modeling the hydromechanical responses of sandwich structure faults during underground fluid injection injection or withdrawal of fluid at depth may trigger felt seismicity at a site with natural faults or artificial fracturing cracks understanding how to avoid triggering felt earthquakes plays a crucial role in the success of underground anthropogenic activities such as subsurface energy storage and co2 geological sequestration with the application of a hydromechanical coupling finite element model we examine the potential of injection induced pore pressure change and the responding fault slippage along the fault surface in two injection scenarios multi layer injection mli and single layer injection sli the results indicate that fault slippage buildup is mainly located along the portion of the fault intersecting the reservoir mli leads to a relatively lower level of induced seismicity compared with sli in the fault zone the external zone and core of the fault exert a connection effect and barrier effect on the hydraulic connectivity respectively after 20 years of injection the pore pressure in the core presents a smaller change than that in the internal and external zones furthermore an uncertainty analysis was performed to evaluate the statistical significance of permeability in the caprock reservoir and fault core internal zone and external zone and only the permeability in the reservoir significantly affects the level of fault slippage based on the fault zone hydromechanics associated with subsurface energy storage and co2 geological sequestration

the link between the north pacific climate variability and the north atlantic oscillation via downstream propagation of synoptic waves abstractthe north atlantic oscillation nao response to the northeast pacific climate variability is examined using the era 40 dataset the main objective is to validate a mechanism involving downstream wave propagation processes proposed in a recent idealized companion study a low frequency planetary scale ridge trough anomaly located in the eastern pacific north american sector induces more equatorward poleward propagation of synoptic scale wave packets on its downstream side which favors the occurrence of anticyclonic cyclonic wave breakings in the atlantic sector and the positive negative nao phase the mechanism first provides an interpretation of the canonical impact of the el nino southern oscillation on the nao in late winter the wintertime relationship between the pacific north american oscillation pna and the nao is also investigated for out of phase fluctuations between the pna and nao indices i e the most recurrent situation in late winter the eastern pacific pna ridge trou

a note on induced stress changes in hydrocarbon and geothermal reservoirs abstract earthquakes have been induced by oil and gas production where pore pressures have decreased in some cases by several tens of mpa it has previously been suggested that such earthquakes are caused by poroelastic stressing of crust surrounding the reservoir induced earthquakes are also common in geothermal fields such as the geysers where strong correlations between both steam production and condensate injection and earthquake activity have been observed over the last several decades stress measurements within hydrocarbon reservoirs show that the least horizontal stress decreases with declining reservoir pressure as predicted by poroelasticity for circular disk shaped reservoirs isothermal reduction in pore pressure induces a relative horizontal tension within the reservoir production induced stressing may promote frictional sliding on pre existing faults within the reservoir itself normal faulting is promoted if the regional stress is extensional and the biot coefficient is sufficiently large 0 85 for reasonable coefficients of friction on the other hand dilatant fracturing and normal faulting are always promoted in extensional environments near the edge of the reservoir or in regions of high pore pressure gradient it is suggested that such fracturing could enhance fracture permeability in tight rocks adjacent to portions of the reservoir that experience large reductions in pore pressure due to production in regional compressional environments production modestly favors reverse faulting above and below the reservoir the ratio of thermal to poroelastic stress can be quite large in geothermal reservoirs such as the geysers reservoir wide energy balance considerations suggest that the average temperature has declined at the geysers by 6 c during the past 20 years reservoir average stress changes are thus on the order of 2 mpa and are certainly much larger near injection wells and steam producing fractures

gas bubble transport and emissions for shallow peat from a northern peatland the role of pressure changes and peat structure gas bubbles are an important pathway for methane release from peatlands the mechanisms controlling gas bubble transport and emissions in peat remain uncertain the effects of hydrostatic pressure and peat structure on the dynamics of gas bubbles in shallow peat were therefore tested in laboratory experiments a peat monolith was retrieved from a raised bog and maintained in a saturated state three distinct layers were identified from noninvasive permittivity measurements supported by soil physical properties porosity bulk density phase i of the experiment involved monitoring for the accumulation of gas bubbles under steady pressure and temperature conditions the data showed evidence for gas bubbles being impeded by a shallow semiconfining layer at depths between 10 and 15 cm visible gas bubbles observed on the side of the sample box were recorded over time to estimate changes in the vertical distribution of volumetric gas content porosity estimates derived using the complex refraction index model crim suggest that gas bubbles enlarge the pore space when the exerted pressure is high enough phase ii involved triggering release of trapped bubbles by repeatedly increasing and decreasing hydrostatic pressure in an oversaturated condition comparison of changes in pressure head and methane density in the head space confirmed that the increasing buoyancy force during drops in pressure is more important for triggering ebullition than increasing mobility during increases in pressure our findings demonstrate the importance of changes in hydrostatic pressure on bubble size and variations in resistance of the peat fabric in regulating methane releases from peatlands

nao implicated as a predictor of northern hemisphere mean temperature multidecadal variability 1 the twentieth century northern hemisphere mean surface temperature nht is characterized by a multidecadal warming cooling warming pattern followed by a flat trend since about 2000 recent warming hiatus here we demonstrate that the north atlantic oscillation nao is implicated as a useful predictor of nht multidecadal variability observational analysis shows that the nao leads both the detrended nht and oceanic atlantic multidecadal oscillation amo by 15 20 years theoretical analysis illuminates that the nao precedes nht multidecadal variability through its delayed effect on the amo due to the large thermal inertia associated with slow oceanic processes an nao based linear model is therefore established to predict the nht which gives an excellent hindcast for nht in 1971 2011 with the recent flat trend well predicted nht in 2012 2027 is predicted to fall slightly over the next decades due to the recent nao decadal weakening that temporarily offsets the anthropogenically induced warming

impacts of two types of la nina on the nao during boreal winter the present work identifies two types of la nina based on the spatial distribution of sea surface temperature sst anomaly in contrast to the eastern pacific ep la nina event a new type of la nina central pacific or cp la nina is featured by the sst cooling center over the cp these two types of la nina exhibit a fundamental difference in sst anomaly evolution the ep la nina shows a westward propagation feature while the cp la nina exhibits a standing feature over the cp the two types of la nina can give rise to a significantly different teleconnection around the globe as a response to the ep la nina the north atlantic na western european we region experiences the atmospheric anomaly resembling a negative north atlantic oscillation nao pattern accompanied by a weakening atlantic jet it leads to a cooler and drier than normal winter over western europe however the cp la nina has a roughly opposing impact on the na we climate a positive nao like climate anomaly is observed with a strengthening atlantic jet and there appears a warmer and wetter than normal winter over western europe modeling experiments indicate that the above contrasting atmospheric anomalies are mainly attributed to the different sst cooling patterns for the two types of la nina mixing up their signals would lead to difficulty in seasonal prediction of regional climate since the la nina related sst anomaly is clearly observed during the developing autumn the associated winter climate anomalies over western europe could be predicted a season in advance

