositing canal at baishuitai yunnan sw china implications for the formation of biannual laminae in travertine and for climatic reconstruction travertine or tufa is a chemical sedimentary deposit that is precipitated from springfed stream water supersaturated with calcite due to co2 degassing liu et al 1995 pentecost 1995 ford and pedley 1996 these deposits have biannual laminations which could provide valuable paleoclimatic and paleoenvironmental information with high resolution liu et al 2006 kano et al 2007 brasier et al 2009 lojen et al 2009

the future of pore pressure prediction using geophysical methods predrill pressure prediction has historically been done using very simple models and overly simplistic estimates of the earth s velocity field the methods usually incorporate a locally calibrated set of curves for pressure the advent of the effective stress concept and pressure prediction methods that developed from that concept have led to a much needed inclusion of fundamental physics into the art of pressure prediction the use of effective stress methods has become the standard for pressure prediction this technique has many variants including the eaton method the bowers method and the sperry sun method

finite difference modeling of faults and fractures for the purposes of seismic propagation a slip fault may be regarded as a surface across which the displacement caused by a seismic wave is discontinuous while the stress traction remains continuous the simplest assumption is that this slip and the stress traction are linearly related such a linear slip interface condition is easily modeled when the fault is parallel to the finite difference grid but is more difficult to do for arbitrary nonplanar fault surfaces to handle such situations we introduce equivalent medium theory to model material behavior in the cells of the finite difference grid intersected by the fault virtually identical results were obtained from modeling the fault by 1 an explicit slip interface condition fault parallel to the grid and 2 using the equivalent medium theory when the finite difference grid was rotated relative to the fault and receiver array no additional computation time is needed except for the preprocessing required to find the relevant cells and their associated moduli the formulation is sufficiently general to include faults in and between arbitrary anisotropic materials with slip properties that vary as a function of position

co2 outgassing and calcite precipitation in falling spring creek virginia u s a abstract chemical and physical observations are presented to explain the evolution of stream water chemistry along the flow path in warm river cave and falling spring creek mixing of two chemically and thermally distinct waters in the cave defines the initial stream water chemistry all along the flow path the observed composition is influenced by outgassing of co2 the water is supersaturated with respect to co2 both in the subsurface and at the surface of the earth but outgassing is greatest in turbulent stretches of the stream above and at the waterfalls the loss of co2 drives the solution to high degrees of supersaturation with respect to calcite physical evidence for calcite precipitation exists in the rimstone dams observed inside the cave and in the travertine deposits above and below the falls along falling spring creek travertine precipitation in the streambed consumes ca2 and hco 3 with most of the precipitation occurring where the stream experiences the most rapid co2 loss the stream water maintains a significant level of supersaturation with respect to calcite over the entire length of the flow path studied

vertical and lateral fluid flow related to a large growth fault south eugene island block 330 field offshore louisiana data from sediments in and near a large growth fault adjacent to the giant south eugene island block 330 field offshore louisiana indicate that the fault has acted as a conduit for fluids whose flux has varied in space and time core and cuttings samples from two wells that penetrated the same fault about 300 m apart show markedly different thermal histories and evidence for mass flux sediments within and adjacent to the fault zone in the u s department of energy pennzoil pathfinder well at about 2200 m sstvd subsea true vertical depth showed little paleothermal or geochemical evidence for throughgoing fluid flow the sediments were characterized by low vitrinite reflectances ro averaging 0 3 ro moderate to high d18o and d13c values and little difference in major or trace element composition between deformed and undeformed sediments in contrast faulted sediments from the a6st well which intersects the a fault at 1993 m sstvd show evidence for a paleothermal anomaly 0 55 ro and depleted d18o and d13c values sodium is depleted and calcium is enriched in a mudstone gouge zone at the top of the fault cut in the well this effect diminishes with distance from this gouge zone cuttings from other wells in south eugene island block 330 show slightly elevated vitrinite reflectance in fault intercepts relative to sediments outside the fault zone overall indicators of mass and heat flux indicate the main growth fault zone in south eugene island block 330 has acted as a conduit for ascending fluids although the cumulative fluxes vary along strike this conclusion is corroborated by oil and gas distribution in downthrown sands in blocks 330 and 331 which identify the fault system in northwestern block 330 as a major feeder simple modeling of coupled heat and mass flux indicates the paleothermal anomaly in the fault zone intersected by a6st well was short lived having a duration less than 150 yr the anomaly could have been produced by a 2 106 m3 pulse of fluid ascending the fault at an actual velocity of over 1 km yr darcy flux of 330 m yr from 3 km deeper in the basin simple darcy law computation indicates a transient fault permeability on the order of 110 md during this flow pulsing of fluid up the fault was probably the norm although most flow did not produce such strong thermal anomalies as the one detected in the a6st well analysis of fluid pressures shows that the main fault is a profound lateral permeability barrier having up to 1800 psi of water pressure differential across it the hydrocarbon sealing capacity of the fault depends on the pressure difference across the fault fault permeability is best understood in terms of effective stress under ambient conditions the fault is at high pressure relative to downthrown reservoirs a pulse of high pressure fluid ascending end page 244 the fault lowers effective stress in the fault zone sufficiently to produce a significant transient increase in permeability if the fluid is in an area of the fault adjacent to downthrown relatively low pressure reservoir sands the fluid will discharge into them permeability in and adjacent to the fault then decreases such that fluid cannot reenter the fault zone and escape from the reservoir

