a diffusion adsorption model of uranium uptake by archaeological bone abstract an argument for the possibility of uranium uptake by buried bone taking place through the adsorption of uranyl species on bone mineral is advanced in the light of this a diffusion adsorption model for uranium uptake by buried bone is developed the necessary constants are evaluated from the literature and from laboratory measurements of the partition coefficient between solution and bone mineral the geochemical and hydrological parameters which control uptake are discussed the predictions of the model are shown to be in general accordance with the timescale magnitude and distribution of uranium uptake in archaeological bone using the model specific predictions of the variation of apparent uranium series ages in bone can be made and bone is shown clearly not to conform to the closed system assumption when the model is extended to tooth enamel it is found to be incompatible with the early uptake model used for esr dating but to fall between the early uptake and linear uptake models similarly it suggests that uranium series dates on enamel assuming a closed system are liable to underestimate the true age by at least one third

ultracataclasite structure and friction processes of the punchbowl fault san andreas system california abstract the punchbowl fault is an exhumed 40 km displacement fault of the san andreas system in the devil s punchbowl the fault contains a continuous ultracataclasite layer along which the punchbowl formation sandstone and an igneous and metamorphic basement complex are juxtaposed the fabric of the ultracataclasite layer and surrounding rock indicate that nearly all of the fault displacement occurred in the layer by analogy with nearby active faults we assume that the punchbowl fault was seismogenic and that the ultracataclasite structure records the passage of numerous earthquake ruptures we have mapped the ultracataclasite layer at 1 1 and 1 10 to determine the mode of failure and to constrain the processes of seismic slip on the basis of color cohesion fracture and vein fabric and porphyroclast lithology two main types of ultracataclasite are distinguished in the layer an olive black ultracataclasite in contact with the basement and a dark yellowish brown ultracataclasite in contact with the sandstone the two are juxtaposed along a continuous contact that is often coincident with a single continuous nearly planar prominent fracture surface pfs that extends the length of the ultracataclasite layer in all exposures no significant mixing of the brown and black ultracataclasites occurred by offset on anastomosing shear surfaces that cut the contact or by mobilization and injection of one ultracataclasite into the other the ultracataclasites are cohesive throughout except for thin accumulations of less cohesive reworked ultracataclasite along the pfs structural relations suggest that 1 the black and brown ultracataclasite were derived from the basement and sandstone respectively 2 the black and brown ultracataclasites were juxtaposed along the pfs 3 the subsequent final several kilometers of slip on the punchbowl fault occurred along the pfs and 4 earthquake ruptures followed the pfs without significant branching or jumping to other locations in the ultracataclasite by comparison with rock friction experiments the slip localization along the pfs in the ultracataclasite implies rate weakening behavior with a critical slip distance similar to laboratory values and thus relatively small nucleation and breakdown dimensions for earthquake ruptures of the various mechanisms proposed to explain the low strength of the san andreas and to produce dynamic weakening of faults those that require or assume extreme localization of slip are most compatible with our observations

finite fault source study of the great 1994 deep bolivia earthquake we apply an empirical green s function method to recordings of teleseismic body waves from the june 9 1994 deep focus bolivia earthquake the far field source time functions reveal that rupture occurred on a sub horizontal fault plane the main rupture began some 25 km to the east of the hypocenter consists of three large subevents and covers a total area of about 60 40 km the static stress drop is quite large more than 150 mpa and the obtained rupture velocity is less than 2 km sec while the total dimensions of the earthquake are large compared to estimates of the width of a metastable olivine wedge at this depth the moment release is confined largely to a narrow band along an ew axis and may be compatible with phase transitions within a thickened slab core

conodont color and textural alteration an index to regional metamorphism contact metamorphism and hydrothermal alteration experimental and field data are used to extend the utility of conodonts as semi quantitative thermal indices into the regimes of regional and contact metamorphism as well as hydrothermal alteration an arrhenius plot of data from induced conodont color alteration by pyrolysis in air at 1 atm was used to generate the geologic temperatures for conodont color alteration indices cai above 300 c that is for cai values of 5 through 8 such cai values occur in very low to medium grade regionally metamorphosed contact metamorphosed and hydrothermally altered rocks the uniformity or variability of cai values within a sample together with conodont texture can help to distinguish grades and environments of metamorphism particularly in metacarbonate sequences induced cai by pyrolysis in a water methane mixture at kbar results in retardation of cai and in a disparate mixture of both low and high cai values within each experimental sample in this system color alteration processes above a cai of 2 to 3 seem to change from predominantly carbonization to predominantly loss of organic matter presumably by oxidation and volatilization of oxides these experiments approximate the type of cai mixture characteristically found in conodonts recovered from hydrothermally altered rocks these data indicate that cai values of 6 to 8 cannot be used to assess precise temperatures of hydrothermally altered rocks but may serve as useful indicators of potential mineralization

scaling of the critical slip distance for seismic faulting with shear strain in fault zones theoretical and experimentally based laws for seismic faulting contain a critical slip distance1 5 dc which is the slip over which strength breaks down during earthquake nucleation on an earthquake generating fault this distance plays a key role in determining the rupture nucleation dimension6 the amount of premonitory and post seismic slip7 10 and the maximum seismic ground acceleration1 11 in laboratory friction experiments d c has been related to the size of surface contact junctions2 5 12 thus the discrepancy between laboratory measurements of dc 10 5m and values obtained from modelling earthquakes 10 2m has been attributed to differences in roughness between laboratory surfaces and natural faults5 this interpretation predicts a dependence of dc on the particle size of fault gouge2 breccia and wear material but not on shear strain here we present experimental results showing that dc scales with shear strain in simulated fault gouge our data suggest a new physical interpretation for the critical slip distance in which dc is controlled by the thickness of the zone of localized shear strain as gouge zones of mature faults are commonly 102 103 m thick12 17 whereas laboratory gouge layers are 1 10 mm thick our data offer an alternative interpretation of the discrepancy between laboratory and field based estimates of dc

seismicity triggered by fluid injection induced aseismic slip anthropogenic fluid injections are known to induce earthquakes the mechanisms involved are poorly understood and our ability to assess the seismic hazard associated with geothermal energy or unconventional hydrocarbon production remains limited we directly measure fault slip and seismicity induced by fluid injection into a natural fault we observe highly dilatant and slow 4 micrometers per second m s aseismic slip associated with a 20 fold increase of permeability which transitions to faster slip 10 m s associated with reduced dilatancy and micro earthquakes most aseismic slip occurs within the fluid pressurized zone and obeys a rate strengthening friction law 0 67 0 045ln v v with v 0 1 m s fluid injection primarily triggers aseismic slip in this experiment with micro earthquakes being an indirect effect mediated by aseismic creep

source characteristics of large deep earthquakes constraint on the faulting mechanism at great depths 1 source parameters for eight large deep earthquakes are inferred from broadband data of the global seismograph network we completed an unprecedented data set composed of 14 largest deep earthquakes mw ranging from 7 0 to 8 3 from six different subduction zones studied with identical source parameter inversion methods the study confirms earlier findings that deep earthquake source properties vary with the temperature of the subducting plate events in cold slabs show high aftershock activity high rupture velocity and high seismic efficiency in contrast earthquakes in warm slabs generally have low aftershock productivity low rupture velocity and low seismic efficiency however the 23 june 1991 argentina event has source properties that are rather characteristic for earthquakes in cold slabs and that differ from those of other large south american events we suggest that two types of deep earthquakes occur in south america and we interpret these results in the context of the shear instability model for the mechanism of deep earthquakes in this model shear failure occurs through a thermal runaway mechanism within a shear zone at great depth and low seismic efficiency might indicate significant energy dissipation due to heating and possible melt production the 1991 argentina event located in a pocket of deep seismicity within the slab core may represent a shear instability with little melting as would be the case for large deep earthquakes in cold subduction zones whereas the other known south american large deep events such as 1963 peru 1970 colombia and 1994 bolivia are located in relatively aseismic regions and may involve significant melting within warmer regions of the slab