tropical extratropical connection in the atlantic atmosphere ocean variability a question how sea surface temperature sst fluctuations in the tropical atlantic affect the north atlantic atmosphere ocean system is addressed using an atmospheric general circulation model coupled to a 50 m deep slab ocean mgcm and a linear baroclinic model a comparison of a 60 yr integration in which the tropical ssts are prescribed to observations toga ml with the control mgcm run shows that the tropical sst variations selectively enhance variance of the north atlantic oscillation nao by about 80 this is partly due to extratropical height anomalies associated with two dominant modes monopole and dipole of tropical atlantic sst variability because they both have a projection on to the nao in the toga ml run the simulated midlatitude height anomalies accompany characteristic sst anomaly patterns beneath and they are consistent with observations analyses of steady linear responses to the heating in the tropical atlantic and transient eddy feedback in midlatitude reveal that the substantial part of height anomalies is generated by the latter rather than the former the anomalous eddy forcing is however organized not only by the heating in the tropical atlantic but also by the tropical pacific sst variations through the teleconnection over north america it is suggested that the tropical atlantic sst variability does affect the north atlantic atmosphere and ocean but the dynamical connection is tenuous in comparison with the pacific

north american rainfall and temperature prediction response to the diversity of enso research has shown that there is significant diversity in the location of the maximum sea surface temperature anomaly ssta associated with the el nino southern oscillation enso in one extreme warm ssta peak near the south american coast often referred to as eastern pacific of ep el nino and at the other extreme warm ssta peak in the central pacific central pacific or cp el nino due to the differing tropical pacific ssta and precipitation structure there are differing extratropical responses particularly over north america recent work involving the north american multi model ensemble nmme system for intra seasonal to inter annual prediction on prediction of the differences between el nino events found excess warming in the eastern pacific during cp el nino events this manuscript investigates the ensemble and observational agreement of the nmme system when forecasting the north american response to the diversity of enso focusing on regional land based 2 meter temperature and precipitation nmme forecasts of north american precipitation and t2m agree with observations more often during ep events ensemble agreement of nmme forecasts is regional for instance ensemble agreement in southeast north america demonstrates a strong connection to nino3 precipitation and ssta amplitude during warm enso events ensemble agreement in northwest north america demonstrates a weak connection to nino4 precipitation and ssta amplitude during warm enso events still other regions do not show a strong connection between ensemble agreement and strength of warm enso events

the impacts of mechanical stress transfers caused by hydromechanical and thermal processes on fault stability during hydraulic stimulation in a deep geothermal reservoir we performed a series of 3d thermo hydro mechanical thm simulations to study the influences of hydromechanical and thermal processes on the development of an enhanced geothermal system strongly influenced by a network of short fault zones the model we developed was calibrated by comparing the simulated thm responses to field observations including ground surface deformations well pressure and microseismic activity of particular importance was the comparison between the observed temporal and spatial distribution of microseismic activity and the calculated shear reactivation of preexisting fractures inferred from simulated elasto plastic mechanical responses in the short fault zones using this approach we could identify when fault zones were reactivated as manifested in the field by a surge of local microseismic activity within the fault zone and we could back calculate the in situ stress field as being close to the stress conditions required for shear reactivation our results show that the main mechanisms of inducing seismicity are related to injection induced pressure increase and cooling during injection the reservoir expansion caused by the pressure increase led to mechanical stress transfer through the reservoir which prevented or delayed the reactivation of preexisting fractures after injection stopped there was an inversion of the mechanical stress transfers that favored shear reactivation which may explain why microseismic activity occurred after the cessation of the injection

the 2002 m5 au sable forks ny earthquake sequence source scaling relationships and energy budget 1 we find invariant high stress drops and radiated energies for a sequence of intraplate earthquakes we estimate source parameters of the au sable forks ny earthquake m5 2002 and aftershocks this intraplate earthquake was the largest to occur in eastern north america since 1988 and the largest to be recorded by regional broadband networks we use the empirical green s function egf method and define a set of qualitative and quantitative rules for the selection of egf earthquake pairs and for the quality verification of the obtained egf spectral ratio we use a multitaper code that performs the complex spectral division with minimum frequency leakage and allows transformation back to the time domain to check the validity of the egf event we estimate source parameters for 22 earthquakes m1 m5 in the sequence the median stress drop of 104 mpa using madariaga source model and 19 mpa for a brune model is significantly larger than estimates for interplate earthquakes the lower crustal strain rates and longer fault healing times in intraplate environments may be responsible for this high average value we find constant stress drop between m2 and m5 up to the bandwidth resolution limit 80 hz of the study and no evidence of stress drop breakdown for m2 to m1 earthquakes we find consistently high radiated seismic energy and apparent stress and a median radiated energy to seismic moment ratio of 9 10 5 this is significantly larger than estimates for interplate earthquakes 2 10 5 and consistent with higher stress drops and stronger faults

reservoir creep and induced seismicity inferences from geomechanical modeling of gas depletion in the groningen field the groningen gas field in the netherlands experienced an immediate reduction in seismic events in the year following a massive cut in production this reduction is inconsistent with existingmodels of seismicity predictions adopting compaction strains as proxy since reservoir creep would then result in a more gradual reduction of seismic events after a production stop we argue that the discontinuity in seismic response relates to a physical discontinuity in stress loading rate on faults upon the arrest of pressure change the stresses originate from a combination of the direct poroelastic effect through the pressure changes and the delayed effect of ongoing compaction after cessation of reservoir production both mechanisms need to be taken into account to this end we employed finite element models in a workflow that couples kelvin chain reservoir creep with a semi analytical approach for the solution of slip and seismic moment from the predicted stress change for ratios of final creep and elastic compaction up to 5 the model predicts that the cumulative seismic moment evolution after a production stop is subject to a very moderate increase 2 10 times less than the values predicted by the alternative approaches using reservoir compaction strain as proxy this is in agreement with the low seismicity in the central area of the groningen field immediately after reduction in production the geomechanical model findings support scope for mitigating induced seismicity through adjusting rates of pressure change by cutting down production the author s 2017 published by oxford university press on behalf of the royal astronomical society

flow path oscillations in transient ground water simulations of large peatland systems abstract transient numerical simulations of the glacial lake agassiz peatland near the red lakes in northern minnesota were constructed to evaluate observed reversals in vertical ground water flow seasonal weather changes were introduced to a ground water flow model by varying evapotranspiration and recharge over time vertical hydraulic reversals driven by changes in recharge and evapotranspiration were produced in the simulated peat layer these simulations indicate that the high specific storage associated with the peat is an important control on hydraulic reversals seasonally driven vertical flow is on the order of centimeters in the deep peat suggesting that seasonal vertical advective fluxes are not significant and that ground water flow into the deep peat likely occurs on decadal or longer time scales particles tracked within the ground water flow model oscillate over time suggesting that seasonal flow reversals will enhance vertical mixing in the peat column the amplitude of flow path oscillations increased with increasing peat storativity with amplitudes of about 5 cm occurring when peat specific storativity was set to about 0 05 m 1