diagenesis of laminated travertine crusts arbuckle mountains oklahoma abstract waterfall travertine deposits in the arbuckle mountains of oklahoma contain laminated coarsely crystalline neomorphic crusts which retain virtually no evidence of their organic origin the precursors of these neomorphic crusts are laminated organic crusts composed of two basic alternating layers 1 layers of filamentous cyanophyte bushes in which each bush is encased by one or more elongate spar crystals and 2 layers in which equidimensional crystals surround individual cyanophyte filaments as diagenesis commences the elongate calcite spar crystals enclosing algal bushes grow into the overlying more finely crystalline calcite layers by aggradational neomorphism forming coarse columnar crystals oriented perpendicularly to the laminae as the crystals increase in size they incorporate the inclusion rich laminae of the surrounding material thereby maintaining the laminated appearance the end product of this diagenetic process is a dense laminated crust composed almost entirely of columnar calcite crystals which commonly reach lengths of more than 1 cm such crusts display almost no evidence of their former constituents however after immersion in hydrochloric acid and application of an organic stain remains of algal bushes are visible within thin sections many other nonmarine carbonate deposits include similar looking crusts that appear to be solely inorganic in origin in light of the present findings caution certainly is warranted before automatically assuming an inorganic origin for such crusts

dating fluvial terraces by 230th u on pedogenic carbonate wind river basin wyoming abstract reliable and precise ages of quaternary pedogenic carbonate can be obtained with 230th u dating by thermal ionization mass spectrometry applied to carefully selected milligram size samples datable carbonate can form within a few thousand years of surface stabilization allowing ages of quaternary deposits and surfaces to be closely estimated pedogenic carbonate clast rinds from gravels of glacio fluvial terraces in the wind river basin have median concentrations of 14 ppm u and 0 07 ppm 232th with median 230th 232th 270 making them well suited for 230th u dating horizons as thin as 0 5 mm were sampled from polished slabs to reduce averaging of long 105 yr and sometimes visibly discontinuous depositional histories dense translucent samples with finite 230th u ages preserve within rind stratigraphic order in all cases ages for terraces wr4 167 000 6 400 yr and wr2 55 000 8600 yr indicate a mean incision rate of 0 26 0 05 m per thousand years for the wind river over the past glacial cycle slower than inferred from cosmogenic nuclide dating terrace wr3 which formed penecontemporaneously with the final maximum glacial advance of the penultimate rocky mountain bull lake glaciation has an age of 150 000 8300 yr indicating that it is broadly synchronous with the penultimate global ice volume maximum