a method for coupled esr u series dating of teeth showing post depositional u loss abstract an additional method of coupled esr u series dating is developed for teeth showing post depositional u loss the fundamental parameter for the dose rate calculation is the present day 230th activity because of the geochemical immobility of thorium uranium and thorium concentrations are measured by thermal ionization mass spectrometry tims due to the unknown uranium history an average saturation uptake of uranium is assumed leading to an average development of u series activities therewith an internal dose rate and the age t can be calculated on condition that the modelled 230th activity at time t corresponds with the measured value using this new method teeth found in archaeological sites in france gramat bramefond could be dated even though they show u loss after u uptake two teeth from gramat could be dated to 128 3 8 6 and 130 5 10 ka two teeth from bramefond have ages of 104 4 8 4 and 115 1 10 2 ka both sites can be ascribed to oxygen isotope stage 5

the 1996 june 17 flores sea and 1994 march 9 fiji tonga earthquakes source processes and deep earthquake mechanisms summary source parameters of the 1996 flores sea and 1994 fiji tonga deep earthquakes are derived from teleseismic body waves recorded by the global seismic network of broad band seismograph stations both events consisted of several subevents models to approximate the spatial and temporal extent of the source process include point sources propagating point sources and a combination of these for the flores sea event rupture lasted about 23 s and terminated some 70 km east of the nucleation point as inferred from the duration of p wave pulses in agreement with the findings reported by other investigators our preferred model suggests bilateral rupture propagation it consists of four point sources that have variable double couple radiation patterns and source time histories and explains well the large compensated linear vector dipole clvd component inferred from the harvard centroid moment tensor cmt solution the main moment release in the fiji tonga event lasted only about 15 s our best model consists of two point sources with a total moment release of 2 8 1020 n m rupture propagated subhorizontally from the nucleation point to the north the termination of rupture was located about 40 km to the north of rupture initiation the inferred velocity of moment release in the flores sea and fiji tonga events was 3 5 km s 1 a value which is higher than that inferred for the great bolivian earthquake of june 1994 other derived source parameters static stress drop radiated seismic energy and maximum seismic efficiency are also significantly different from those inferred for the 1994 bolivian event suggesting that deep earthquake processes do not follow an easily detectable common mechanism

can fossil bones and teeth be dated using fission track analysis the high proportion of crystallized apatite forming the bones and teeth should theoretically allow the use of fission track analysis to date vertebrate fossils when burying of the fossiliferous series did not subject them to temperatures exceeding 60 degrees c however several major obstacles arise such as the complexity of fossils internal structures diagenetic modifications and substitutions of the hydroxyapatite by other minerals and massive u uptake or loss during diagenesis in this work those various problems are addressed combining optical microscope observation of the fossils a systematic fission track analysis of the best samples alpha and gamma spectrometry and secondary ion mass spectrometry even if the problems caused by internal structures and mineral substitutions can be solved u movements in and out of the fossils are generally too complex to allow fission track analysis dating of the fossils

impact of co2 geological sequestration on the nucleation of earthquakes 1 can co2 storage cause earthquakes what is the maximum possible earthquake magnitude resulting from co2 injection here as a theoretical case study we investigate these questions using coupled hydromechanical modeling with multiphase flow and seismological variables for quantifying earthquake magnitude and energy our simulations consider transient fluid flow and stress coupling and the evolution of fault properties we simulate co2 injection into a reservoir caprock system bounded by a subvertical normal fault subjected to different extensional stress regimes and over a range of initial fault permeability values for our assumed system and injection rate the simulation results show that sudden stress drop and fault slip primarily initiated along the fault portion intersecting the storage reservoir after a few months of injection when a sufficiently high reservoir pressure has been reached the size of the rupture area and consequently the earthquake magnitude and energy depends on initial horizontal to vertical stress ratio and fault permeability which strongly influences the size of the pressurized area and subsequent stress variations

uranium and thorium in fossil bones activity ratios and dating we have analysed fossil bones by u series disequilibrium from five different sites ngandong and sonde indonesia gold ox hill people s republic of china pestera romania and ksar akil lebanon two samples were taken from all bones one representing surface material and one from the inner bone bulk material in general the u concentration at the surface was found to be lower than in the bulk resulting in higher ages this might be explained by leaching of u from the surface area of the bone which means that the surface material of the bones cannot be considered to be a closed system an exception is the gold ox hill series where we found that the u concentration is roughly constant throughout the bone also the ages are roughly the same for surface and bulk material in this case

frictional properties of shale reservoir rocks 1 we performed laboratory friction experiments on shale samples from three hydrocarbon reservoirs to assess compositional controls on fault slip behavior accompanying hydraulic stimulation the samples span a range of clay and total organic content from 10 to 60 by weight and demonstrate fine scale heterogeneity within each reservoir friction measurements demonstrate strong dependence of strength and rate state constitutive properties on shale composition shale samples with clay and organic content above 30 by weight show coefficient of friction 0 4 and consistently velocity strengthening behavior while those below this threshold show increased strength and velocity weakening behavior this transition in frictional strength and stability suggests a change in the shale grain packing framework from rigid clast supported to clay mineral and organic matter supported measurements of gouge dilatancy further support these relationships showing a negative correlation with clay and organic content critical slip evolution distance exhibits similar dependence on composition implying common micromechanical mechanisms for the observed transition in frictional behavior we performed microstructural characterization of the experimental samples and observe changes in the gouge load bearing framework and shear localization features consistent with these mechanical data while these results can be applied generally to infer slip stability on faults in clay bearing sedimentary rock we employ the experimental data to place constraints on microearthquake magnitudes and the occurrence of slow shear slip on preexisting faults during hydraulic stimulation in shale reservoirs

source and aftershock properties of the 1996 flores sea deep earthquake the june 17 1996 mw 7 8 flores sea event is one of the largest deep earthquakes and also produced an energetic aftershock sequence allowing study of the spatial relationships between moment release aftershock occurrence and slab geometry the locations of the mainshock rupture sub events aftershocks and background slab seismicity are determined using a relative location algorithm the event occurred at the edge of a 150 km wide aseismic region that represents a tear in the lowermost part of the indonesian slab the aftershocks define a 90 km long region parallel to slab strike two other aftershocks locate 10 25 km outside the previously active slab the mainshock propagated about 75 km eastward along an e w striking southward dipping plane with a rupture velocity of 3 5 km s there was a substantial change in focal mechanism during the rupture with the fault plane strike changing by about 25 overall the flores sea deep earthquake shows numerous large aftershocks and a moderate rupture velocity 3 5 km s and stress drop 7 20 mpa these characteristics are similar to the 1994 tonga deep earthquake but different from the 1994 bolivia event suggesting they may be typical of large deep earthquakes in colder slabs systematic differences in rupture parameters for deep earthquakes in warm and cold slabs suggest that the rupture mechanism is highly sensitive to temperature

origin of illite in the lower paleozoic of the illinois basin evidence for brine migrations in the lower paleozoic of the illinois basin three illite polytypes are found 2m 1 of detrital origin and 1m d and 1m of diagenetic origin illite polytype quantification of detrital 2m 1 illite and diagenetic 1m d and 1m illite combined with k ar age dating allows extrapolation to apparent detrital and diagenetic illite ages kinetic modeling of smectite illitization combined with the calculated age of illitization can evaluate different origins of illite the diagenetic illite in the lower paleozoic of the illinois basin is interpreted not to have formed solely by burial diagenesis but mainly during multiple brine events the upper ordovician maquoketa group contains diagenetic illite dominantly 1m d with minor 1m with an extrapolated age of 360 m y 356 377 m y and formed from smectite at temperatures of 50 100 c this age falls within the span of dates for illite smectite i s in k bentonites from the upper mississippi valley and is interpreted to be a combined result of illitization by burial diagenesis and either a hydrothermal brine from the southern and deeper part of the basin or a k rich brine from the michigan basin upper mississippi valley area or forest city basin in ordovician and cambrian shale partings and sandstone older than the maquoketa group the diagenetic illite 1m d in shale and 1m in sandstone has an age of 300 m y and formed at temperatures 140 c this late paleozoic age falls within the range of illites from sandstone in the upper mississippi valley and k bentonites of the appalachian basin it coincides with the alleghany orogeny and is interpreted as having formed by gravity driven flow from the uplifted alleghanian ouachita orogenic belt that drove hot 140 c fluids through the illinois basin

