isotopic analysis of tooth enamel carbonate from modern north american feral horses implications for paleoenvironmental reconstructions the accuracy of paleoenvironmental reconstructions based on isotope analyses of equid teeth is currently uncertain because the exact relationship between the isotope composition of modern feral equids and their environment has not been thoroughly studied we analyzed the carbon and oxygen isotope values 13c and 18o of tooth enamel carbonate and the 13c values of fecal samples from modern feral horses we compared those values with the 13c values of local vegetation and the 18o values of local waters herds were studied in two contrasting localities eastern oregon where grasslands consisted of 100 c3 species and new mexico where 95 of the grasses were c4 species carbon isotope analyses of fecal material and tooth enamel suggest that horses consumed primarily grass but some new mexico horses also consumed significant amounts of shrubs and or forbs microhistological analyses of fecal samples show that oregon horses consumed 95 grass and oregon enamel 13c values are consistent with a diet containing 100 c3 plants microhistological analyses of fecal samples from new mexico indicate a diet averaging 75 grass while enamel 13c values suggest that diets averaged 85 c4 plants range 72 97 thus reconstructions of the c3 c4 ratio of grasses in ancient grasslands that are based on the 13c values of fossil equid teeth may underestimate the abundance of c4 grasses the mean 18o values of tooth enamel paralleled the trends observed in the mean 18o values of precipitation however the mean 18o values of enamel carbonate from oregon and new mexico differed by only 3 3 which is less than the difference in the mean 18o values of precipitation 6 5 in addition the range of 18o values within new mexico enamel samples 6 5 was greater than the difference between mean enamel 18o values at each site calculated values for the 18o of water ingested by horses are 2 3 more positive than mean 18o values for corresponding precipitation suggesting that horses consumed waters that were enriched in 18o due to evaporation while our results confirm that local climatic and hydrological conditions can influence the 18o values of equid enamel they also show that the 18o values of equid teeth are not always a direct proxy for the isotope ratios of precipitation

stable isotope patterns found in early eocene equid tooth rows of north america implications for reproductive behavior and paleoclimate abstract reproductive behaviors of early eocene equids were likely different from their modern day counterparts as a result of their small body size and warmer global temperatures to better understand the paleoenvironment and ecology of these early horses teeth of protorohippus montanum jaws from a single stratigraphic locality in early eocene sedimentary deposits of wyoming were sampled for oxygen and carbon isotope analysis these enamel data along with an inferred body mass of 7 5 kg suggests that adult teeth formed over a matter of months with the final tooth erupting at 1 3 years of age thus average isotopic values from each tooth likely represent environmental conditions from a single season results indicate two isotopic patterns in the case of isotopic pattern a the second forming molars m 2s have significantly lower isotopic ratios compared to other teeth within the jaw this suggests the first forming molars m 1s formed during a cooling fall season followed by formation of the m 2s during the cooler winter in the case of isotopic pattern b results suggest that m 1s formed during a warming spring season while the m 2s formed during the following warmer summer season isotopic ratios of m 1s may represent an individual s birth season or in utero season depending on timing of initial formation implying that early equids experienced at least two birth seasons per year these results suggest that temperatures and reduced seasonality of the early eocene played a strong role in early equid birth cycles lastly the mean and variance in carbon and oxygen isotope ratios was not significantly different across tooth positions of all jaws when all individuals were grouped together regardless of their pattern assignment such results indicate that isotopic data from fourth premolars first molars second molars and third molars can be combined for purposes of temporal reconstructions of paleoclimate thus increasing the potential sample sizes for these types of studies

basin wide magnetostratigraphic framework for the bighorn basin wyoming new paleomagnetic data from six different sections in the bighorn basin are combined with previously published results to construct a basin wide magnetostratigraphic framework geomagnetic polarity reversals between chrons c26r c26n c25r c25n c24r and c24n have been identifi ed among multiple stratigraphic sections in different parts of the basin using the new magnetostratigraphic framework paleontological paleobotanical and isotopic information from these varied locations in the basin can now be correlated and compared to similar records from elsewhere in the world these new data resolve previous uncertainty concerning the timing of an important episode of faunal turnover known as biohorizon b which is slightly below the chron c24r c24n boundary close to the position of the elmo isotope excursion in marine records backstripping analysis using these new magnetostratigraphic data helps defi ne the time transgressive onset of basin formation and shows the different subsidence histories of the northern and southern parts of the basin

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

photo acoustic study of subshear and supershear ruptures in the laboratory abstract we report supershear ruptures observed in spontaneously nucleating laboratory microearthquakes and describe the signature of the associated mach wavefront radiation transducers detect the wavefield both close and at a distance from the fault the rupture velocities are inferred from either photoelastic high speed imaging or using the acoustic recordings both methods yield compatible estimates of sufficient accuracy to discriminate between sub shear and supershear ruptures the acoustic records allow to characterize the mach wavefront radiated from the supershear rupture front in particular its amplitude and decay velocity functions recovered by integrating high frequency accelerometer signals in the case of supershear ruptures consist in a double pulse function a first pulse traveling at supershear velocity followed by a second pulse traveling at the rayleigh wave velocity conversely the sub shear event is characterized by a single pulse finally we perform numerical simulations of our experiment using a prescribed supershear rupture velocity the synthetic waveforms obtained from these simulations including the mach wave amplitude and phase yield a satisfactory fit to the experimental results

characterization of nucleation during laboratory earthquakes 1 we observe the nucleation phase of in plane ruptures in the laboratory we show that the nucleation is composed of two distinct phases a quasi static and an acceleration stage followed by dynamic propagation we propose an empirical model which describes the rupture length evolution the quasi static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time the transition from quasi static to accelerating rupture is related to the critical nucleation length which scales inversely with normal stress in accordance with theoretical predictions and to a critical surfacic power which may be an intrinsic property of the interface finally we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions