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

insights from closed form expressions for injection and production induced stresses in displaced faults we consider fluid induced seismicity and present closed form expressions for the elastic displacements strains and stresses resulting from injection into or production from a reservoir with displaced faults we apply classic inclusion theory to two dimensional finite width and infinite width reservoir models first we simplify the fault model to the bare minimum while still maintaining its essential features a vertical fault in a homogeneous reservoir of infinite width in an infinite domain we confirm and sharpen findings from earlier numerical studies and furthermore conclude that the development of infinitely large elastic shear stresses in a displaced fault at the internal and external reservoir fault corners implies that even small amounts of injection or production will result in some amount of slip or other nonelastic deformation another finding is that there is a marked difference between the shear stress patterns resulting from injection and production in a reservoir with a displaced fault in both situations two slip patches emerge but at the start of injection some amount of slip occurs immediately in the overburden and underburden whereas during production the slip may remain inside the reservoir region next we derive similar but more complicated expressions for displaced inclined normal or reverse faults and conclude that our findings for vertical faults also apply to inclined faults

seismic rupture and ground accelerations induced by co2 injection in the shallow crust seismic rupture and ground accelerations induced by co 2 injection in the shallow crust f rederic cappa 1 2 and jonny rutqvist 1 lawrence berkeley national laboratory earth sciences division university of california berkeley usa geoazur university of nice sophia antipolis cote d azur observatory france e mail addresses cappa geoazur unice fr f cappa jrutqvist lbl gov j rutqvist july 23 2012

teleconnections in the geopotential height field during the northern hemisphere winter abstract contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth referred to as teleconnections in this paper are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the northern hemisphere winter with time scales on the order of a month or longer a review of existing literature on the subject reveals the existence of at least four such patterns the north atlantic and north pacific oscillations identified by walker and bliss 1932 a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes first noted by lorenz 1951 and what we will refer to as the pacific north american pattern which has been known to operational long range forecasters in this country since the 1950 s a data set consisting of nmc monthly mean sea level pressure and 500 mb height analyses for a 15 year period is used as a basis fo

scaling of seismicity induced by nonlinear fluid rock interaction 1 fluid injections into geothermal systems sometimes can produce significant seismic events with magnitudes of up to 4 however in the case of hydraulic fracturing of hydrocarbon reservoirs such events occur extremely seldom in the last case in contrast to the former one the structure of rocks is being actively destroyed e g opening of tensile fractures and the fluid rock interaction is strongly nonlinear e g a strong increase of permeability what is the role of this nonlinearity we consider nonlinear pore pressure diffusion to explain features of seismicity we formulate seismicity triggering front its propagation is sensitive to a grade of nonlinearity to spatial dimension of diffusion and to the injection rate we show that a probability of an event with a magnitude larger than a given one increases proportionally to the injected mass an increase of this probability with time is insensitive to the nonlinearity we compare different borehole experiments in some cases the injection produces clearly nonlinear impact on rocks in others this impact is approximately linear we find a well agreement with our theory we observe an insensitivity of temporal increments of magnitude probability to the grade of nonlinearity these increments are controlled by injection rate increments in contrast nonlinear fluid rock interactions are characterized by a strong dominance of small earthquakes defects activated by a nonlinear diffusion possibly obey gutenberg richter statistics with anomalous high b values in the case of a linear diffusion magnitude distributions of events are probably inherited from pre existing fracture systems

skilful predictions of the winter north atlantic oscillation one year ahead the north atlantic oscillation profoundly influences european and north american winter weather dynamical model predictions now exhibit skill in prediction of the winter north atlantic oscillation more than one year in advance

a nonstationary enso nao relationship due to amo modulation abstractmany previous studies have demonstrated a high uncertainty in the relationship between el nino southern oscillation enso and the north atlantic oscillation nao in the present work dec

chemical dilation and the dual porosity of humified bog peat abstract hydrochloric acid and calcium and sodium chloride salt solutions were passed through humified bog peat cores of 1 m length to determine values for the material properties governing solute transport hydraulic conductivity effective porosity and dispersivity chloride passed through the bulk peat cores about twice as fast as predicted from chloride breakthrough curves measured by selective ion probes inserted into the peat 30 cm downgradient from the source with continued experimentation chloride dispersion increased chloride also passed through the peat up to 12 times slower than predicted by conservative advection the retardation of the chloride fronts the faster bulk transport and the asymmetry of the breakthrough curves all indicate that humified bog peat is a dual porosity medium solutes both move preferentially through active macropore spaces and diffuse into dead pore space in the peat matrix neither the effective porosity nor the dispersivity of the peat could be precisely determined from the breakthrough curves because the hydraulic conductivity and discharge rates through the peat increased as much as five times when the chloride tracer solutions passed through the cores in initial experiments the shapes of the breakthrough curves were generally consistent with dispersivities of the order of centimeters similar to that observed for granular soils at the same length scale meters whereas they flattened considerably during later experiments suggesting dispersivities in the range of tens of centimeters we hypothesize that the interaction of the chloride tracer solutions with organic acid functional groups may cause macropores to dilate and micropores to close off

influence of atlantic meridional overturning circulation on the east asian winter monsoon east asian summer monsoon precipitation varied on millennial timescales during the last glacial period sediment records and climate modelling suggest that the winter monsoon was also affected by millenial scale variability and that the abrupt changes were driven by changes in the strength of the atlantic meridional overturning circulation

tropical atlantic sst forcing of coupled north atlantic seasonal responses recent observational studies reveal that a fall pan atlantic sea surface temperature sst anomaly composed of a horseshoe like dipole in the north atlantic and a southern center in the equatorial atlantic tends to precede the winter north atlantic oscillation nao and its related sst tripole by several months this study seeks to understand this relationship using large ensembles of atmospheric general circulation model agcm experiments and experiments with the agcm coupled to a mixed layer ocean agcm ml the models are forced either by the north atlantic horseshoe nah or by the tropical sst anomalies over the boreal winter months the agcm results show that the nah anomaly induces a baroclinic response in geopotential heights throughout the winter with little projection on the nao since the nah anomaly is ineffective in forcing the wintertime nao it cannot account for observations that the nah sst leads the nao in contrast in the agcm ml the coupled north atlantic response forced by the tropical anomaly exhibits a strong seasonal dependence in early winter the positive anomaly induces a trough east of newfoundland with a wave train to the northeast and in late winter the response projects strongly on a negative nao correspondingly the extratropical sst response features an nah like pattern in early winter and a tripole in late winter these results suggest that tropical atlantic sst anomalies can significantly influence the coupled extratropical variability the observed relationship between the fall nah sst and the winter nao or the sst tripole may be a consequence of persistent forcing of the seasonally varying atmosphere by tropical sst anomalies comparisons with the parallel agcm results indicate that the largely sign symmetric nao responses developed in the agcm ml are in part due to active extratropical sst feedbacks diagnostic experiments using a linear model further illustrate that in the absence of transient eddy feedbacks an idealized tropical heating induces anomalous flows that are qualitatively similar in early and late winter with a trough southeast of newfoundland and a ridge to the northeast the enhanced seasonality in the sst induced coupled response likely arises from the seasonal modulation of transient eddy feedbacks on the heatingforced anomalous flow