a deep earthquake aftershock sequence and implications for the rupture mechanism of deep earthquakes a distinguishing characteristic of deep earthquakes has been the absence of observable aftershock sequences1 2 here we report the first extensive deep earthquake aftershock sequence to be observed it was recorded by an array of eight broadband seismographs following the 9 march 1994 deep tonga earthquake the aftershocks show a power law decay with time following the main shock as is typical of shallow events most of the well located aftershocks are concentrated along a steeply dipping plane consistent with one of the nodal planes of the main shock mechanism and the mechanisms of three large aftershocks assuming these aftershocks denote the main shock rupture area they define a 50 65 km fault plane extending across the entire width of the active seismic zone and into the surrounding aseismic region the width of the aftershock zone is wider than the expected width of the metastable olivine wedge demonstrating that either the width of the metastable olivine material exceeds previous estimates or the aftershocks are not confined in such a wedge

the complex rupture process of the 1996 deep flores indonesia earthquake mw 7 9 from teleseismic p waves from an analysis of broadband teleseismic p waveforms we resolve the complex rupture characteristics of the 17 june 1996 flores earthquake mw 7 9 depth 590 km this earthquake was unusual because of its large rupture length and the variation in rupture velocity and stress drop the 20 s rupture propagated westward at 2 3 km s to a distance of 20 30 km and eastward at 4 km s over 75 km average static stress drop was about 15 mpa but the stress drop during the episode of main moment release was a factor of four higher the oblique normal faulting mechanism is consistent with the stress expected as the slab descends along a highly curved trench significant rupture complexity is also reported for the deep 1994 mw 7 7 fiji and mw 8 2 bolivia earthquakes and is evident in the source time functions of many smaller earthquakes at depths 300 km thus source complexity of deep earthquakes appears no less common than for shallow earthquakes and requires the existence of significant heterogeneity in the physical properties that control deep earthquake rupture

nucleation of dynamic slip on a hydraulically fractured fault this work is concerned with the relationship between hydraulic fracturing injection into a fault and the possibility of a seismic slip the results of this study show that the nucleation of dynamic slip on a fault with slip weakening friction is only weakly dependent on the magnitude of the stress perturbation ahead of the propagating hydraulic fracture hf or the hf propagation regime and is mainly controlled by the hydraulic fracture length i e the size of the fully unloaded fault segment at a given time the growth of the fault slipping patch remains stable when the background shear stress 0 is smaller than the residual fault strength r under ambient conditions otherwise 0 r nucleation of dynamic slip takes place when the hydraulic fracture grows to the critical size lc which is vanishingly small p 0 for critically stressed faults i e when the background stress approaches the fault peak strength 0 p and is diverging as 1 0 r when the stability boundary is approached 0 r our solution for the critical hf size allows to infer the corresponding fluid injection volume that may lead to slip instability on a fault with given frictional properties and background stress

trace elements and their isotopes in bones and teeth diet environments diagenesis and dating of archeological and paleontological samples fossil biogenic apatites display trace element te compositions that can record environmental and biological signals give insights into past water compositions or be used for dating paleontological and archeological bones and teeth processes of te incorporation into apatites of skeletal phosphatic tissues are described ranging from those active in living organisms to those active during diagenesis abiotic incorporation mechanisms have been modeled theoretically and experimentally and include crystallographic controls on te partitioning coefficients inorganic surface adsorption and adsorption mediated through chelation and diffusion adsorption processes each leading to specific fractionation patterns predictions from models and experiments have been tested against te concentration and isotopic composition data on remains of contemporary wild or raised animals and on fossils of various ages and environments in living organisms tes incorporated in apatite are separated in two categories non essential elements whose concentration is compared to that of an essential element with similar behavior e g sr ca to reconstruct trophic chains and essential elements whose isotopic ratios are used to trace metabolic activity diet etc in fossils elements are incorporated during diagenesis such as rare earth elements rees and trace diagenetic processes ranging from very early quantitative adsorption likely mediated by chelators protracted diagenesis through inorganic adsorption and fractionation through diffusion adsorption to low grade metamorphism associated with intense redistribution under crystal chemical control these different steps are also identified as steps of radionuclide incorporation only samples where the earliest steps can be deciphered are useful for determining stratigraphic and archeological ages other samples will date late diagenetic events that have obliterated the paleoenvironmental signals but may be of geological significance e g tectonic events

is apatite u th zonation information necessary for accurate interpretation of apatite u th he thermochronometry data abstract new u th and ce concentration maps were acquired by la icpms for 70 apatites from 18 cratonic basement samples from the canadian shield to characterize the nature and variability in apatite u th zonation all apatites are zoned in effective u concentration eu with 80 exhibiting modest zoning that varies from a factor of 1 2 to 2 4 zonation patterns include those with a general eu decrease from core to rim 25 a core to rim eu increase 35 and patchy or irregularly shaped eu distributions 40 most samples consist of individual apatites with markedly different and opposing eu profiles cathodoluminescence cl images were obtained for 258 apatites in 25 samples comparison of eu and cl patterns reveals no consistent relationships suggesting that cl is not a reliable proxy for apatite eu zonation we explore the implications of eu zonation for the interpretation of apatite u th he ahe data by comparing the age predictions for representative and endmember apatite eu profiles with those for unzoned apatites we focus on thermal histories that should magnify eu zonation effects including slow monotonic cooling extended he partial retention zone heprz residence and protracted reheating and cooling application of incorrect ejection correction f t factors different he concentration gradients and variable intracrystalline retentivity due to heterogeneous radiation damage may cause the ahe dates for zoned and unzoned apatites to differ in our dataset the magnitude of the f t effect is by simulating zoned apatite suites from representative and endmember samples we find that eu zoning adds modest age dispersion to most samples the commonly opposing eu profiles for individual apatites in most samples have different effects on the predicted age and partially cancel each other out to reduce the predicted age difference at the sample level higher predicted age differences between zoned and unzoned samples generally correlate with greater sample scatter the age deviations generally fall within the 15 uncertainty range that we commonly apply for sample ahe dates except in unusual circumstances it appears unlikely that the unzoned apatite assumption will lead to misinterpretation of ahe datasets thus precluding the need to routinely acquire apatite eu zonation information as part of ahe dating studies

polygenetic history of paleosols in middle upper pennsylvanian cyclothems of the illinois basin u s a part i characterization of paleosol types and interpretation of pedogenic processes abstract the morphology mineralogy and stratigraphic and lateral distribution of paleosols in pennsylvanian moscovian gzhelian atokan virgilian mixed marine and terrestrial coal bearing strata i e cyclothems in the illinois basin ib are presented in order to determine the dominant pedogenic processes within a cyclic sedimentary depositional framework paleosol morphologies range from weakly developed horizons with evidence of rooting and little to no pedogenic structure to horizons with well developed pedogenic slickensides angular blocky structure and carbonate nodules and tubules the majority of paleosols in strata from the basin interior preserve 1 low chroma gley colors 2 identifiable fossil plant organic matter that increases in abundance upward in the profiles toward the interpreted paleo surface and 3 calcite sphaerosiderite and pyrite cements and nodules in contrast paleosols in strata from the northern margin of the basin display high chroma colors and calcite cements and less fossil organic matter sphaerosiderite and pyrite cements these observations indicate that paleosols from the basin interior underwent a multi phase pedogenic and shallow burial history characterized by an initial better drained oxygenated stage under seasonal precipitation followed by a poorly drained reducing stage while paleosols from the northern margin of the basin experienced overall more oxygenated conditions throughout their pedogenic development x ray diffraction analysis of the 2 m size fraction of paleosol matrix in ib paleosols indicates a mineralogical composition dominated by kaolinite randomly interstratified r0 and ordered r1 illite smectite i s and subordinate amounts of chlorite these combined observational and mineralogical datasets indicate that significant changes in paleohydrology were largely responsible for the development of pennsylvanian soil profiles in the ib