evidence for rapid climate change in north america during the latest paleocene thermal maximum oxygen isotope compositions of biogenic phosphate from the bighorn basin wyoming oxygen isotope records of cenozoic sea water temperatures indicate that a rapid warming event known as the latest paleocene thermal maximum lptm occurred during the otherwise gradual increase in world temperatures during the late paleocene and early eocene oxygen isotope analysis of the carbonate and phosphate components of hydroxyapatite found in mammalian tooth enamel and body scales of river dwelling fish from the bighorn basin in wyoming were made to investigate corresponding changes in the terrestrial climate a comparison of carbonate and phosphate isotope data from modern and fossil material indicates that some diagenetic alteration of the fossil material has occurred although systematically larger intra tooth ranges in the oxygen isotope composition of carbonate indicate that it is more likely to have been affected than phosphate carbonate and phosphate from the ecologically diverse mammals and fishes both record a shift to higher oxygen isotope ratios at the same time and of the same duration as the lptm these shifts reflect a change in the isotopic composition of regional precipitation which in turn provides the first evidence for continental climate change during the lptm assuming the present day relation between the oxygen isotope composition of precipitation and temperature applies to conditions in the past and that animal physiology and behavior is relatively invariant over time the isotopic shift is equivalent to an increase of surface temperature in western north america of several degrees this result is consistent with the magnitude of high latitude ocean warming and provides a basis for relating marine and terrestrial oxygen isotope records to records of terrestrial biotic change 1998 elsevier science b v all rights reserved

fossil herbivores and crocodiles as paleoclimatic indicators of environmental shifts from bed i and bed ii times of the olduvai gorge tanzania abstract the radiation and extinction of east african vertebrate fauna have been linked to key climatic changes that occurred around 2 8 1 7 and 0 5 ma this study tests whether vegetation and diet changes were linked to these climatic fluctuations focusing on the climatic event at 1 7 ma by analyzing the stable isotope composition of fossil herbivore teeth from the olduvai gorge s stratigraphic units upper bed i lower bed ii and upper bed ii which span 1 83 1 33 ma in order to detect shifts in herbivore diets from c3 to c4 plants in conjunction with their drinking water isotope geochemistry c4 grass expansion is known to be favored by drier conditions and this study tests whether herbivore diets permanently changed to being c4 dominant at this key time period crocodile teeth are also tested to determine if they may be used as paleoclimatic indicators since large crocodiles prey on terrestrial herbivores in wetland settings vertebrate bioapatite carbon and oxygen isotope compositions are found to show similar trends as those of pedogenic carbonates carbonate nodules or concretions which reflect a drying environment that was changing from c3 woodland dominated habitats to mixed woodlands and grasslands from bed i to lower bed ii times while a return to more humid conditions in upper bed ii might be expected to lead to a reversal to c3 plants herbivores in this study retained mostly the drought resistant c4 plant diet these results indicate that vegetation and ecosystems passed through a threshold to c4 domination that did not revert to c3 despite partial reversals in climate that occurred later crocodile teeth were not useful in tracking c4 plant expansion but the oxygen isotopes from modern crocodiles are able to show seasonal cycles from wet to dry seasons and bed i crocodiles indicate a degree of annual seasonal changes during that time as well

time dependent cracking and brittle creep in crustal rocks a review abstract rock fracture under upper crustal conditions is driven not only by applied stresses but also by time dependent chemically activated subcritical cracking processes these subcritical processes are of great importance for the understanding of the mechanical behaviour of rocks over geological timescales a macroscopic manifestation of time dependency in the brittle field is the observation that rocks can deform and fail at constant applied stresses a phenomenon known as brittle creep here we review the available experimental evidence for brittle creep in crustal rocks and the various models developed to explain the observations laboratory experiments have shown that brittle creep occurs in all major rock types and that creep strain rates are extremely sensitive to the environmental conditions differential stress confining pressure temperature and pore fluid composition even small changes in any of these parameters produce order of magnitude changes in creep strain rates and times to failure three main classes of brittle creep model have been proposed to explain these observations phenomenological statistical and micromechanical statistical and micromechanical models explain qualitatively how the increasing influence of microcrack interactions and or the increasing accumulated damage produces the observed evolution of macroscopic deformation during brittle creep however no current model can predict quantitatively all of the observed features of brittle creep experimental data are limited by the timescale over which experiments are realistically feasible clearly an extension of the range of available laboratory data to lower strain rates and the development of new modelling approaches are needed to further improve our current understanding of time dependent brittle deformation in rocks

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

the long precursory phase of most large interplate earthquakes many earthquakes are preceded by foreshocks however the mechanisms that generate foreshocks and the reason why they occur before some earthquakes and not others are unknown here we use seismic catalogues from the best instrumented areas of the north pacific to analyse the foreshock sequences preceding all earthquakes there between 1999 and 2011 of magnitude larger than 6 5 and at depths shallower than 50 km the data set comprises 31 earthquakes at plate boundaries and 31 in plate interiors we find that there is a remarkable contrast between the foreshock sequences of interplate compared with intraplate earthquakes most large earthquakes at plate interfaces in the north pacific were preceded by accelerating seismic activity in the months to days leading up to the mainshock in contrast foreshocks are much less frequent in intraplate settings we suggest that at plate boundaries the interface between the two plates begins to slowly slip before the interface ruptures in a large earthquake this relatively long precursory phase could help mitigate earthquake risk at plate boundaries