climatic change recorded by stable isotopes and trace elements in a british holocene tufa combined stable isotope d18o and d13c and trace element mg sr geochemistry from bulk tufa calcite and ostracod shell calcite from an early holocene british tufa reveal clear records of holocene palaeoclimatic change variation in d18o is caused principally by change in the isotopic composition of holocene rainfall recharge itself caused mainly by change in air temperature the d13c variability through much of the deposit reflects increasing influence of soil zone co2 owing to progressive woodland soil development bulk tufa mg ca and sr ca are controlled by their concentrations in the spring water importantly mg ca ratios are not related to d18o values and thus show no temperature dependence first order sympathetic relationships between d13c values and mg ca and sr ca are controlled by aquifer processes residence times co2 degassing and calcite dissolution reprecipitation and probably record intensity of palaeorainfall recharge effects stable isotope records from ostracod shells show evidence of vital effects relative to bulk tufa data the ostracod isotopic records are markedly spiky because the ostracods record snapshots of relatively short duration years whereas the bulk tufa samples record averages of longer time periods probably decades the d18o record appears to show early holocene warming a thermal maximum at ca 8900 cal yr bp and the global 8200 yr bp cold event combined d13c mg ca and sr ca data suggest that early holocene warming was accompanied by decreasing rainfall intensity the mg ca data suggest that the 8200 yr bp cold event was also dry warmer and wetter conditions were re established after the 8200 yr bp cold event until the top of the preserved tufa sequence at ca 7100 cal yr bp

spectral element modelling of fault plane reflections arising from fluid pressure distributions the presence of fault plane reflections in seismic images besides indicating the locations of faults offers a possible source of information on the properties of these poorly understood zones to better understand the physical mechanism giving rise to fault plane reflections in compacting sedimentary basins we numerically model the full elastic wavefield via the spectral element method sem for several different fault models using well log data from the south eugene island field offshore louisiana we derive empirical relationships between the elastic parameters e g p wave velocity and density and the effective stress along both normal compaction and unloading paths these empirical relationships guide the numerical modelling and allow the investigation of how differences in fluid pressure modify the elastic wavefield we choose to simulate the elastic wave equation via sem since irregular model geometries can be accommodated and slip boundary conditions at an interface such as a fault or fracture are implemented naturally the method we employ for including a slip interface retains the desirable qualities of sem in that it is explicit in time and therefore does not require the inversion of a large matrix we perform a complete numerical study by forward modelling seismic shot gathers over a faulted earth model using sem followed by seismic processing of the simulated data with this procedure we construct post stack time migrated images of the kind that are routinely interpreted in the seismic exploration industry we dip filter the seismic images to highlight the fault plane reflections prior to making amplitude maps along the fault plane with these amplitude maps we compare the reflectivity from the different fault models to diagnose which physical mechanism contributes most to observed fault reflectivity to lend physical meaning to the properties of a locally weak fault zone characterized as a slip interface we propose an equivalent layer model under the assumption of weak scattering this allows us to use the empirical relationships between density velocity and effective stress from the south eugene island field to relate a slip interface to an amount of excess pore pressure in a fault zone

vegetation climate and sea level in the past 55 000 years hanjiang delta southeastern china abstract pollen in quaternary deposits from the subtropical hanjiang delta records three major phases in the local vegetation and climate history during the last 55 000 yr 1 a prevalent cool to temperate and humid climate at ca 24 000 14c yr b p is indicated by abundant pollen of temperate trees including conifers 2 between 20 000 and 15 000 14c yr b p a cold dry environment was associated with low sea level during the last glaciation leading to subaerial exposure weathering and interruption of sedimentation as well as departure from the region of dacrydium and sonneratia 3 a short term expansion of grassland at ca 10 300 14c yr b p reduced the predominant lauraceae fagaceae evergreen forest possibly corresponding to the younger dryas cooling the combined data indicate a maximum sea level rise in the mid holocene 7500 4000 14c yr b p and a marine influence in the late pleistocene at 45 000 20 000 14c yr b p the holocene warming however did not bring back moisture sensitive taxa indicating high seasonal aridity probably caused by renewed monsoon conditions