the influence of atlantic sea surface temperature anomalies on the north atlantic oscillation abstract the influence of atlantic sea surface temperature sst anomalies on the atmospheric circulation over the north atlantic sector during winter is investigated by performing experiments with an atmospheric general circulation model these consist of a 30 yr run with observed sst anomalies for the period 1961 90 confined geographically to the atlantic ocean and of a control run with climatological ssts prescribed globally a third 30 yr integration with observed ssts confined to the south atlantic is made to confirm present findings the simulated interannual variance of 500 hpa wintertime geopotential heights over the north atlantic attains much more realistic values when observed atlantic ssts are prescribed circulation patterns that resemble the positive phase of the north atlantic oscillation nao become more pronounced in terms of the leading eof of winter means and a cluster analysis of daily fields the variance of an interannual nao index increases by fivefold over its control value atl

dynamic winter climate response to large tropical volcanic eruptions since 1600 1 we have analyzed the mean climate response pattern following large tropical volcanic eruptions back to the beginning of the 17th century using a combination of proxy based reconstructions and modern instrumental records of cold season surface air temperature warm anomalies occur throughout northern eurasia while cool anomalies cover northern africa and the middle east extending all the way to china in north america the northern portion of the continent cools with the anomalies extending out over the labrador sea and southern greenland the analyses confirm that for two years following eruptions the anomalies strongly resemble the arctic oscillation northern annular mode ao nam or the north atlantic oscillation nao in the atlantic eurasian sector with our four century record the mean response is statistically significant at the 95 confidence level over much of the northern hemisphere land area however the standard deviation of the response is larger than the mean signal nearly everywhere indicating that the anomaly following a single eruption is unlikely to be representative of the mean both the mean response and the variability can be successfully reproduced in general circulation model simulations driven by the solar heating induced by the stratospheric aerosols these models produce enhanced westerlies from the lower stratosphere down to the surface the climate response to volcanic eruptions thus strongly suggests that stratospheric temperature and wind anomalies can affect surface climate by forcing a shift in the ao nam or nao

static stress drop associated with brittle slip events on exhumed faults 1 we estimate the static stress drop on small exhumed strike slip faults in the lake edison granodiorite of the central sierra nevada california the subvertical strike slip faults were exhumed from 4 to 15 km depth and were chosen because they are exposed in outcrop along their entire tip to tip lengths of 8 12 m slip nucleated on joints and accumulated by crystal plastic shearing forming quartz mylonites from early quartz vein filling in joints and successive brittle faulting forming epidote bearing cataclasites the occurrence of thin 300 m wide pseudotachylytes along some small faults throughout the study area suggests that some if not all of the brittle slip on the study area faults may have been seismic we suggest that the contribution of brittle cataclastic slip to the total slip along the studied cataclasite bearing small faults may be estimated by the length of epidote filled rhombohedral dilatational jogs rhombochasms distributed quasi periodically along the length of the faults the interpretation that slip recorded by rhombochasms occurred in single events is based on evidence that 1 epidote crystals are randomly oriented and undeformed within the rhombochasm 2 cataclasite in principal slip zones does not include clasts of previous cataclasite and 3 rhombochasm lengths vary systematically along the length of the faults with slip maximum occurring near the fault center tapering to the fault tips we thereby constrain both the rupture length and slip on the basis of these measurements we calculate stress drops ranging over 90 250 mpa i e one to two orders of magnitude larger than typical seismological estimates for earthquakes but similar in magnitude to seismological estimates of small m2 earthquakes from the san andreas fault observatory at depth safod the slip events described in the present study occurred along small deep seated faults and given the calculated stress drops and observations that brittle faults exploited joints sealed by quartz bearing mylonite we conclude that these were strong faults

hydraulic reversals and episodic methane emissions during drought cycles in mires reversals in hydraulic head and extensive methane losses were identified during a drought cycle from changes in hydraulic head and dissolved methane profiles in peat glacial lake agassiz peatlands northern minnesota united states during the summer 1990 drought regional groundwater discharged from the mineral soil underlying the peat to raised bogs concentrations of dissolved ch sub 4 in pore water showed that much of the peat column was supersaturated with respect to a reference standard of 1 atm partial pressure ch sub 4 by the summer of 1991 the severity of the drought lessened and the upper peat column was resaturated with precipitation derived water groundwater flow was then controlled by local precipitation driven recharge flow systems and the regional groundwater that was discharged affected only the peat immediately above the mineral soil peat interface in contrast during the summer of 1991 concentrations of dissolved ch sub 4 in pore water were all undersaturated with respect to 1 atm partial pressure ch sub 4 23 refs 5 figs

groundwater flow patterns in a large peatland abstract groundwater flow patterns and geochemistry were studied in the mer bleue bog near ottawa ontario canada groundwater flow patterns alternated between recharge i e head gradients producing flow from the surface of the peatland to the deeper peat and discharge i e head gradients indicating flow from the deeper peat towards the surface of the peatland during the summer of 1998 the patterns were controlled by changes in precipitation evapotranspiration and the differential head response of catotelm peat lower layer to changes in water table elevation above average rainfall in the spring created recharge patterns of groundwater flow in the peatland evapotranspiration exceeded precipitation for a three week period in mid summer decreasing head at the water table and reversing flow from recharge to discharge a sustained moisture deficit maintained a flow reversal for 32 days until a 46 mm rainfall raised the water table reversing the vertical hydraulic gradients and restoring recharge flow the mixing of meteoric water and deeper groundwater controls geochemical profiles diffusion modeling shows that the peatland is a long term recharge system however electrical conductivity and cation concentrations in peat pore waters increase up to 60 during a flow reversal and then decrease when recharge conditions are re established the redistribution of substrates from reversed flow is likely important to peatland biogeochemical function

surface temperature patterns associated with the southern oscillation abstract the typical global and large scale regional temperature patterns associated with the low warm and high cold phases of the southern oscillation so are investigated a total of 12 separate regions were found to have consistent temperature patterns associated with low phase of the so while 11 areas were found to have temperature patterns associated with the high phase of these areas 9 have expected temperature patterns during both phases of the so in the tropics temperature anomalies are of the same sign as the so related sea surface temperature sst anomaly in all land regions except for one area in the west pacific three extratropical responses to the low phase of the so are found over north america and one is found in japan high so temperature patterns were found in the extratropies for japan western europe and northwestern north america the identified temperature responses are more consistent in tropical regions than in the extratropies the so can influence the estimation of