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

rare earth element evolution of phanerozoic seawater recorded in biogenic apatites rare earth element ree contents of marine biogenic apatites have been shown to record seawater compositions a data base of available and newly acquired rare earth element ree contents of marine biogenic apatites has been created to assess the past seawater ree compositions to ensure that this data base contains only the pristine ree signals altered samples characterized by very low la sm n ratios where n stands for ree ratios normalized to the nasc composition gromet et al 1984 geochim cosmochim acta 48 1984 2469 acquired during apatite recrystallization are not included in the database these data reveal a change in the tethyan seawater composition between 110 and 80 ma after the end of the cretaceous the ree chemistry of seawater remains constant until present day this seawater composition change is likely due to concurrent changes in ree scavenging processes a strong correlation between decreasing sm yb n ratios and increasing ce anomalies for samples deposited during the 80 110 ma period is observed as ce anomalies are attributed to ocean water redox conditions changes in ree scavenging could reflect an evolution from stratified and poorly oxygenated waters towards well mixed and oxygenated waters this could have resulted from changing current patterns stemming from the contemporaneous opening of the atlantic ocean a major change in middle and late cretaceous oceanic circulation linked to plate tectonics would have favored the colonization of pelagic environments

protracted diagenetic alteration of ree contents in fossil bioapatites direct evidence from lu hf isotope systematics abstract fossil bones and teeth are potentially important repository for geochemical proxy data and a target for radiometric dating the concentration of many trace elements in bones and teeth increases by orders of magnitude after death and it is this diagenetic incorporation that forms the basis for several areas of geochemical study the use of bones and teeth in this context relies on two assumptions first that target metal ions are incorporated rapidly after death reflecting a known environmental signal and second that after early incorporation the bone or tooth remains as an essentially closed system resistant to later diagenetic change a wide literature has developed exploring these assumptions but relatively little direct evidence has been used to assess the long term diagenetic stability of trace elements within bones and teeth in this study we use the lu hf isotope system to show that bones and teeth of cretaceous and triassic age from both terrestrial and marine settings experience continued long term diagenetic change most likely through gradual addition of trace elements modelling suggests that diagenetic addition after initial recrystallisation may account for 50 of the total ree content in the sampled bones the extent depending on initial uptake conditions tooth enamel and enameloid may be more resistant to late diagenetic changes but dentine is probably altered to the same extent as bone these results have significant implications for the use of bones and teeth as hosts of chronological palaeoceanographic palaeoenvironmental and taphonomic information particularly in mesozoic and palaeozoic contexts

conodont u th he thermochronology initial results potential and problems abstract we performed he diffusion experiments and u th he age determinations on conodonts from a variety of locations to explore the potential of conodont u th he thermochronology to constrain thermal and exhumation histories of some sedimentary rock dominated terrains based on two diffusion experiments and age results from some specimens he diffusion in conodont elements appears to be similar to that in durango apatite fragments of similar size and closure temperatures are approximately 60 70 c for cooling rates of 10 c m y u th he ages of conodonts from some locations yield reproducible ages consistent with regional thermal history constraints and in at least two cases require a closure temperature lower than 80 c other samples however yield irreproducible ages and in one case yield ages much younger than expected based on regional geologic considerations these irreproducible samples show inverse correlations between parent nuclides and age consistent with late stage open system u th behavior

regional fluid flow as a factor in the thermal history of the illinois basin constraints from fluid inclusions and the maturity of pennsylvanian coals vitrinite reflectance measurements on pennsylvanian coals in the illinois basin indicate significantly higher thermal maturity than can be explained by present day burial depths an interval of additional sedimentary section now removed by erosion has been suggested to account for the discrepancy although burial could indeed account for the observed maturity levels of organic matter fluid inclusion temperatures provide a stringent additional constraint in this article we combine measurements of coal maturity with fluid inclusion temperatures from three sites to constrain the basin s thermal and burial history the fluorspar district at the illinois basin s southern margin the upper mississippi valley zinc district at the basin s northern margin and a north central location two dimensional numerical modeling of a north south cross section through the basin tests scenarios both with and without regional fluid flow vitrinite reflectance values can be matched assuming burial by 1 8 2 8 km of southward thickening additional post pennsylvanian sedimentary section in the central and northern illinois basin however these burial depths and temperatures are not sufficient to account for the fluid inclusion data to account for both parameters with burial alone does not appear feasible in contrast our best hypothesis assumes a wedge of post pennsylvanian sediment thickening southward to about 1 2 km and a brief period of magmatism in the fluorspar district significant advective heat redistribution by northward regional fluid flow accounts for fluid inclusion temperatures and coal maturities throughout the basin the modeling results demonstrate the potential contribution of advective heat transport to the thermal history of the illinois basin begin page 258

the march 9 1994 m w 7 6 deep tonga earthquake rupture outside the seismically active slab we investigate the rupture process of the march 9 1994 m w 7 6 deep tonga earthquake and its relationship to the background seismicity of the subducted tonga slab variations in observed p and s wave pulse duration indicate that the rupture propagated to the nne and extended well beyond the background seismicity we inverted 47 p and sh waveforms including regional broadband waveforms from the southwest pacific seismic experiment using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane the results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10 20 km beyond the bounds of the previous seismic activity a significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region rupture along the near vertical nne striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near horizontal nodal plane this result combined with the planar alignment of aftershocks and the general nne directivity of the waveforms provides strong evidence that the rupture occurred on the near vertical plane thermal modeling of the tonga slab indicates that the rupture terminated in material about 200 c warmer than the temperature that normally limits the occurrence of smaller earthquakes additionally aftershocks seem to be suppressed in the outer regions of the rupture which contain about half of the moment release but only 1 of the 15 well located aftershocks we suggest that slabs may be composed of an inner cold core where seismic rupture initiates and small earthquakes occur and a thermal halo of warmer material which can sustain rupture and only a few aftershocks the mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core nucleation may occur through a process limited to the cold core such as transformational faulting and propagation through the warmer material may occur through ductile faulting or plastic instabilities isolated deep earthquakes in other subduction zones such as the 1994 bolivia event may occur almost completely within the warmer zone accounting for the lack of background seismicity and the dearth of aftershocks

the quiet daily variation in the total magnetic field global curves most magnetic mapping exercises involve measuring the amplitude or total field component of earth s magnetic field removing the time varying part of the field is a task of data reduction and the most common time variation is the quiet daily variation sq it is thus valuable to have for reference type curves of the quiet daily variation in the total field to meet this need global data obtained during the 1965 international year of the quiet sun have been used to derive type curves describing the sq variation of the total magnetic field in addition to the traditional field components as for the traditional components the total field curves show significant seasonal and latitudinal variability in amplitude and phase the effect of the equatorial electro jet is clearly evident in both hemispheres of the globe there are bands of reduced amplitude in total field signal between the equator and the path of the sq focus these bands here termed the total field doldrums persist throughout the year