factors controlling tufa deposition in natural waters at waterfall sites abstract study on calcite precipitation has major implications for both the hydrochemical evolutions of river systems and the global carbon cycle the precipitation of calcite generally requires the water to be 5 10 times supersaturated with respect to calcite which is usually achieved by the removal of co2 formation of waterfall tufa has been often simply described as the result of water turbulence in fast flowing water in this paper the formation mechanisms of waterfall tufa are discussed and a series of laboratory experiments are designed to simulate the hydrological conditions at waterfall sites the influences of the air water interface the water flow velocity and the solid water interface on co2 outgassing and calcite precipitation are compared and evaluated quantitatively the results show that the principal cause of waterfall tufa formation is the enhanced inorganic carbon dioxide outgassing resulted from the sudden hydrological changes occurring at waterfall sites rather than organisms evaporation or the solid water interface the air water interface area and the water flow velocity are greatly increased at waterfall sites as a result of the aeration effect low pressure effect and jet flow effect which greatly accelerate co2 outgassing inorganic co2 outgassing drives the waters to become highly supersaturated with respect to calcite and consequently results in much calcite deposition the solid water interface is less important as the air water interface in affecting calcite precipitation at waterfall sites field measurements showed that conductivity ca2 and hco3 concentrations along tianhe creek and hot creek decrease downstream while ph rises field observations also showed that tufa deposition occurred mainly at waterfall sites

fault properties from seismic q summary a previously published investigation of seismic q from a north sea vertical seismic profiling data set has revealed an abrupt increase in attenuation to a q value of 45 which is associated with a region between 1000 and 2000 m depth where the borehole intersects a major fault zone dipping at approximately 50 this q anomaly is modelled using a linear slip theory fractures are considered to be imperfectly bonded interfaces where displacement is not required to be continuous the resulting apparent attenuation of the vertically propagating incident p wave is shown to be very dependent on the dip angle of the fault due to the relatively high predicted values of shear compliance compared to the normal compliance of fluid filled fractures at 30 60 mpa confining pressure the observed frequency independence of q is satisfactorily reproduced in addition to the low pass filtering of the downgoing p wave a difference in the frequency content of the first arriving p wave on the vertical compared to the horizontal components has been observed this difference is interpreted as resulting from rayleigh scattering from 3 d inhomogeneities within the fault zone no assumptions are required concerning the existence of intrinsic q although our results do not preclude this possibility

climatic significance of seasonal trace element and stable isotope variations in a modern freshwater tufa abstract we present a continuous 14 yr long 1985 to 1999 high resolution record of trace element mg sr ba u and stable isotope 13c 18o variations in a modern freshwater tufa from northwestern queensland australia by utilizing the temperature dependence of the 18o signal an accurate chronology was developed for the sampled profile which allowed a comparison of the chemical records with hydrological and meteorological observations as a consequence it was possible to constrain the relevant geochemical processes relating climate variables such as temperature and precipitation to their chemical proxies in the tufa record temperatures calculated from the mg concentrations of the tufa samples provide close approximations of average annual water temperature variations furthermore we demonstrate that temporal changes in mg ca water can be estimated using an empirically derived equation relating mg ca water to the sr ba ratio measured in the tufa samples by means of this relationship it is theoretically possible to determine the mg ca ratio of paleowaters and hence to derive reliable estimates of former water temperatures from the mg concentrations of fossil tufas from the study area sympathetic variations in sr ba and 13c along the sampled profile record changes in water chemistry which are most probably caused by variable amounts of calcite precipitation within the vadose zone of the karst aquifer this process is thought to be markedly subdued whenever the amount of wet season precipitation exceeds a given threshold accordingly distinct minima in sr ba and 13c are interpreted to reflect years with above average rainfall the pronounced seasonal and annual variability of the u concentration along the profile is thought to primarily record changes in the u flux from the soil to the water table we suggest that during intensive rain events u is transported to the phreatic zone by complexing organic colloids giving rise to conspicuous u maxima in the tufa after above average wet seasons this study demonstrates the potential of freshwater tufas to provide valuable information on seasonal temperature and rainfall variations if tufa deposits turn out to be reasonably resistant to secondary processes combined investigation of speleothems and tufas from the same area could become a promising approach in future research while speleothems offer continuous records of long term paleoenvironmental changes tufas could provide high resolution time windows into selected periods of the past