free phase gas processes in a northern peatland inferred from autonomous field scale resistivity imaging the mechanisms that control free phase gas fpg dynamics within peatlands and therefore estimates of past present and future gas fluxes to the atmosphere remain unclear electrical resistivity imaging eri is capable of autonomously collecting three dimensional data on the centimeter to tens of meter scale and thus provides a unique opportunity to observe fpg dynamics in situ we collected 127 3 d eri data sets as well as water level soil temperature atmospheric pressure and limited methane flux data at a site in a northern peatland over the period july august 2013 to improve the understanding of mechanisms controlling gas releases at a hitherto uncaptured field scale our results show the ability of eri to image the spatial distribution of gas accumulation and infer dynamics of gas migration through the peat column at high i e hourly temporal resolution furthermore the method provides insights into the role of certain mechanisms previously associated with the triggering of fpg releases such as drops in atmospheric pressure during these events buoyancy driven gas release primarily occurs in shallow peat as proposed by the shallow peat model releases from the deeper peat are impeded by confining layers and we observed a large loss of fpg in deep peat that may likely represent a rupture event where accumulated fpg escaped the confining layer as suggested by the deep peat model negative linear correlations between water table elevation and resistivity result from hydrostatic pressure regulating bubble volume although these variations did not appear to trigger fpg transfer or release

dependence of slip weakening distance dc on final slip during dynamic rupture of earthquakes summary in this study we aim to understand the dependence of the critical slip weakening distance d c on the final slip d tot during the propagation of a dynamic rupture and the consistency of their inferred correlation to achieve this goal we have performed a series of numerical tests suitably designed to validate the adopted numerical procedure and to verify the actual capability in measuring d c we have retrieved two kinematic rupture histories from spontaneous dynamic rupture models governed by a slip weakening law in which a constant d c distribution on the fault plane as well as a constant d c d tot ratio are assumed respectively the slip velocity and the shear traction time histories represent the synthetic real target data which we aim to reproduce we use a 3 d traction at split nodes numerical procedure to image the dynamic traction evolution by assuming our modelled slip velocity as a boundary condition on the fault plane we assume a regularized yoffe function as source time function in our modelling attempts and we measure the critical slip weakening distance from the inferred traction versus slip curves at each point on the fault we compare the inferred values with those of the target dynamic models our numerical tests show that fitting the slip velocity functions of the target models at each point on the fault plane is not enough to retrieve good traction evolution curves and to obtain reliable measures of d c we find that the estimation of d c is very sensitive to any small variation of the slip velocity function an artificial correlation between d c d tot is obtained when a fixed shape of slip velocity is assumed on the fault i e constant rise time and constant time for positive acceleration which differs from that of the target model we point out that the estimation of fracture energy breakdown work on the fault is not affected by biases in measuring d c

the nonlinear association between enso and the euro atlantic winter sea level pressure a nonlinear projection of the tropical pacific sea surface temperature anomalies ssta onto the northern hemisphere winter sea level pressure slp anomalies by neural networks nn was performed to investigate the nonlinear association between el nino southern oscillation enso and the euro atlantic winter climate while the linear impact of enso on the euro atlantic winter slp is weak the nn projection reveals statistically significant slp anomalies over the euro atlantic sector during both extreme cold and warm enso episodes suggesting that the euro atlantic climate mainly responds to enso nonlinearly the nonlinear response mainly a quadratic response to the ssta reveals that regardless of the sign of the ssta positive slp anomalies are found over the north atlantic stretching from eastern canada to europe with anomaly center located just northwestward of portugal and negative anomalies centered over scandinavia and norwegian sea consistent with the excitation of the positive north atlantic oscillation pattern

contributions of downstream eddy development to the teleconnection between enso and the atmospheric circulation over the north atlantic abstractthe spatiotemporal evolution of various meteorological phenomena associated with el nino southern oscillation enso in the north pacific north american north atlantic sector is examined using both ncep ncar reanalyses and output from a 2000 yr integration of a global coupled climate model particular attention is devoted to the implications of downstream eddy developments on the relationship between enso and the atmospheric circulation over the north atlantic the el nino related persistent events are characterized by a strengthened pacific subtropical jet stream and an equatorward shifted storm track over the north pacific the wave packets that populate the storm tracks travel eastward through downstream development the barotropic forcing of the embedded synoptic scale eddies is conducive to the formation of a flow that resembles the negative phase of the north atlantic oscillation nao the more frequent and higher persistence of those episodes during el nino winters contribute to the prevale

north pacific north atlantic relationships under stratospheric control 1 based on a linear regression correlation analysis of monthly mean atmospheric sea level pressure slp data from the national centers for environmental prediction ncep reanalysis 1948 2000 we find a significant anticorrelation between pressure in the northern north atlantic and north pacific only if the stratospheric circulation is in the strong polar vortex regime but not when the vortex is weak since some general circulation models gcms e g european center hamburg echam4 are biased toward the strong vortex regime svr they tend to reproduce this anticorrelation already in the mean the pattern of the arctic oscillation ao is shown to be consistent with the mean surface pressure differences between the two stratospheric regimes the typical sw ne tilt of the node line of the north atlantic oscillation nao found with linear analyses i e with such that do not take the regime character of the stratospheric circulation into account in northern hemisphere winter is due to a superposition of correlation patterns based on physical processes working in the troposphere a strictly meridional dipole and the pattern resulting from planetary wave refraction in the strong vortex regime and those produced by the rapid transition from one stratospheric regime to the other with subsequent downward propagation of the signal this result underlines the necessity of the application of nonlinear statistics or the restriction of linear statistics to variations in the stable quasi linear environment of natural regimes

surface deformations as indicators of deep ebullition fluxes in a large northern peatland 1 peatlands deform elastically during precipitation cycles by small 3 cm oscillations in surface elevation in contrast we used a global positioning system network to measure larger oscillations that exceeded 20 cm over periods of 4 12 hours during two seasonal droughts at a bog and fen site in northern minnesota the second summer drought also triggered 19 depressuring cycles in an overpressured stratum under the bog site the synchronicity between the largest surface deformations and the depressuring cycles indicates that both phenomena are produced by the episodic release of large volumes of gas from deep semi elastic compartments confined by dense wood layers we calculate that the three largest surface deformations were associated with the release of 136 g ch4 m 2 which exceeds by an order of magnitude the annual average chamber fluxes measured at this site ebullition of gas from the deep peat may therefore be a large and previously unrecognized source of radiocarbon depleted methane emissions from northern peatlands