the potential of crinoids as u th sm he thermochronometers abstract u th sm he dating of crinoid fossils from late paleozoic rocks from a variety of sites in north america suggest that this material can be used as a useful low t thermochronometer under certain conditions available data suggest that common he is present in crinoids and at the low production rates typical of calcite this common he will be sufficient to produce spurious ages in crinoid samples of any age heating of samples to temperatures in excess of the opening temperature will expel all he common and radiogenic with all subsequent he accumulation being exclusively radiogenic such samples have potential for being robust thermochronometers sensitive to temperatures in the range of 60 110 c without common he analysts can take advantage of the large size of crinoid columnals and produce reliable ages even when young miocene or with low production rates eu 0 1 ppm samples not subjected to post depositional heating above the opening temperature burial to depths of 2 to 4 km depending on geothermal gradient sedimentation rate and residence time at peak t will likely retain common he and be unsuitable for thermochronologic studies

laboratory study of fault healing and lithification in simulated fault gouge under hydrothermal conditions abstract seismic data geologic observations and laboratory friction studies suggest that faults lithify and strengthen heal during the interseismic period of the earthquake cycle we report on experiments to investigate the influence of healing duration and temperature on the strength and healing rate of simulated faults layers of m sized quartz powder were used to simulate granular fault gouge gouge layers were sheared within sioux quartzite in a triaxial pressure vessel at elevated pressures p c 250 mpa elevated temperatures 230 636 c and in the presence of water p h 2 o 75 mpa we performed hold slide experiments in which samples were subjected to a period of healing under hydrostatic load followed by shear deformation healing times ranged from a few hundred s to 10 5 s to isolate the effects of lithification and temperature on friction we ran two types of hold slide experiments 1 samples were subjected to healing and shear deformation at elevated temperature and 2 following healing at elevated temperature temperature was reduced prior to shear we also ran slide hold slide experiments in which samples were healed under shear load at elevated temperature 636 c experiments in which deformation was carried out at a lower temperature than healing show that both the static and sliding coefficient of friction increase with heal time samples healed and deformed at higher temperature showed lower peak strengths and within the scatter of our data did not exhibit time dependent strengthening these data are interpreted to result from enhanced lithification rates at elevated temperature samples healed under shear load exhibited a log linear decrease in friction with hold time modeling using rate and state dependent friction laws indicates velocity strengthening behavior with friction a b values of 0 05 to 0 08 our data indicate that at high temperature the effective state evolution term b is negative moreover the data indicate the rate of frictional strengthening varies significantly for hydrostatic and non hydrostatic load conditions our measurements of healing are consistent with seismic estimates of fault healing rates if stress drop is a fraction of the total shear stress and effective fault normal stress is 60 to 100 mpa

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

seismological constraints on the mechanism of deep earthquakes temperature dependence of deep earthquake source properties abstract seismological observations are of vital importance for understanding the mechanism of deep earthquakes most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms deep earthquake aftershock sequences show temporal decay rates and magnitude frequency relations b values that are similar to shallow earthquakes with the aftershocks occurring preferentially along the mainshock fault planes in addition deep earthquake rupture velocities are similar to those of shallow earthquakes however there are also some observations that are clearly distinctive relative to shallow earthquakes stress drops of deep earthquakes show a large variation but are larger on average than shallow earthquakes different deep seismic zones show very different b values in contrast to shallow earthquakes which show similar b values worldwide in addition deep earthquakes show fewer aftershocks than shallow earthquakes a variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab both deep earthquake b values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs suggesting that these factors are temperature controlled to an extent much greater than with shallow earthquakes large deep earthquakes in warm slabs show slower rupture velocities larger stress drops and lower seismic efficiencies than similar earthquakes in cold slabs the width of deep seismic zones is also probably temperature controlled but deep earthquake rupture can propagate outside the normal limits of benioff zone seismicity simple thermal models for the tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200 c warmer than the temperature that limits smaller earthquakes in the slab these observations can be used to evaluate physical models for deep earthquakes including brittle slip along fluid weakened faults transformational faulting and thermal and perhaps melt lubricated shear instabilities it is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model since hydrated materials are expected in only a narrow depth zone at the top of the slab and the fault planes do not have the expected orientations for reactivated faults the lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model in which events should be confined within a narrow metastable wedge seismological studies have also failed to find evidence for the existence of metastable olivine in slabs the temperature dependence of deep earthquakes argues in favor of a temperature activated phenomenon such as thermal shear instabilities and perhaps fault zone melting

rupture of the large mw 7 8 deep earthquake of 1973 beneath the japan sea with implications for seismogenesis the earthquake of 29 september 1973 beneath the western edge of japan sea is the largest deep earthquake along the western pacific m w 7 8 focal depth 575 km the source region is near the tip of the japan wadati benioff zone where a continuous belt of seismicity clearly defines a unique shallow dipping 30 slab down to depths about 600 km this unique geometry of slab predicts a limited vertical extent available for earthquake rupture in direct conflict with previous reports of a large subvertical fault for the 1973 earthquake using a comprehensive data set of nearly 20 p and sh waveforms recorded at teleseismic distances we investigate rupture associated with this event in some detail by inversion of waveforms in particular we use wave trains with all relevant depth phases pp sp and ss that are crucial for resolving the vertical extent of rupture in addition to a northward component of rupture as in previous reports we find clear evidence for a significant component of eastward rupture propagation that cannot fit within a subvertical north south striking nodal plane meanwhile all regions of major slip are at essentially the same depth thus the true fault plane appears subhorizontal with the entire region of slip extending only about 15 6 km across the slab9s thickness the spatio temporal distribution of slip is well explained by a simple circular rupture front propagating at a speed of 3 4 km sec a comparison among the largest deep earthquakes shows that most rupture planes are more or less dimensionally equal with high aspect ratios ranging approximately from 0 5 to 1 while some events have fairly restricted source regions others seem to extend more than 30 km into the thickness of the slabs

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

low temperature thermochronology of anticosti island a case study on the application of conodont u th he thermochronology to carbonate basin analysis abstract the ordovician to silurian carbonate succession preserved on anticosti island is a near continuous record of the changing depositional and tectonic history of the early paleozoic laurentian margin these strata are a prime natural laboratory to test the applicability of conodont apatite u th he thermochronology cahe for carbonate basin analysis in a setting with a well understood burial history and abundant paleotemperature constraints we report 63 cahe dates from seven samples collected from two boreholes x ray micro computed tomography xr ct was used to assess individual conodont element size volume and alpha ejection correction cahe dates are negatively correlated with effective uranium concentration eu with single conodont dates enigmatically older than the age of deposition to conodont dates as young as 2 7 0 2 ma we rationalize this dispersion as a product of several factors including diagenetic processes and pervasive conodont crystalline porosity and permeability additionally one basement and two sandstone samples were collected for apatite and zircon u th he ahe zhe and apatite fission track aft thermochronology in the precambrian basement zhe dates range from 581 7 46 5 ma to 761 2 60 9 ma n 4 and ahe dates range from 26 7 1 6 to 48 4 2 9 ma n 4 ahe dates from two lower ordovician sandstones n 8 are positively correlated with eu increasing from 13 7 0 8 ma to 105 2 6 3 ma over a range of 186 ppm eu thermal history modeling of these data suggests two plausible thermal histories both of which are novel as they imply that far field effects related to the opening of the atlantic ocean have influenced the thermal history of anticosti island ultimately these data highlight the complexities associated with and necessity of low temperature thermochronology in carbonate basins