stable isotope stratigraphy and paleoclimatology of the paleogene bighorn basin wyoming usa abstract climatic warming from the late paleocene into the early eocene had profound effects on atmospheric and marine circulation marine thermal gradients and benthic biota in addition marine carbon isotope values decreased substantially in both surface and deep waters because carbon is rapidly exchanged between reservoirs at the earth s surface such as marine surface water the atmosphere land plants and materials that obtain carbon from plants e g soil minerals and herbivores carbon isotope fluctuations provide time lines linking the marine and continental records we analyzed the carbon isotope composition of paleosol carbonates and mammalian tooth enamel from stratigraphic sections in northwestern wyoming carbon isotope correlation demonstrates that a short interval of extreme high latitude warming coincided precisely with the first appearance of several important modern mammalian orders in addition oxygen isotope analyses of fossils and paleosol carbonates provide information about climatic conditions on land in the paleogene we reconstructed the oxygen isotope composition of local meteoric water using biogenic minerals paleocene eocene meteoric water was significantly 18 o depleted indicating substantial loss of water vapor during its transport to the region soil temperature was calculated as a proxy for mean annual temperature assuming oxygen isotope equilibrium between soil carbonate and meteoric water calculated temperatures were plausible 10 25 c but highly variable prohibiting high resolution analysis of local temperature variations in response to global climatic warming

earthquake nucleation on rate and state faults aging and slip laws we compare 2 d quasi static earthquake nucleation on rate and state faults under both aging and slip versions of the state evolution law for both versions mature nucleation zones exhibit 2 primary regimes of growth well above and slightly above steady state corresponding respectively to larger and smaller fault weakening rates well above steady state aging law nucleation takes the form of accelerating slip on a patch of fixed length this length is proportional to b 1 and independent of a where a and b are the constitutive parameters relating changes in slip speed and state to frictional strength under the slip law the nucleation zone is smaller and continually shrinks as slip accelerates the nucleation zone is guaranteed to remain well above steady state only for values of a b that are low by laboratory standards near steady state for both laws the nucleation zone expands the propagating front remains well above steady state giving rise to a simple expression for its effective fracture energy g c this fracture energy controls the propagation style for the aging law g c increases approximately as the square of the logarithm of the velocity jump this causes the nucleation zone to undergo quasi static crack like expansion to a size asymptotically proportional to b b a 2 for the slip law g c increases only as the logarithm of the velocity jump and crack like expansion is not an option instead the nucleation zone grows as an accelerating unidirectional slip pulse under both laws the nucleation front propagates at a velocity larger than the slip speed by roughly b divided by the logarithm of the velocity jump where is the effective elastic shear modulus for this prediction to be consistent with observed propagation speeds of slow slip events in subduction zones appears to require effective normal stresses as low as 1 mpa

correlation between isotope records in marine and continental carbon reservoirs near the palaeocene eocene boundary changes in the isotope content of the large marine carbon reservoir can force shifts in that of the smaller carbon pools in the atmosphere and on land the carbon isotope compositions of marine carbonate sediments from the late palaeocene vary considerably exhibiting a sudden decrease close to the palaeocene eocene boundary which coincides with deep sea benthic extinctions1 and with changes in ocean circulation here we report that these fluctuations in the marine carbon isotope record are closely tracked by the terrestrial records provided by palaeosol carbonates and mammalian tooth enamel in using palaeosol carbonates to reconstruct the co2 content of the ancient atmosphere2 isotope shifts of this sort will have to be taken into account the sharp decrease in 13c 12c ratios in the late palaeocene provides a datum for precise correlation of marine and continental records and suggests that abrupt climate warming at this time may have played an important role in the evolution of land mammals

fossil horses and carbon isotopes new evidence for cenozoic dietary habitat and ecosystem changes in north america abstract the transformation from low crowned to high crowned horse teeth during the miocene has traditionally been interpreted as an adaptive response to the spread of savanna grasslands during the middle miocene by about 15 m y ago carbon isotope data from 50 north american horse teeth spanning eocene to pleistocene age indicate that savanna grasslands presumably dominated by c 4 grasses as they are today first became widespread much later than do horses with high crowned teeth prior to 7 m y horses had a c 3 based diet and after 7 m y horses started eating c 4 grasses this change in diet occurred when the major drop in the diversity of horses occurs during the late miocene the change in vegetation reflected in horses diet may be related to a significant reduction in atmospheric co 2 level toward the end of the miocene which provided the c 4 grasses with an adaptive advantage and led to their expansion at the expense of c 3 plants

biologic and climatic signals in the oxygen isotopic composition of eocene oligocene equid enamel phosphate abstract oxygen isotope results for tooth enamel phosphate p from late eocene early oligocene fossil horses are presented to determine if paleobiologic and paleoclimatologic information is recorded in fossil tooth enamel chemistry teeth from jaws of mesohippus and miohippus from the white river formation or group in the western great plains are well preserved and have excellent geochronologic control although there is clear evidence for post depositional alteration of the enamel a hydroxylapatite mineralogy is preserved and isotopic exchange of oxygen does not appear to be significant there are distinctive patterns of p variation among individual teeth from the same jaw these patterns reflect season of birth and timing of enamel mineralization most of the horses were born in the spring and mineralization of the enamel is complete after 1 1 5 years these results show that paleoclimate reconstruction from enamel p must account for tooth position and timing of mineralization the observation that there are different intrajaw patterns of p variation among mesohippus and miohippus may provide a basis to reconstruct changes in climate seasonality in ancient environments