palaeo reversals in groundwater flow and peatland development at lost river minnesota usa raised bogs represent end products of peatland development that are generally assumed to be decoupled from the regional groundwater system however the development of two peat mounds in northern minnesota was driven by three major reversals in its groundwater flow regime a 4000 year record of groundwater flow was reconstructed by stratigraphic studies that utilized both botanical indicators of the surface water chemistry and mineral indicators of the pore water chemistry and chemical weathering regime the mineral stratigraphy indicated that the peat mounds were initially formed under a discharge upward flow regime during the succeeding moist period these peat mounds developed into raised bogs as their water table mounds drove local recharge downward flow into the mineral substratum a major reversal to a prevailing discharge regime at 1200 bp however converted the smaller peat mound into a spring fen and weathered the silicate minerals within the peat profile of the larger mound this reversal

variations in free phase gases in peat landforms determined by ground penetrating radar 1 the spatial variability of biogenic gas produced by methanogenic archaea is difficult to assess within saturated peat soils and is often poorly quantified this study uses ground penetrating radar to noninvasively estimate the vertical distribution of biogenic free phase gas fpg in two distinct peat landform types in the glacial lake agassiz peatland minnesota a near crest bog and a midslope lawn i e both as defined by surface vegetation communities ground penetrating radar velocities retrieved from common midpoint surveys were modeled using the complex refractive index model to estimate free phase gas volumes along one dimensional vertical profiles near crest bog landforms are characterized by vertical variability in gas content with accumulations along the peat column up to 24 by volume of fpg localized within the deep peat e g 2 4 m depth midslope lawn sites show lower total volumes up to 12 free phase gas and less variability that result in a more even free phase gas distribution throughout the vertical profile high resolution data also suggests the presence of thinner layers 1 m containing up to 40 free phase gas at a near crest site our results suggest that spatial distribution of free phase gas along the peat column may vary in the glacial lake agassiz peatlands depending on peat landform statistical analysis supports vertical variability of ground penetrating radar velocity data with a 95 confidence limit our results could potentially be upscaled using surface vegetation communities to estimate landscape scale gas storage potential in peat landforms

anisotropy and depth related heterogeneity of hydraulic conductivity in a bog peat ii modelling the effects on groundwater flow anisotropy and heterogeneity of hydraulic conductivity k are seldom considered in models of mire hydrology we investigated the effect of anisotropy and heterogeneity on groundwater flow in bog peat using a steady state groundwater model in five model simulations four sets of k data were used the first set comprised measured k values from an anisotropic and heterogeneous bog peat these data were aggregated to produce the following simplified data sets an isotropic and heterogeneous distribution of k an isotropic and homogeneous distribution and an anisotropic and homogeneous distribution we demonstrate that where anisotropy and heterogeneity exist groundwater flow in bog peat is complex fine scale variations in k have the potential to influence patterns and rates of groundwater flow however for our data at least it is heterogeneity and not anisotropy that has the greater influence on producing complex patterns of groundwater flow we also demonstrate that patterns and rates of groundwater flow are simplified and reduced when measured k values are aggregated to create a more uniform distribution of k for example when measured k values are aggregated to produce isotropy and homogeneity the rate of modelled seepage is reduced by 28 we also show that when measured k values are used the presence of a drainage ditch can increase seepage through a modelled cross section our work has implications for the accurate interpretation of hydraulic head data obtained from peat soils and also the understanding of the effect of drainage ditches on patterns and rates of groundwater flow

shallow groundwater surface water interactions in pond peatland complexes along a boreal plains topographic gradient shallow pond peatland complexes are important water resources and waterbird habitat on the boreal plains of canada and are potentially threatened by accelerated resource development we examined two shallow pond peatland complexes located in contrasting topographic positions within a clay rich till region one on a moraine and one on a lowland clay plain to evaluate the influence of landscape position on wetland groundwater surface water interactions hydrologic function and potential response to disturbance hydrometric and geochemical measurements indicated that precipitation and evaporation dominated the annual water balance of both ponds forested mineral uplands adjacent to the pond peatland complexes contributed no runoff inputs small recharge rates 5 mm year 1 from the ponds through low permeable clay to the underlying aquifer were measured at both topographic locations shallow groundwater fluxes to from the pond were controlled by water storage in and exchange with the adjacent riparian peatlands shallow groundwater fluxes may have contributed as much as 23 of inputs and outputs and influenced pond chemistry and permanence shallow groundwater exchange between the pond and adjacent peatlands varied seasonally and differed between the moraine and lowland complex groundwater flow reversals occurred around the entire perimeter of the moraine pond recharge from the moraine pond to the peatland during dry periods was off set by discharge to the pond from the peatland during wet periods in contrast shallow groundwater flow through conditions were observed at the lowland pond for most of the study however during a high rainfall event a groundwater flow reversal within the outflow peatland initiated discharge back into the pond these results suggest that the hydrologic regimes of some pond peatland complexes in clay rich till within the boreal plains will be more sensitive to local scale disturbances that impact flow dynamics and storage of near shore peatlands rather than larger scale disturbances of mineral upland regions

the impact of climate change tectonism and hydrology on the formation of carboniferous tropical intermontane mires the springhill coalfield cumberland basin nova scotia abstract both climate and tectonism are influential in determining basin and mire hydrology in the rapidly subsiding intermontane basins of euramerica which formed in response to the variscan hercynian alleghenian orogeny however the record of climate change has been muted and may therefore go unrecognized the complex interplay of climate and tectonism in a tropical continental setting is recorded in the 4 km thick westphalian a b fill of the cumberland basin thick peat accumulation was favoured at the southern basin margin where rheotrophic forested mires 4 9 km wide flourished between basin axis rivers and coalesced alluvial fans derived from the cobequid highlands as a result of their geomorphic setting the thicker coals 4 3 m exhibit distinct areal zones piedmont inner mire and riverine evidence that the forest swamps were nourished both by rainfall and by groundwater discharge from alluvial fans includes the stratigraphic affinity of thick coals and basin margin conglomerates interfingering of distal fan sheetflood and mire deposits petrographic evidence of elevated ph partial gelification of tissues and predominance of the arboreous lycopsids lepidodendron and paralycopodites whereas such groundwater supply is crucial to maintenance of modern wetlands in drier periods a subhumid climate is implied coals at joggins further removed from groundwater discharge at the basin margin are less well developed a 600 m thick coal window within the fining and reddening upward basin fill sequence at springhill is inferred to represent a coincidence of optimal groundwater supply and net subsidence the ultimate decline of peat accumulation in the basin may reflect decrease in catchment area and fan size and loss of rainshadow as the cobequid massif was denuded and the basin filled the coal window ca 6 9 10 5 yr is punctuated by non marine cyclothems of coal grey sideritic mudrock and multistorey sandstone that share similar time frames with astronomic cycles of axial precession ca 1 4 10 4 yr the stress of related climate change is inferred to have caused peat accumulation to fall into disequilibrium with sediment flux and basin subsidence the late quaternary history of certain african mires illustrates the dramatic impact of climate change especially temperature and seasonality and orographic effects on tropical continental peat accumulation aspects of the groundwater recharge model may be applicable to the westphalian stephanian intermontane mires of spain and mesozoic coal basins of northern china