conodont thermochronology of exhumed footwalls of low angle normal faults a pilot study in the mormon mountains tule springs hills and beaver dam mountains southeastern nevada and southwestern utah abstract u th he thermochronology is a well established dating technique used to understand the temperature time histories of rocks in a wide range of geologic settings the technique is presently restricted to rocks that contain specific accessory minerals such as apatite or zircon marine carbonates and shales typically lack these accessory phases in quantities and sizes practical for u th he dating and thus present a challenge for application of the method here we explore the utility of biogenic apatite from conodonts as a u th he thermochronometer at a well studied calibration site located in eastern nevada and southwestern utah we performed u th he thermochronometry laser ablation inductively coupled plasma mass spectrometry x ray micro computed tomography and scanning electron microscopy sem on specimens with conodont color alteration indices cai of 1 5 3 extracted from carbonate rocks in the footwalls of low angle normal faults in the mormon mountains tule spring hills and beaver dam mountains conodont u th he che dates have high scatter dates are commonly reproducible to 20 of sample means but can deviate up to 150 all cai 1 5 2 5 conodonts produce che dates younger than 240 ma consistent with thermal resetting of samples however most cai 3 conodonts give ages 2 6 older than mississippian and permian deposition average u th and rare earth element ree concentrations depend on porosity and permeability differences between albid and hyaline conodont tissue and range from we propose that microstructural changes associated with increasing cai influence che dates parent isotope loss occurs during the post cooling stage either in the outcrop or in the laboratory our hypothesis is that the double buffered formic acid procedure for dissolving dolomitized carbonates may accelerate this loss in thermally altered higher cai conodonts

the dallas fort worth earthquake sequence october 2008 through may 2009 this paper describes a sequence of small earthquakes m b 3 3 that occurred at the dallas fort worth airport texas between 30 october 2008 and 31 may 2009 analysis of records at regional station wmok identified more than 180 earthquakes in the sequence about 90 percent occurred in four clusters on 30 october 1 november 20 november 26 december and 15 17 may after the sequence began a six station temporary local network obtained high quality three component records for 11 earthquakes occurring between 20 november and 1 december analysis of these data demonstrated that all 11 earthquakes originated from a focus near 32 855 n 97 051 w with an estimated depth 4 4 km this location is less than 0 5 km from a well completed in august 2008 that extends to a depth of 4 2 km drilled to dispose of brines collected during flowback of hydraulic fracturing fluids associated with the production of natural gas brine disposal commenced at the well on 12 september 2008 seismograms and s p intervals for the earthquakes are similar though not identical and relative locations indicate they occurred along a north northeast south southwest trend with horizontal and vertical dimensions of 1 1 km and 0 2 km respectively this trend is approximately coincident with that of a mapped normal fault in the subsurface and consistent with the maximum horizontal in situ stress direction because of the absence of previous historical earthquakes the proximity of the brine disposal well and the similarity with other documented cases of induced seismicity it seems likely that fluid injection induced the 2008 2009 sequence

depth dependence of rupture velocity in deep earthquakes 1 explaining deep seismicity is a long standing challenge in earth science between 300 and 700 km depth earthquakes are scarce except between 530 and 600 km where the majority of events occur by imaging the seismic rupture process for a set of recent deep earthquakes using the back projection of teleseismic p waves we found that the rupture velocities are less than 60 of the shear wave velocity except in the depth range of 530 to 610 km we propose that large fracture surface energy gc values for deep earthquakes generally prevent the acceleration of dynamic rupture propagation and generation of earthquakes between 300 and 700 km depth whereas small gc value in the exceptional depth range promote dynamic rupture propagation and explain the seismicity peak near 600 km

nucleation and arrest of dynamic slip on a pressurized fault 1 elevated pore pressure can lead to reactivation of slip on pre existing fractures and faults when the static coulomb failure is reached locally as the pressurized region spreads diffusively slip can accumulate quasi statically paced by the pore fluid diffusion or dynamically in this work we consider a prestressed fault with a locally peaked diffusively spreading pore pressure field to study 1 conditions leading to the escalation of slip and nucleation of dynamic rupture and 2 rupture run out distance before it is arrested nucleation appears in this model when the fault friction decreases from its peak value with slip while arrest of dynamic propagation is imminent on aseismic faults i e such that prestress b is less than the residual fault strength rat ambient conditions when fluid overpressure is a small to moderate fraction of the ambient value of normal effective stress and prestress is large enough for fault slip to be activated by overpressure dynamic rupture always nucleates and the nucleation length increases with decreasing prestress practically independently of the overpressure value transition from the ultimately unstable b r to the ultimately stable b r fault loading is marked by a strong increase of the nucleation length 1 b r 2 as b approaches r from above for aseismic faults b r no dynamic rupture is nucleated at large fluid overpressures for all but the smallest values of prestress the largest run out distances of dynamic slip on aseismic faults correspond to overpressure prestress just sufficient for slip activation in such cases the dynamically accumulated slip can lead to enhanced dynamic fault weakening resulting in a sustained dynamic rupture and generating a large earthquake this is consistent with field observations when the largest injection induced seismicity occurred after fluid injection ended

source retrieval for mantle earthquakes by iterative deconvolution of long period p waves abstract long period p waves of eighteen deep and intermediate depth earthquakes in a moment range 1 10 26 3 10 28 dyn cm are analysed by the method of kikuchi and fukao this issue to iteratively derive point dislocations on a plane the fault plane is defined as the plane of point dislocations that gives the best fit between the synthetic and observed waveforms for dip slip type earthquakes the fault plane is found to dip more steeply than the auxiliary plane in general point dislocations tend to cluster into one or a few subevents we characterize the largest subevent by its maximum moment rate m 0 m and its half duration from which the associated stress drop can be roughly estimated for earthquakes shallower than 400 km half duration is approximately constant 5 s so that stress drop increases from 1000 bar as m 0 m increases the value of for the great banda sea earthquake of 1963 h 100 km is exceptionally large 10 s for which 700 bar for earthquakes deeper than 400 km increases from 3 s to 9 s as m 0 m increases so that is approximately constant 700 bar stress drop so obtained approximates to an overall stress drop with a possible exception of the several largest earthquakes the distribution of point dislocations and the moment rate function indicate that rupture most often expands in a fan shaped fault area and that a large part of the rupture front ceases propagation in a coherent manner a sharp contrast in stress drop behavior across a depth of 400 km may be related to the olivine spinel transition within the downgoing slab of lithosphere

implications of spatial and temporal development of the aftershock sequence for the mw 8 3 june 9 1994 deep bolivian earthquake on june 9 1994 the mw 8 3 bolivia earthquake 636 km depth occurred in a region which had not experienced significant deep seismicity for at least 30 years the mainshock and aftershocks were recorded in bolivia on the banjo and seda broadband seismic arrays and on the san calixto network we used the joint hypocenter determination method to determine the relative location of the aftershocks we have identified no foreshocks and 89 aftershocks m 2 2 for the 20 day period following the mainshock the frequency of aftershock occurrence decreased rapidly with only one or two aftershocks per day occuring after day two the temporal decay of aftershock activity is similar to shallow aftershock sequences but the number of aftershocks is two orders of magnitude less additionally a mb 6 apparently triggered earthquake occurred just 10 minutes after the mainshock about 330 km east southeast of the mainshock at a depth of 671 km the aftershock sequence occurred north and east of the mainshock and extends to a depth of 665 km the aftershocks define a slab striking n68 w and dipping 45 ne the strike dip and location of the aftershock zone are consistent with this seismicity being confined within the downward extension of the subducted nazca plate the location and orientation of the aftershock sequence indicate that the subducted nazca plate bends between the nnw striking zone of deep seismicity in western brazil and the n s striking zone of seismicity in central bolivia a tear in the deep slab is not necessitated by the data a subset of the aftershock hypocenters cluster along a subhorizontal plane near the depth of the mainshock favoring a horizontal fault plane the horizontal dimensions of the mainshock beck et al this issue silver et al 1995 and slab defined by the aftershocks are approximately equal indicating that the mainshock ruptured through the slab

conodont u th he thermochronology a case study from the illinois basin abstract acquisition of thermochronologic data in carbonates and shales has traditionally been elusive owing to the paucity of dateable minerals in these lithologies conodont apatite u th he cahe thermochronology has the potential to fill this need we acquired 50 u th he dates for conodonts with cai values 1 5 from seven pennsylvanian shale and limestone samples from two drillcores in the illinois basin we also obtained x ray microcomputed tomography microct results for 8 conodonts to evaluate the accuracy of alpha ejection corrections the simplified geometric corrections yield corrected dates within 5 of those derived from 3d characterization using microct nearly all of the conodont cahe dates are substantially younger than their depositional age indicating that maximum post depositional temperatures of 90 c caused he loss over geologic timescales the youngest and most reproducible dates consist of whole platform elements from shales and may record a regional late cretaceous early tertiary cooling and erosion event the remainder of the data exhibit strong negative date u and date th correlations characterized by higher and more variable th u than the conodonts with reproducible dates these patterns are best explained by u loss with more limited th loss the results suggest that whole platform elements and higher u th conodont materials are the most promising targets for cahe analysis