paleoseasonality inferred from equid teeth and intra tooth isotopic variability abstract this research presents a study of intra tooth isotopic variability measured in hypsodont equid teeth as a means of determining paleoseasonality the purpose of this investigation is to determine whether sivapithecus a miocene hominoid from the siwaliks of pakistan lived in habitats characterized by a seasonal rainfall regime and if so what kind of forest is associated with this regime forest type such as evergreen semi evergreen moist deciduous and dry deciduous is largely determined by length of the dry period reconstruction of the rainfall regime therefore allows inferences of forest type in the fossil record seasonal rainfall regimes can be determined from oxygen stable isotopes for oxygen isotopes track amount of rainfall in tropical regions sampling oxygen isotopes along the length of an equid tooth which takes over a year to develop therefore provides a record of seasonal isotopic changes in body water which in turn reflect drinking water and ultimately precipitation carbonate oxygen results from siwalik equid teeth spanning 10 0 6 3 ma indicate a rainfall regime similar to that experienced by southern china monsoon forests today with a monsoonal rainy season and a five to six month dry season furthermore results suggest a seasonal birthing pattern for siwalik equids and therefore predictable seasonal regime these results indicate that sivapithecus lived under a seasonal regime with dry periods longer than the maximum of 4 months that most great apes experience today therefore sivapithecus may have experienced longer periods of low fruit availability than do modern apes either sivapithecus differed in habitat requirements or the forest differed in terms of the spatio temporal variability of fruit compared to modern analogues over geologic time annual rainfall appears to have decreased in the siwaliks beginning around the time sivapithecus became extinct but annual seasonality of rainfall does not change

integration of channel and floodplain suites i developmental sequence and lateral relations of alluvial paleosols abstract the lower eocene willwood formation of the bighorn basin northwest wyoming consists of about 770 m of alluvial rocks that exhibit extensive mechanical and geochemical modifications resulting from eocene pedogenesis willwood paleosols vary considerably in their relative degrees of maturity maturity is defined as stage of development as a function of the amount of time required to form five arbitrary stages are proposed to distinguish these soils of different maturities in the willwood formation stage 1 soils the least mature are entisols stage 2 and stage 3 soils are intermediate in maturity and are probably alfisols and stage 4 and stage 5 soils the most mature are spodosols these stages are not only time progressive elements of an in situ maturation sequence for w llwood soil formation but in the lateral dimension they are also usually distributed sequentially study of willwood paleosols indicates that an inverse relationship exists between soil maturity and short term sediment accumulation rate the least mature willwood paleosols formed in areas of relatively high net rates of sediment accumulation on 1 channel levee and crevasse splay sediments of the proximal alluvial ridge and 2 deposits filling large and small paleovalleys formed by major episodes of gullying lowered baselevels in contrast the fine grained sediments of the distal floodplain where net sediment accumulation rates were relatively low experienced development of much more mature soils soils of intermediate maturities occur in the order of their stage on intervening proximal floodplain and distal alluvial ridge sediments adjacent bodies of sedimentary rock that differ in their ancient soil properties because of distance from areas of relatively high sediment accumulation are denoted by the new term pedofacies the remarkable sequence of paleosols in the willwood formation clearly illustrates several important principles of soil sediment interrelationships in aggrading alluvial systems that have broad application to other deposits this is especially true in view of the widespread distribution of paleosols in nearly all ancient fluvial rocks further study of willwood paleosols will not only enable precise lateral correlation of coeval alluvial sediments and thereby fluvial sedimentary events from the distal to the proximal realms of the floodplain but will also contribute to increasingly informative evaluations of the nature tempo and mode of alluvial succession

rupture preparation process controlled by surface roughness on meter scale laboratory fault abstract we investigate the effect of fault surface roughness on rupture preparation characteristics using meter scale metagabbro specimens we repeatedly conducted the experiments with the same pair of rock specimens to make the fault surface rough we obtained three experimental results under the same experimental conditions 6 7 mpa of normal stress and 0 01 mm s of loading rate but at different roughness conditions smooth moderately roughened and heavily roughened during each experiment we observed many stick slip events preceded by precursory slow slip we investigated when and where slow slip initiated by using the strain gauge data processed by the kalman filter algorithm the observed rupture preparation processes on the smooth fault i e the first experiment among the three showed high repeatability of the spatiotemporal distributions of slow slip initiation local stress measurements revealed that slow slip initiated around the region where the ratio of shear to normal stress was the highest as expected from finite element method fem modeling however the exact location of slow slip initiation was where became locally minimum probably due to the frictional heterogeneity in the experiment on the moderately roughened fault some irregular events were observed though the basic characteristics of other regular events were similar to those on the smooth fault local stress data revealed that the spatiotemporal characteristics of slow slip initiation and the resulting drop for irregular events were different from those for regular ones even under similar stress conditions on the heavily roughened fault the location of slow slip initiation was not consistent with anymore because of the highly heterogeneous static friction on the fault which also decreased the repeatability of spatiotemporal distributions of slow slip initiation these results suggest that fault surface roughness strongly controls the rupture preparation process and generally increases its complexity with the degree of roughness

a constitutive scaling law and a unified comprehension for frictional slip failure shear fracture of intact rock and earthquake rupture 1 it is widely recognized that some of the physical quantities inherent in a rupture are scale dependent and the scale dependence is one of the most no facts and features of rupture phenomena the paper addresses how such scale dependent shear rupture of a broad range from laboratory scale frictional slip failure and shear fracture of intact rock to field scale rupture as an earthquake source can be unified by a single constitutive law noting that the earthquake rupture is a mixed process between frictional slip failure and the shear fracture of intact rock it is concluded that the constitutive law for the earthquake rupture be formulated as a unifying law that governs both frictional slip failure and shear fracture of intact rock it is demonstrated that the slip dependent constitutive law is such a unifying law and a constitutive scaling law is derived from laboratory data on both frictional slip failure and shear fracture of intact rock this constitutive scaling law enables one to provide a consistent and unified comprehension for scale dependent physical quantities inherent in the rupture over a broad range from small scale frictional slip failure and shear fracture in the laboratory to large scale earthquake rupture in the field