evidence for a recent change in the link between the north atlantic oscillation and arctic sea ice export evidence for a recent change in the link between the north atlantic oscillation nao and arctic sea ice export through fram strait during wintertime djfm is presented from the analysis of simulated arctic sea ice and observations for the period 1958 1997 whereas no correlation between the two time series is found from 1958 to 1977 r 0 1 the correlation increased significantly thereafter r 0 7 1978 1997 the increased coherency between the nao and arctic sea ice export through fram strait during the last two decades resulted from anomalous meridional wind components near fram strait which were associated with the more easterly position of the nao s centers of interannual variability compared to 1958 1977

role and character of seasonal peat soil deformation on the hydrology of undisturbed and cutover peatlands 1 this paper describes the nature and magnitude of peat soil volume changes and its relation to seasonal changes in water table at an undisturbed bog peatland and on two cutover sections of the same peatland and its effect on hydraulic conductivity in the latter two sites operations had ceased 2 and 7 years prior to this study respectively the water table dropped to a maximum depth of 40 50 and 68 cm respectively at the undisturbed 2 year and 7 year abandoned sites and a resulted in subsidence of 0 96 3 84 and 2 65 cm m 1 respectively at the undisturbed site surface elevation changes did not always correspond to water table changes but peat underwent a period of swelling even as the water table fell probably due to the accumulation of methane in soil pores at all sites most volume change occurred in the upper 50 cm layer with maximum strain of 5 15 and 5 at the undisturbed 2 year and 7 year abandoned sites respectively and was strongly related to water table decline a model of peat deformation in the zone of saturation 100 cm depth based on changes in saturated soil moisture 6 grossly overestimated strain 1 in the saturated zone and again methane accumulation was the suspected cause of the soil moisture decrease peat compression and perhaps methane accumulation caused hydraulic conductivity to decrease over two orders of magnitude at 75 125 and 170 cm depth the decrease in hydraulic conductivity as a peatland dries may be an important self preservation mechanism i e against further water loss

tropical atlantic sea surface temperature variability and its relation to el nino southern oscillation past analyses of tropical atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics across the intertropical convergence zone itcz by analyzing an improved 43 year 1950 1992 record of sst smith et al 1996 and other data derived from the comprehensive ocean atmosphere data set coads it is shown that the regions north and south of the itcz are statistically independent of each other at the seasonal to interannual timescales dominating the data confirming the conclusions of houghton and tourre 1992 some dipole behavior does develop weakly during the boreal spring season when there is a tendency for sst anomaly west of angola to be opposite of that in the tropical north atlantic it is further shown that tropical atlantic sst variability is corre lated with pacific e1 nifio southern oscillation enso variability in several regions the ma jor region affected is the north atlantic area of ne trades west of 40ow along 10on 20on and extending into the caribbean there about 50 80 of the anomalous sst variability is associ ated with the pacific enso with atlantic warmings occurring 4 5 months after the mature phases of pacific warm events an analysis of local surface flux fields derived from coads data shows that the enso related atlantic warmings occur as a result of reductions in the surface ne trade wind speeds which in turn reduce latent and sensible heat losses over the region in ques tion as well as cooling due to entrainment this enso connection is best developed during the boreal spring following the most frequent season of maximum enso anomalies in the pacific a region of secondary covariability with enso occurs along the northern edge of the mean itcz position and appears to be associated with northward migrations of the itcz when the north at lantic warmings occur although easterly winds are intensified in the western equatorial atlantic in response to pacific warm events they do not produce strong local changes in sst contrary to expectations from studies based on equatorial dynamics these teleconnected wind anomalies do not give rise to significant correlations of sst in the gulf of guinea with the pacific enso as the teleconnection sequence matures strong se trades at low southern latitudes follow the de velopment of the north atlantic sst anomaly and precede by several months the appearance of weak negative sst anomalies off angola and stronger positive anomalies extending eastward from southern brazil along 15o 30os

off fault plasticity and earthquake rupture dynamics 1 dry materials or neglect of fluid pressure changes 1 we analyze inelastic off fault response during earthquakes spontaneous crack like rupture with slip weakening is modeled in 2 d plane strain using an explicit dynamic finite element procedure a mohr coulomb type elastic plastic description describes the material bordering the fault we identify the factors which control the extent and distribution of off fault plasticity during dynamic rupture those include the angle with the fault of the maximum compressive prestress the seismic s ratio and the closeness of the initial stress state to mohr coulomb failure plastic response can significantly alter the rupture propagation velocity delaying or even preventing a transition to supershear rupture in some cases plastic straining also alters the residual stress field left near the fault in part 1 we consider dry materials bordering the fault or at least neglect pore pressure changes within them part 2 addresses the effects of fluid saturation showing that analysis procedures of this part can describe undrained fluid saturated response elastic plastic laws of the type used are prone to shear localization resulting in an inherent grid dependence in some numerical solutions nevertheless we show that in the problems addressed the overall sizes of plastic regions and the dynamics of rupture propagation seem little different from what are obtained when we increase the assumed plastic hardening modulus or dilatancy parameter above the theoretical threshold for localization obtaining a locally smooth numerical solution at the grid scale evidence for scaling of some localization features with a real nongrid length scale in the model is also presented

impact of enso on the atmospheric variability over the north atlantic in late winter role of transient eddies abstractthe dynamical mechanism for the late winter teleconnection between el nino southern oscillation enso and the north atlantic oscillation nao is examined using the output from a 2000 yr integration of a coupled general circulation model gcm the coupled model captures many salient features of the observed behavior of both enso and nao as well as their impact on the surface climate in late winter both the observational and model data indicate more occurrences of negative phase of nao in late winter during el nino events and positive nao in la nina episodes the potential role of high frequency transient eddies in the above teleconnection is diagnosed during el nino winters the intensified transient disturbances along the equatorward shifted north pacific storm track extend their influences farther downstream the eddy induced negative height tendencies are found to be more coherent and stronger over north atlantic than that over north pacific these negative height tendencies over the north

assessing fault reactivation tendency within and surrounding porous reservoirs during fluid production or injection abstract this paper investigates fault reactivation tendency within and surrounding reservoirs during fluid injection or production induced stress analysis is performed using eshelby s theory of inclusions for a poroelastic material and the concept of coulomb failure stress change is implemented as a criterion for fault reactivation tendency a methodology is developed to find the range of fault dip angles that tend towards reactivation in either thrust or a normal fault stress regimes the results demonstrate that during production from a reservoir in a normal fault stress regime fault reactivation is likely to occur within the reservoir and adjacent to its flanks for a thrust fault stress regime only faults located in rocks overlying and underlying the reservoir tend towards reactivation the results for the analogous case of fluid injection are exactly the opposite sensitivity analyses show that the locations of the boundaries defining these regions of reactivation or stabilization tendency are not highly sensitive to the fault friction coefficient the use of the proposed methodology is illustrated for a synthetic case study