pressure and porosity influences on vp vs ratio in unconsolidated sands elevated pore pressures commonly encountered in the shallow unconsolidated section of the sedimentary column present a significant hazard during the drilling and completion of offshore wells the porosity of the ocean floor sediments is high and a cover of low permeability clay can prevent the underlying sediments from draining even at very shallow depths below the seafloor as further deposition loads the sediment the entrapped fluids impede normal compaction by becoming pressurized the lowered effective stress that results from the higher pore fluid pressure produces a proportional drop in the strength of non cohesive sediments resulting in very weak shallow sands this weakness can result in washouts or fracturing in the shallow strata potentially leading to the loss of the well and of neighboring wells

transmission of seismic waves across single natural fractures fractures and other nonwelded contacts are important mechanical and hydrological features of rock masses their effects on seismic wave propagation can be modeled as a boundary condition in the seismic wave equation seismic stress is continuous across such a boundary but seismic particle displacement and seismic particle velocity are not the complete solutions for seismic wave reflection conversion and transmission across a displacement and velocity discontinuity between two half spaces with different densities and elastic properties are derived for all angles of the incident wave the ratio between the seismic stress across this boundary and the seismic particle displacement and velocity are described by a specific stiffness and a specific viscosity respectively a displacement discontinuity results in frequency dependent reflection and transmission coefficients and a frequency dependent group time delay the velocity discontinuity results in frequency independent coefficients and zero delay results of laboratory experiments on compressional and shear wave transmission across three different natural fractures in a quartz monzonite are described measurements were made at different effective stresses under dry and saturated conditions at room temperature it is shown that the effect of these fractures on the spectral amplitudes for compressional and shear pulses transmitted across these fractures are described well by a displacement discontinuity for compressional pulses under dry and saturated conditions and by a combined displacement and velocity discontinuity for shear wave pulses under dry and saturated conditions values of specific stiffness and specific viscosity vary between fractures and increase with increasing effective stress as does the static specific stiffness of these fractures changes in the spectral amplitudes of transmitted pulses are also analyzed in terms of attenuation using the seismic quality factor q which is found to be a function of frequency

spectral element modeling of spontaneous earthquake rupture on rate and state faults effect of velocity strengthening friction at shallow depths we develop a spectral element methodology sem for simulating dynamic rupture on rate and state faults and use it to study how the rupture is affected by a shallow fault region of steady state velocity strengthening friction our comparison of the developed sem and a spectral boundary integral method bim for an anti plane two dimensional test problem shows that the two methods produce virtually identical solutions for the finest resolution we use and that the convergence with grid reduction of the developed sem methodology is comparable to that of bim we also use the test problem to compare numerical resolution required for different state evolution laws and for linear slip weakening friction using our three dimensional implementation of the methodology we find that a shallow velocity strengthening fault region can significantly alter dynamic rupture and ground motion the velocity strengthening region suppresses supershear propagation at the free surface occurring in the absence of such region which could explain the lack of universally observed supershear rupture near the free surface in addition the velocity strengthening region promotes faster fall off of slip velocity behind the rupture front and decreases final slip throughout the entire fault causing a smaller average stress drop the slip decrease is largest in the shallow parts of the fault resulting in the depth profile of slip qualitatively consistent with observations of shallow co seismic slip deficit the shallow velocity strengthening region also reduces the amplification of strong ground motion due to a low velocity bulk structure

the seismic response to overpressure a modelling study based on laboratory well and seismic data we investigate the seismic detectability of an overpressured reservoir in the north sea by computing synthetic seismograms for different pore pressure conditions the modelling procedure requires the construction of a geological model from seismic well and laboratory data seismic inversion and avo techniques are used to obtain the p wave velocity with higher reliability than conventional velocity analysis from laboratory experiments we obtain the wave velocities of the reservoir units versus confining and pore pressures laboratory experiments yield an estimate of the relationship between wave velocities and effective pressure under in situ conditions these measurements provide the basis for calibrating the pressure model overpressures are caused by different mechanisms we do not consider processes such as gas generation and diagenesis which imply changes in phase composition but focus on the effects of pure pore pressure variations the results indicate that changes in pore pressure can be detected with seismic methods under circumstances such as those of moderately deep north sea reservoirs