effective friction law for small scale fault heterogeneity in 3d dynamic rupture 1 we address the problem of modeling dynamic rupture on multiscale heterogeneous faults in 3d under the assumption of slip weakening friction we numerically construct effective friction laws that integrate the effects of small scale heterogeneity during the rupture this homogenization process is based on the description of the initial phase of the rupture by the dominant unstable spectral mode its dynamics is influenced by the geometry of the fault the static friction heterogeneities and the friction law we first define a periodic small scale heterogeneous model introducing heterogeneity in the distribution of the static friction coefficient we then describe a method for constructing this effective friction law applying this new law homogeneously on the fault permits to reproduce the dynamic evolution of the heterogeneous fault furthermore we show that the effective friction law can be used to replace small scale heterogeneities in two scale heterogeneous models while preserving their effects we study three kinds of two scale models with growing complexity first periodic at both scales then periodic only at small scale and finally irregular at both scales this homogenization method can be adapted to the case where the heterogeneity is introduced in the initial stress rather than in the static friction value finally we show in a simple example that the effective friction law permits to reproduce the transition between subshear and supershear rupture propagation originally produced by heterogeneities on the fault

isotopic records of climate seasonality in equid teeth abstract we investigate how oxygen isotopes in equid teeth can be used as a record of seasonality first we use in situ laser ablation and conventional microsampling techniques to understand time averaging of environmental signals in intra tooth isotope profiles in modern feral horse teeth n 5 from mongolia where there is a large seasonal gradient in the oxygen isotopic composition of precipitation we demonstrate that laser ablation can be used to sample inner middle and outer enamel layers in large mammalian herbivore teeth the inner enamel layer records less attenuated isotopic signals than other layers as predicted by the mineralization patterns but intra tooth signal amplitude is similar for laser and conventional sampling methods second we use modern zebra teeth n 21 collected in eastern africa to evaluate how intra tooth oxygen isotope variation relates to rainfall patterns in the tropics we show that the intra tooth isotopic range increases with intra annual precipitation isotopic range which in turn relates to aridity in the equatorial bimodal rainfall region but is influenced by other hydroclimate processes in the region as a whole finally we address isotopic seasonality during the early pleistocene in eastern africa using oxygen isotopes in fossil equid teeth from southwestern kenya n 11 and northern tanzania n 5 we find variable isotopic seasonality in the past similar to present day eastern africa consistent with the notion that hominins and other mammals were able to accommodate environmental variability on intra annual scales in addition to well documented orbital cycles

terrestrial carbon isotope excursions and biotic change during palaeogene hyperthermals the earth s climate between 60 and 50 million years ago was punctuated by several abrupt warming events the largest of these being the palaeocene eocene thermal maximum carbon isotope records from the bighorn basin wyoming document these events in the terrestrial realm and show a consistent scaling between marine and terrestrial records across the three main events

extended nucleation of the 1999 mw 7 6 izmit earthquake laboratory and theoretical studies suggest that earthquakes are preceded by a phase of developing slip instability in which the fault slips slowly before accelerating to dynamic rupture we report here that one of the best recorded large earthquakes to date the 1999 moment magnitude mw 7 6 izmit turkey earthquake was preceded by a seismic signal of long duration that originated from the hypocenter the signal consisted of a succession of repetitive seismic bursts accelerating with time and increased low frequency seismic noise these observations show that the earthquake was preceded for 44 minutes by a phase of slow slip occurring at the base of the brittle crust this slip accelerated slowly initially and then rapidly accelerated in the 2 minutes preceding the earthquake

effects of physical fault properties on frictional instabilities produced on simulated faults laboratory studies of large scale simulated faults show that physical properties of the fault specifically normal stress and fault roughness significantly influence the unstable shear failure behavior of the fault in addition the experiments provide insights into important length scaling effects that are useful for assessing concepts such as critical crack length or rupture nucleation dimension stick slip shear failures have been generated along a 2 m long simulated fault in a block of sierra white granite experiments were conducted at normal stresses between 0 6 and 4 mpa with two different prepared roughnesses of the simulated fault a smooth fault with a profilometer measured roughness of 0 2 m and a rough fault with a measured roughness of 80 m high speed records of shear strains and slip velocities clearly show both the propagation of the stick slip failures and details of the local deformations and motions from which slip weakening of the fault at the onset of sliding can be resolved both the dynamic stress drops of the stick slip events and the apparent fracture energies of the events determined from the records of the fault slip weakening increase with increasing normal stress although critical slip weakening displacements are insensitive to normal stress they increase with increased fault roughness such that the apparent fracture energies are also larger on the rough fault than on the smooth fault slip velocities in these experiments are influenced by roughness and are proportional to stress drop although the onset of slip is sharply defined the termination of slip is more gradual without a clear stopping phase measured rupture velocities on the rough fault are lower than those on the smooth fault when a length scale based on rupture nucleation dimensions directly proportional to critical displacements is introduced the data suggest that at fault lengths close to the nucleation lengths rupture velocity and slip velocity increase with increasing fault length