injection induced seismicity poroelastic and earthquake nucleation effects the standard model of injection induced seismicity considers changes in coulomb strength due solely to changes in pore pressure we consider two additional effects full poroelastic coupling of stress and pore pressure and time dependent earthquake nucleation we model stress and pore pressure due to specified injection rate in a homogeneous poroelastic medium stress and pore pressure are used to compute seismicity rate through the dieterich 1994 model for constant injection rate the time to reach a critical seismicity rate scales with t r2 cfc where r is distance from the injector c is hydraulic diffusivity and fc is a factor that depends on mechanical properties and weakly on r the seismicity rate decays following a peak consistent with some observations during injection poroelastic coupling may increase or decrease the seismicity rate depending on the orientation of the faults relative to the injector if injection induced stresses inhibit slip abrupt shut in can lead to locally sharp increases in seismicity rate tapering the flux mitigates this effect the maximum magnitude event has been observed to occur postinjection we suggest the seismicity rate at a given magnitude depends on the nucleation rate the size distribution of fault segments and if the background shear stress is low the time varying volume of perturbed crust this leads to a rollover in frequency magnitude distribution for larger events with a corner that increases with time larger events are absent at short times but approach the background frequency with time larger events occurring post shut in are thus not unexpected

stability of dike intrusion along preexisting fractures we investigate the ability of magma to propagate along preexisting fractures oblique to the least compressive stress relaxation of the preexisting shear stress to zero over the portion of the fracture dilated by magma the dike results in slip for some distance along the closed portion of the fracture ahead of the dike tip and a stress concentration near the dike tip this could lead to the production of new tensile cracks oblique to the parent dike that could capture the flow if the shear stress resolved on the fracture plane is perpendicular to the fracture front mode i ii the front may deviate along its entire length if the shear stress is parallel to the fracture front mode i iii the front may splay into segments for mode i ii dikes the maximum tensile stress occurs at the dike tip and is parallel to the dike if the effective tensile stress exceeds the rock tensile strength then the intruding magma rather than dilating the existing fracture is expected to propagate into a self generated crack analogous to the wing cracks observed to form at the tips of pure mode ii fractures for mode i iii dikes the maximum tensile stress lies within the plane of the dike and is oriented at some angle that depends upon the far field boundary conditions even if the magma pressure exceeds the ambient normal stress it appears to be very difficult for dikes to intrude into preexisting fractures unless one or more of the following conditions is satisfied 1 the fracture is nearly perpendicular to the least compressive stress 2 the resolved shear stress on the fracture is small compared to the excess magma pressure i e the ratio of shear to opening of the dike walls is small 3 the effective ambient dike normal stress is small compared to the rock tensile strength this indicates that it may be quite difficult for dikes emerging from midcrustal to lower crustal depths to follow faults

investigation of injection induced seismicity using a coupled fluid flow and rate state friction model we describe a numerical investigation of seismicity induced by injection into a single isolated fracture injection into a single isolated fracture is a simple analog for shear stimulation in enhanced geothermal systems egs during which water is injected into fractured low permeability rock triggering slip on preexisting large scale fracture zones a model was developed and used that couples 1 fluid flow 2 rate and state friction and 3 mechanical stress interaction between fracture elements based on the results of this model we propose a mechanism to describe the process by which the stimulated region grows during shear stimulation which we refer to as the sequential stimulation ss mechanism if the ss mechanism is realistic it would undermine assumptions that are made for the estimation of the minimum principal stress and unstimulated hydraulic diffusivity we investigated the effect of injection pressure on induced seismicity for injection at constant pressure there was not a signifi

effect of slab temperature on deep earthquake aftershock productivity and magnitude frequency relations deep earthquakes generally show fewer aftershocks and a larger variability in magnitude frequency relations than shallow earthquakes1 5 these characteristics and the factors that control them may place constraints on the mechanism of deep earthquakes which is still uncertain6 8 here we show that systematic variations in aftershock productivity and magnitude frequency relations are related to the temperature of the downgoing slab as inferred from the vertical velocity and age of the subducting lithosphere deep earthquakes with substantial aftershock sequences are found only in colder slabs and earthquakes in warmer slabs consistently show few aftershocks magnitude frequency relations for deep earthquakes also show systematic variations as a function of slab thermal structure with warmer slabs showing fewer small earthquakes lower b values the statistical properties of deep earthquakes thus appear to be temperature sensitive to an extent not observed for shallow earthquakes and not adequately explained by simple geometrical limitations on fault width

radiation damage control on apatite u th he dates from the grand canyon region colorado plateau individual detrital apatite grains from the esplanade coconino and moenkopi formations in the grand canyon region of the colorado plateau yield u th he dates from 104 to 5 ma the range of dates within each unit far exceeds analytical uncertainty but correlates with both he concentration he and effective u concentration eu these dates are all signifi cantly younger than the sandstone units indicating partial to complete he loss following deposition recently published laboratory diffusion data suggest that he retentivity in apatite increases with radiation damage forward models predict that the consequences of this effect will be manifested most clearly as a correlation between u th he dates and the he and eu in suites of apatites that 1 are characterized by a large span of eu and 2 had thermal histories in which suffi cient time elapsed for the apatite he diffusion kinetics to diverge prior to reheating and partial resetting apatites in the sedimentary units investigated fi t these cri teria using geologically reasonable deposition burial and unroofi ng histories simulations that include the effect of radiation damage on apatite he retentivity can reproduce the observed distributions of apatite dates and correlations with parent and daughter concentrations these results suggest that a span of u th he dates positively correlated with eu may provide important information regarding a sample s thermal history

systematic determination of earthquake rupture directivity and fault planes from analysis of long period p wave spectra summary if an earthquake has a primarily unilateral rupture the pulse width observed on seismograms will vary depending on the angle between the rupture direction and the takeoff vector to the station we have developed a method to estimate the amount of pulse broadening from the spectrum and apply it to a long period database of large globally distributed earthquakes that occurred between 1988 and 2000 we select vertical component p waves at epicentral distances of 20 98 w ecompute the spectrum from a 64 s long window around each p wave arrival each spectrum is the product of source receiver and propagation response functions as well as local source and receiver side effects since there are multiple receivers for each source and multiple sources for each receiver we can estimate and remove the source and receiverside terms by stacking the appropriate p log spectra for earthquakes deeper than 200 km source effects dominate the residual spectra we use our pulse width estimates to determine the best rupture direction and to identify which nodal plane of the harvard centroid moment tensor cmt solution is most consistent with this rupture direction for 66 events in about 30 per cent of the cases one of the two nodal planes produces a much better fit to the data and can be identified as the true fault plane when results from previous studies are available for comparison our rupture directions are usually consistent with their results particularly for earthquakes with simple rupture histories

rupture propagation behavior and the largest possible earthquake induced by fluid injection into deep reservoirs earthquakes caused by fluid injection into deep underground reservoirs constitute an increasingly recognized risk to populations and infrastructure quantitative assessment of induced seismic hazard however requires estimating the maximum possible magnitude earthquake that may be induced during fluid injection here i seek constraints on an upper limit for the largest possible earthquake using source physics simulations that consider rate and state friction and hydro mechanical interaction along a straight homogeneous fault depending on the orientation of the pressurized fault in the ambient stress field different rupture behavior can occur 1 uncontrolled rupture front propagation beyond the pressure front or 2 rupture front propagation arresting at the pressure front in the first case fault properties determine the earthquake magnitude and the upper magnitude limit may be similar to natural earthquakes in the second case the maximum magnitude can be controlled by carefully designing and monitoring injection and thus restricting the pressurized fault area