oxygen isotope and paleobotanical estimates of temperature and δ18o latitude gradients over north america during the early eocene empirical estimates of climate parameters such as mean annual temperature mat are essential to describe both ancient climate and to ground truth climate model simulations of past climates in terrestrial settings no single proxy record is able to provide detailed temporal yet geographically widespread information and it thus becomes essential that paleoclimatic information obtained from different proxies be comparable as an example of how to compare estimates and to illustrate what can be learned by doing so we measured oxygen isotope ratios of phosphate in the tooth enamel of fossil mammals and the body scales of freshwater fish in order to estimate the mean annual temperature mat in which they lived the samples were collected from paleogene rocks of the bighorn green river and powder river basins of wyoming earliest eocene samples provide mat estimates that are similar to mat estimates for the same times and areas obtained from leaf margin analysis furthermore isotopic data from the paleocene eocene thermal maximum petm which has not yet yielded fossil floras indicate a significantly higher mat of 26 c we also used temperature estimates obtained by both methods to investigate paleoelevation and to reconstruct geographic patterns of mat over north america for the early eocene larger laramide basins have higher temperatures and lower inferred elevations compared to associated volcanic highlands where cooler temperatures are consistent with substantial elevations in turn latitudinal gradients in temperature indicate that warming centered on polar regions played an important role during the eocene hothouse lastly we show that the reconstructed eocene relation between oxygen isotope ratios of precipitation and mat is significantly different from the modern relation between these two variables because the relation has changed over time the modern correlation should not be used to estimate paleo mat from measurements of oxygen isotope ratios in fossil material

fluid evolution in an oceanic core complex a fluid inclusion study from iodp hole u1309 d atlantis massif 30 n mid atlantic ridge in the detachment mode of slow seafloor spreading convex upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the seafloor large long lived hydrothermal systems such as tag are situated off axis and may be controlled by fluid flow up a detachment fault with the source of magmatic heat being as deep as 7 kmbsf the consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated microthermometry on fluid inclusions trapped in diabase gabbro and trondjhemite recovered at the atlantis massif oceanic core complex 30 n mid atlantic ridge reveals evidence for magmatic exsolution phase separation and mixing between hydrothermal fluids and previously phase separated fluids four types of fluid inclusions were identified ranging in salinity from 1 4 to 35 wt nacl although the most common inclusions have salinities close to seawater 3 4 wt nacl homogenization temperatures range from 160 to 400 c with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low salinity inclusions trapped in quartz veins the fluid history of the atlantis massif is interpreted in the context of published thermochronometric data from the massif and a comparison with the inferred circulation pattern beneath the tag hydrothermal field to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by iodp hole u1309d fluids have been trapped

properties of the seismic nucleation phase abstract near source observations show that earthquakes begin abruptly at the p wave arrival but that this beginning is weak with a low moment rate relative to the rest of the main shock we term this initial phase of low moment rate the seismic nucleation phase we have observed the seismic nucleation phase for a set of 48 earthquakes ranging in magnitude from 1 1 8 1 the size and duration of the seismic nucleation phase scale with the total seismic moment of the earthquake suggesting that the process responsible for the seismic nucleation phase carries information about the eventual size of the earthquake the seismic nucleation phase is characteristically followed by quadratic growth in the moment rate consistent with self similar rupture at constant stress drop in this paper we quantify the properties of the seismic nucleation phase and offer several possible explanations for it

a mechanism for preseismic steady rupture fronts observed in laboratory experiments it has been shown that the onset of frictional instability is characterized by a transition from stable quasi static rupture growth to unstable inertially controlled high speed rupture in particular slow rupture fronts propagating at a steady speed v slow of the order of 5 of the s wave speed have been observed prior to the onset of dynamic rupture in recent fault friction laboratory experiments however the precise mechanism governing this v slow stage is unknown here we reproduce this phenomenon in numerical simulations of earthquake sequences that incorporate laboratory derived rate and state friction laws our simulations show that the v slow stage originates from a stress concentration inherited from the coalescence of interseismic slow creep fronts its occurrence is limited to a narrow range of the parameter space but is found in simulations with two commonly used state variable evolution laws in the rate and state formulation the sensitivity of the speed v slow to the model parameters suggests that the propagation speed v slow reported in laboratory experiments may also be sensitive to parameters of friction and stress conditions our results imply that time and space dimensions associated with the propagation of v slow on natural faults can be as much as a few seconds and several hundred meters respectively hence the detection of such preseismic signals may be possible with near field high resolution observations

experimental observation of stick slip instability fronts summary stick slip dynamic instability is a key mechanism governing frictional processes from microscale physics to earthquake faults and landslides yet challenging questions are still open about its nucleation and propagation dynamics here we present novel observations on laboratory experimental faults where spontaneously nucleating fractures are produced describing 1 an initial quasistatic stable rupture front propagating at about 5 per cent the shear wave velocity vs accelerating to subshear and then to intersonic velocity 2 the arisal of a higher degree of complexity when the friction to prestress ratio is increased on the sliding surface the complex behaviour includes stop and go sequences irregular propagation and rerupturing episodes within short time intervals implying rapid restrengthening of the surface and the formation of self healing pulses reproducing experimentally for the first time a behaviour observed on seismic faults

nucleation process of magnitude 2 repeating earthquakes on the san andreas fault predicted by rate and state fault models with safod drill core data recent laboratory shear slip experiments conducted on a nominally flat frictional interface reported the intriguing details of a two phase nucleation of stick slip motion that precedes the dynamic rupture propagation this behavior was subsequently reproduced by a physics based model incorporating laboratory derived rate and state friction laws however applying the laboratory and theoretical results to the nucleation of crustal earthquakes remains challenging due to poorly constrained physical and friction properties of fault zone rocks at seismogenic depths here we apply the same physics based model to simulate the nucleation process of crustal earthquakes using unique data acquired during the san andreas fault observatory at depth safod experiment and new and existing measurements of friction properties of safod drill core samples using this well constrained model we predict what the nucleation phase will look like for magnitude 2 repeating earthquakes on segments of the san andreas fault at a 2 8 km depth we find that despite up to three orders of magnitude difference in the physical and friction parameters and stress conditions the behavior of the modeled nucleation is qualitatively similar to that of laboratory earthquakes with the nucleation consisting of two distinct phases our results further suggest that precursory slow slip associated with the earthquake nucleation phase may be observable in the hours before the occurrence of the magnitude 2 earthquakes by strain measurements close a few hundreds meters to the hypocenter in a position reached by the existing borehole

scaling of the shear rupture process from nucleation to dynamic propagation implications of geometric irregularity of the rupturing surfaces a series of systematic high resolution laboratory experiments have been performed on the nucleation of propagating slip failure on preexisting faults having different surface roughnesses to demonstrate how the size scale and duration of shear rupture nucleation are affected by geometric irregularity of the rupturing surfaces on the basis of the experimental results it has been discussed theoretically how consistently scale dependent physical quantities inherent in shear rupture are scaled the experiments led to conclusive results that the nucleation process consists of two phases phase i an initial quasi static phase and phase ii a subsequent accelerating phase and that the nucleation process is greatly affected by geometric irregularities on the rupturing surfaces in phase i the rupture grows at a slow steady speed which is independent of the rupture growth length l in contrast during phase ii the rupture develops at accelerating speeds v which increase with an increase in l obeying a power law v vs l c n where vs is the shear wave velocity c is the characteristic length representing the geometric irregularity of the fault surfaces and and n are constants 8 87 10 29 and n 7 31 scale dependency of scale dependent physical quantities including the nucleation zone size and its duration is commonly ascribed to scale dependency of the slip dependent constitutive law parameter dc which is in turn governed by c it has been discussed that a unified comprehension can be provided for shear rupture of any size scale if the constitutive law for shear rupture is formulated as a slip dependent law

foreshocks during the nucleation of stick slip instability 1 we report on laboratory experiments which investigate interactions between aseismic slip stress changes and seismicity on a critically stressed fault during the nucleation of stick slip instability we monitor quasi static and dynamic changes in local shear stress and fault slip with arrays of gages deployed along a simulated strike slip fault 2 m long and 0 4 m deep in a saw cut sample of sierra white granite with 14 piezoelectric sensors we simultaneously monitor seismic signals produced during the nucleation phase and subsequent dynamic rupture we observe localized aseismic fault slip in an approximately meter sized zone in the center of the fault while the ends of the fault remain locked clusters of high frequency foreshocks mw 6 5 to 5 0 can occur in this slowly slipping zone 5 50 ms prior to the initiation of dynamic rupture their occurrence appears to be dependent on the rate at which local shear stress is applied to the fault the meter sized nucleation zone is generally consistent with theoretical estimates but source radii of the foreshocks 2 to 70 mm are 1 to 2 orders of magnitude smaller than the theoretical minimum length scale over which earthquake nucleation can occur we propose that frictional stability and the transition between seismic and aseismic slip are modulated by local stressing rate and that fault sections which would typically slip aseismically may radiate seismic waves if they are rapidly stressed fault behavior of this type may provide physical insight into the mechanics of foreshocks tremor repeating earthquake sequences and a minimum earthquake source dimension

tooth enamel biomineralization in extant horses implications for isotopic microsampling abstract isotopic analyses of tooth enamel from fossil equids are increasingly being used to reconstruct paleoclimatic and paleoenvironmental conditions however the accuracy of these reconstructions is currently limited partly because the precise timing and spatial patterns of enamel mineralization in equids have not been documented we used radiographic analyses of mandibles collected from modern juvenile and adult domestic horses equus caballus to document the timing of enamel mineralization in equid cheek teeth premolars and molars optical and radiographic analyses of thin sections of mature and developing teeth were used to document the spatial pattern of enamel mineralization in each tooth we found that the enamel layers in equine cheek teeth took longer to mineralize than had been previously assumed largely because the enamel continued to mineralize for approximately 6 to 12 months after each tooth had begun to erupt total enamel mineralization times for individual teeth ranged from 1 5 to 2 8 years examination of thin sections revealed that retzius striae extend outward from the enamel dentin junction at an angle of 5 to 10 and run near parallel to the nonocclusal surface of the tooth daily cross striations average 5 m in width suggesting that during the initial phase of enamel mineralization matrix formation new material is added at a rate of 5 m day radiographic analyses demonstrate that the secondary mineralization front enamel maturation front is orientated approximately parallel to the retzius striae but complete maturation i e full mineralization lags behind matrix formation by several weeks to months

mammalian community response to the latest paleocene thermal maximum an isotaphonomic study in the northern bighorn basin wyoming new stratigraphic and paleontological information from the mccullough peaks northern bighorn basin wyoming is incorporated into an isotaphonomic faunal database and used to investigate the impact of the latest paleocene thermal maximum and coincident earliest wasatchian immigration event on local mammalian community structure surface collections from willwood formation overbank deposits provide taphonomically consistent and stratigraphically resolved samples of the medium to large sized components of underlying mammalian communities rarefaction shows that the immigration event caused an abrupt and dramatic increase in species richness and evenness after this initial increase diversity tapered off to more typical wasatchian levels that were still higher than those in the preceding clarkforkian wasatchian immigrants were rapidly incorporated into the new community organization representing 20 of the taxa and 50 of the individuals immigrant taxa generally had larger body sizes and more herbivorous and frugivorous dietary habits compared to endemic taxa causing significant turnover in body size structure and trophic structure there was a significant short term body size decrease in many lineages that may have been prompted by the elevated temperatures and or decreased latitudinal thermal gradients during the latest paleocene thermal maximum rapid short term climatic change transient climates and associated biotic dispersal can have abrupt and long lasting effects on mammalian community evolution