apatite u th sm he thermochronology of rapidly cooled samples the effect of he implantation apatite u th sm he ahe thermochronology is increasingly used for reconstructing geodynamic processes of the upper crust and the surface results of ahe thermochronology however are often in conflict with apatite fission track aft thermochronology yielding an inverted age relationship with ahe dates older than aft dates of the same samples this effect is mainly explained by radiation damage of apatite either impeding he diffusion or causing non thermal annealing of fission tracks so far systematic age inversions have only been described for old and slowly cooled terranes whereas for young and rapidly cooled samples too old ahe dates are usually explained by the presence of undetected u and or th rich micro inclusions we report apatite u th sm he results for rapidly cooled volcanogenic samples deposited in a deep ocean environment with a relatively simple post depositional thermal history robust age constraints are provided independently through sample biostratigraphy all studied apatites have low u contents 5 ppm on average while aft dates are largely in agreement with deposition ages most ahe dates are too old for leg 43 where deposition age of sampled sediment is 26 5 29 5 ma alpha corrected average ahe dates are up to 45 ma indicating overestimations of ahe dates up to 50 this is explained by he implantation from surrounding host u th rich sedimentary components and it is shown that ahe dates can be corrected by mechanically abrading the outer part of grains we recommend that particularly for low u th apatites the possibility of he implantation should be carefully checked before considering the degree to which the alpha ejection correction should be applied

timescales and mechanisms of ree and hf uptake in fossil bones abstract rare earth element ree patterns of fossil bones and teeth are widely used as proxies for provenance taphonomy and palaeoenvironment in order to investigate if fossil bones behave as closed systems over geologic time ree profiles were analysed by la icpms along cross sections of 54 bones from various well characterised and well dated settings these include terrestrial and marine diagenetic environments covering early triassic to holocene ages in general all fossil bones exhibit the highest ree concentrations at the outer rim gradually decreasing by up to four orders of magnitude toward the inner bone cortex intra bone ree concentration gradients decrease significantly from quaternary via tertiary to mesozoic specimens suggesting long term ree uptake and open system behaviour of fossil bone this view is further corroborated by 176lu 176hf dating of selected samples all yielding significantly younger ages than the known chronostratigraphic ages hence there is clear evidence for long term open system behaviour of fossil bones with respect to ree which is in marked contrast to currently accepted models suggesting that ree uptake is only early diagenetic although unexpected statistically significant four to seven point isochrons are observed for four fossil dinosaur bone samples and one upper triassic mastodonsaurus tooth with mswds ranging from 0 083 to 4 5 notably mobility of lu alone cannot account for the observed age patterns assuming constant lu uptake rates over time the radiometric ages should only be as low as half of the chronostratigraphic age however a six point isochron defined by subsamples of a single upper triassic mastodonsaurus tooth yields an age of 65 2 1 1 ma mswd 0 68 much younger than half of the stratigraphic age ca 234 ma hence hf must also undergo late diagenetic exchange likely mechanisms to account for the presence of statistically meaningful isochrons as well as for the late diagenetic exchange of both ree and hf are diffusion adsorption and dissolution reprecipitation processes

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

aftershock locations and rupture characteristics of the 1995 mariana deep earthquake the august 23 1995 mw 7 0 depth 586 km mariana event produced one of the strongest deep earthquake aftershock sequences known we combined teleseismic arrival times with arrival times picked from a local network operating directly above the event in the mariana islands and relocated the main shock and aftershocks we derived the source characteristics from teleseismic body waves by inverting waveform data of the global seismograph network the main shock ruptured downward across the entire width of the active slab along a plane dipping southwestward most of the 17 well located aftershocks occurred along or very near the rupture plane the different focal mechanisms of the main shock and aftershocks indicate that the stress field varies across the width of the mariana slab at the area of the august 1995 sequence

earthquake source characteristics from dynamic rupture with constrained stochastic fault stress one of the challenging tasks in predicting near source ground motion for future earthquakes is to anticipate the spatiotemporal evolution of the rupture process the final size of an event but also its temporal properties propagation velocity slip velocity depend on the distribution of shear stress on the fault plane though these incipient stresses are not known for future earthquakes they might be sufficiently well characterized in a stochastic sense we examine the evolution of dynamic rupture in numerical models of a fault subjected to heterogeneous stress fields with varying statistical properties by exploring the parameter space of the stochastic stress characterization for a large number of random realizations we relate generalized properties of the resulting events to the stochastic stress parameters the nucleation zone of the simulated earthquake ruptures in general has a complex shape but its average size is found to be independent of the stress field parameterization and is determined only by the material parameters and the friction law furthermore we observe a sharp transition in event size from small to system wide events governed mainly by the standard deviation of the stress field a simplified model based on fracture mechanics is able to explain this transition finally we find that the macroscopic rupture parameters e g moment moment rate seismic energy of our catalog of model quakes are generally consistent with observational data

u pb dating of fossil enamel from the swartkrans pleistocene hominid site south africa abstract we demonstrate that young fossil enamel older than the range of the u series 300 ka can be dated by the u pb methods using new models of u and pb loss and uptake contrary to the current hypothesis of u uptake that only considers the adsorption diffusion mechanism we here develop a complete time dependent model which takes gains and losses of the most critical nuclides 238u 234u and 230th into account both during chemical dissolved u and physical th and u recoil processes lead is assumed to be a mixture between two components of common pb and a radiogenic component the proportions of these components are calculated from the pb isotope abundances and u pb ratios we apply this new u pb method to bovid enamel from the swartkrans cave gauteng province south africa this cave has yielded abundant early pleistocene hominid remains attributed to paranthropus and homo as well as various associated archaeological vestiges biochronological comparisons with east africa have provided age estimates ranging between 1 8 and 1 0 ma which however remain poorly constrained after correction for initial 234u disequilibrium and further 238u loss the u and pb isotope data yield ages of 1 83 1 38 1 36 0 29 and 0 83 0 21 ma for the three stratigraphic units members 1 2 and 3 respectively we discuss the consequences of these radiometric results for hominid evolution in south africa

effects of u th rich grain boundary phases on apatite helium ages abstract single grain apatite u th sm he ages he ages from non detrital samples sometimes show larger dispersions than can be explained by known sources of age dispersion such as grain size radiation damage parent zonation fracturing or intracrystalline inclusions we present observations and model age bias effects of an additional source of apatite he age dispersion observed in some cases u th rich grain boundary phases gbps precipitated on apatite crystal surfaces apatite grains from several samples with dispersed apatite he ages are coated or partially coated by reddish orange gbps rich in fe u and th these gbps are heterogeneous in thickness grain coverage and composition and have effective u concentrations eu up to 1000 ppm these phases may have large effects on the bulk eu and 4he compositions of apatite grains and can produce significant he age biases the direction and magnitude of this age bias is primarily a function of four factors 1 the host grain s size 2 the gbps thickness and eu relative to the host grain 3 the timing of gbp formation relative to the cooling age of the host grain and 4 whether the gbps are preserved and analyzed with the grain during he dating some of the most severe effects occur when gbps formed before or near the time of the apatite cooling age but were lost just prior to analysis for example during mineral separation in this case gbps of commonly observed thicknesses 1 10 m with eu 2 10 times that of the host apatite grain implant enough 4he into typical sized apatite grains to produce hundreds of percent positive age biases in contrast when u th rich gbps are preserved and analyzed with apatite grains the resulting he ages can be negatively biased though the heterogeneity variable preservation and ambiguous formation ages of gbps preclude a general quantitative and practical solution to this problem our model demonstrates that observed age dispersions in some samples are consistent with the effects of u th rich grain boundary phases and illustrates the conditions under which and by how much they will bias he ages