intra tooth stable isotope profiles in warthog canines and third molars implications for paleoenvironmental reconstructions abstract intra tooth stable isotope variations have been used to interpret seasonality and aridity in paleoenvironmental reconstructions of paleontological and archeological sites however most intra tooth datasets only permit qualitative interpretations of seasonality because the measured signal is attenuated due to the duration of enamel mineralization process and sampling geometry the common warthog phacochoerus africanus is an ideal organism to investigate stable isotope variation in enamel their canines grow continuously through the life of the individual and are therefore excellent candidates for mathematical modeling of seasonal signals and of signal attenuation further their isotope profiles a series of isotope measurements can be compared to isotope profiles of third molars m3 to provide insights into environmental reconstructions we first obtained paired intra tooth enamel samples from ever growing canines and hypsodont m3s of two extant common warthog specimens from laikipia kenya second from a different set of specimens we collected data on enamel growth patterns and geometry using histological thin sections and transmitted light microscopy and enamel mineralization parameters using micro ct scans in each tooth type third we reconstructed the timeline of unattenuated seasonal 18o signal from canine enamel using growth rate estimates and the inverse model of passey et al 2005 our results demonstrate that canines which capture 1 5 years of time exhibit near constant growth rates and simple enamel maturation geometry whereas m3s which also represent 1 5 years of time exhibit linearly decreasing growth rates and more complex maturation patterns we compare the timelines of unattenuated seasonal 18o signal and measured m3 profiles and find an average signal reduction of 50 in the m3s providing interpretations of the duration of seasonal cycles that are consistent 75 of the time we conclude that warthog canines are well suited for the inverse model approach and we established the model parameters for the forward and inverse methods timeline reconstructions based on m3 histology are promising for investigating the pattern of rainfall seasonality in the past finally we found an unexpected carbon isotopic spacing of 2 between canine and m3 enamel which suggests caution in interpreting 13c results from suid canine or molar enamel alone

slow and fast ruptures on a laboratory fault controlled by loading characteristics recent geodetic observations indicate that a single fault may slip slowly and silently or fast and seismically in different circumstances we report laboratory experiments that demonstrate how the mode of faulting can alternate between fast stick slip events 70 to 500 mm s sliding rates and slow silent events 0 001 to 30 mm s as a result of loading conditions rather than friction properties or stiffness of the loading machine the 760 mm long granite sample is close to the critical nucleation length scale for unstable slip so small variations in nucleation properties result in measurable differences in slip events slow events occur when instability cannot fully nucleate before reaching the sample ends dynamic events occur after long healing times or abrupt increases in loading rate which suggests that these factors shrink the spatial and temporal extents of the nucleation zone arrays of slip strain and ground motion sensors installed on the sample allow us to quantify seismic coupling and study details of premonitory slip and afterslip we find that seismic coupling decreases when slip rates fall below about 70 mm s the slow slip events we observe are primarily aseismic less than 1 of the seismic coupling of faster events and produce swarms of very small m 6 to m 8 events these mechanical and seismic interactions suggest that faults with transitional behavior where creep small earthquakes and tremor are often observed could become seismically coupled if loaded rapidly either by a slow slip front or dynamic rupture of an earthquake that nucleated elsewhere

vertebrate fossil bearing paleosol units willwood formation lower eocene northwest wyoming u s a implications for taphonomy biostratigraphy and assemblage analysis abstract in the lower 300 m of the fluvial willwood formation of northwest wyoming most large concentrations of vertebrate fossils consist of disarticulated and broken skeletal remains that occur in widespread tubular thin 2 cm 1 m greenish and bluish gray mudstones that are low in percentage of free iron aluminum and manganese relatively high in percentages of organic carbon and often show evidence of clay eluviation these units lie atop sesquioxide rich sometimes clay illuviated variegated unit red yellow purple and mottled variants and are relicts of the a horizons of podzolic spodosols aquods the wide distribution of bone fragments the numerical proportions of bone elements weathering of the fragments and biogenic evidence indicate the vertebrate remains accumulated gradually as litter on the surface of the soils and became incorporated into the a horizon the bones were disarticulated broken and otherwise disturbed by the action of scavengers prior to burial occurrence of abundant vertebrate remains in discrete readily identifiable and widespread paleosol units makes these units valuable biostratigraphic markers zonules faunules evidence of geologically short term lag accumulation of the bones is important to assemblage analysis the paleosol concentrations result from attritional mortalities that more closely reflect composition of the life assemblage than do remains concentrated by fluvial transport or predators

foreshock triggering of the 1 april 2014 mw 8 2 iquique chile earthquake abstract on april 1st 2014 a mw 8 2 u s geological survey moment magnitude earthquake occurred in the subduction zone offshore northern chile in the two weeks leading up to the earthquake a sequence of foreshocks starting with a mw 6 7 earthquake on march 16th and including three more mw 6 0 events occurred predominantly south of the april 1st mainshock epicenter and up dip of the area of significant slip during the mainshock using earthquake locations and source parameters derived in a previous study hayes et al 2014 and a coulomb failure stress change analysis of these events we assess in detail the hypothesis that the earthquakes occurred as a cascading sequence each event successively triggering the next ultimately triggering the rupture of the mainshock following the initial mw 6 7 event each of the three largest foreshocks mw 6 4 6 2 and 6 3 as well as the hypocenter of the mainshock occurred in a region of positive coulomb stress change produced by the preceding events indicating these events were brought closer to failure by the prior seismicity in addition we reexamine the possibility that aseismic slip occurred and what role it may have played in loading the plate boundary using horizontal gps displacements from along the northern chile coast prior to the mainshock we find that the foreshock seismicity alone likely does not account for the observed signals we perform a grid search for the location and magnitude of an aseismic slip patch that can account for the difference between observed signals and foreshock related displacement and find that a slow slip region with slip corresponding to a mw 6 8 earthquake located coincident with or up dip of the foreshock seismicity can best explain this discrepancy additionally such a slow slip region positively loads the mainshock hypocentral area enhancing the positive loading produced by the foreshock seismicity