poroelastic stressing and induced seismicity near the lacq gas field southwestern france o hundreds of shallow small to moderate earthquakes have occurred near the lacq deep gas field in southwestern france since 1969 these earthquakes are clearly separated from tectonic seismicity occurring in the pyrenees 25 km to the southwest the induced seismicity began when the reservoir pressure had declined by 30 mpa repeated levehng over the field shows localized subsidence reaching a maximum of 60 mm in 1989 segall 1989 suggested that poroelastic stressing associated with volumetric contraction of the reservoir rocks is responsible for induced seismicity associated with fluid extraction to test this model we compare the observed subsidence and hypocentral distributions with the predicted displacement and stress fields we find that the relationship between average reservoir pressure drop and subsidence is remarkably linear lending support to the linear poroelastic model displacements and stresses are computed based on a priori knowledge of the reservoir geometry materim properties and reservoir pressure changes the computed vertical displacements are found to be in excellent agreement with the subsidence observed from leveling stress perturbations accompanying gas extraction computed using the same parameters are found to be 0 2 mpa or less changes in coulomb failure stress are computed assuming that shp occurs on optimally oriented planes the predicted failure zones correlate very well with the spatial distribution of earthquakes if the perturbing stresses are small in comparison to the ambient regional deviatoric stresses and if the minimum regional compresslye stress axis is vertical accurate determination of focal mechanisms of the induced events would allow a more rigorous test of the poroelastic model and could lead to important inferences about the crustal stress state

extreme threshold failures within a heterogeneous elastic thin sheet and the spatial temporal development of induced seismicity within the groningen gas field measurements of the strains and earthquakes induced by fluid extraction from a subsurface reservoir reveal a transient exponential like increase in seismicity relative to the volume of fluids extracted if the frictional strength of these re activating faults is heterogeneously and randomly distributed then progressive failures of the weakest fault patches account in a general manner for this initial exponential like trend allowing for the observable elastic and geometric heterogeneity of the reservoir the spatio temporal evolution of induced seismicity over 5 years is predictable without significant bias using a statistical physics model of poro elastic reservoir deformations inducing extreme threshold frictional failures of previously inactive faults this model is used to forecast the temporal and spatial probability density of earthquakes within the groningen natural gas reservoir conditional on future gas production plans probabilistic seismic hazard and risk assessments based on these forecasts inform the current gas production policy and building strengthening plans

induced stresses due to fluid extraction from axisymmetric reservoirs earthquakes can be induced by fluid extraction as well as by fluid injection segall 1989 proposed that poroelastic stresses are responsible for inducing earthquakes associated with fluid extraction here i present methods for computing poroelastic stress changes due to fluid extraction for general axisymmetric reservoir geometries the results ofgeertsma 1973 for a thin disk reservoir with uniform pressure drop are recovered as a special case predicted surface subsidence agrees very well with measured leveling changes over the deep lacq gas field in southwestern france the induced stresses are finite if the reservoir pressure changes are continuous computed stress changes are on the order of several bars suggesting that the preexisting stress states in regions of extraction induced seismicity are very close to frictional instability prior to production

geochemical and petrographical characteristics of a domed bog nova scotia a modern analogue for temperate coal deposits a freshwater domed bog in nova scotia canada was investigated for its petrographic mineralogical and geochemical characteristics three cores were taken from the lagg zone margin and centre to examine different stages of evolution in a fen to domed bog sequence based on this study a model was developed which shows the relationship between factors controlling peat geochemistry and petrography all of them being dependent on trophic conditions the bog may represent a modern analogue for temperate coal deposits and thus can be used to help understand spatial variability within coal beds rheotrophic conditions in the early stages of mire development result in the formation of high ash highly humified carex dominated peat while the evolution toward ombrotrophic conditions is reflected in a progressive replacement by low ash and well preserved sphagnum dominated peat the associated decrease in ph reflects a decrease of groundwater influence which is also supported by the mineralogical assemblage and petrographic data decrease in reflectance and change in maceral composition except for cl br i and se which are associated with the organic fraction variations in elemental composition can be explained in terms of mineral matter abundance i e decrease upward and with increasing distance from the margin sea spray is the most probable origin for halogen enrichment in the peat

impact of the atlantic multidecadal oscillation on the asian summer monsoon 1 the impact of the atlantic multidecadal oscillation amo on the asian summer monsoon is investigated using a coupled atmosphere ocean global general circulation model by imposing the amo associated sea surface temperature anomalies in the atlantic as boundary forcing and allowing atmosphere ocean interactions outside the atlantic the positive amo phase characterized by anomalous warm north atlantic and cold south atlantic leads to strong southeast and east asian summer monsoons and late withdrawal of the indian summer monsoon these changes of monsoons are mainly through coupled atmosphere ocean feedbacks in the western pacific and indian oceans and tropospheric temperature changes over eurasia in response to the imposed forcing in the atlantic the results are in agreement with the observed climate changes in china corresponded to the amo phases they suggest a non local mechanism for the asian summer monsoon variability and provide an alternative view to understanding its interdecadal variation during the twentieth century citation lu r b dong and h ding 2006 impact of the atlantic multidecadal oscillation on the asian summer monsoon geophys res lett 33 l24701 doi 10 1029 2006gl027655

simulating vertical flow in large peatlands ground water flow simulations are used to evaluate the importance of three parameters on vertical flow in peatlands regional slope permeability of the mineral soil underlying the peat and peatland topography our results indicate that the extent of vertical ground water flow in peatlands is primarily controlled by mineral soil permeability local ground water flow cells that form under small water table mounds at bog domes can drive peat pore water into permeable mineral soil however when peat forms over low permeability mineral soil vertical movement of peat pore water becomes negligible and lateral flow of water in the upper portion of the peat column dominates the peatland hydrology this effect is not due to the low permeability of humified peat layers as is commonly assumed in many peatland studies field data from the hudson bay lowland canada and the glacial lake agassiz peatlands of northern minnesota confirm the validity of these models these results are relevant in settings other than peatlands where relief is low and small topographic mounds exist

on the physics based processes behind production induced seismicity in natural gas fields author s zbinden d rinaldi ap urpi l wiemer s abstract 2017 american geophysical union all rights reserved induced seismicity due to natural gas production is observed at different sites worldwide common understanding states that the pressure drop caused by gas production leads to compaction which affects the stress field in the reservoir and the surrounding rock formations and hence reactivates preexisting faults and induces earthquakes in this study we show that the multiphase fluid flow involved in natural gas extraction activities should be included we use a fully coupled fluid flow and geomechanics simulator which accounts for stress dependent permeability and linear poroelasticity to better determine the conditions leading to fault reactivation in our model setup gas is produced from a porous reservoir divided into two compartments that are offset by a normal fault results show that fluid flow plays a major role in pore pressure and stress evolution within the fault fault strength is significantly reduced due to fluid flow into the fault zone from the neighboring reservoir compartment and other formations we also analyze scenarios for minimizing seismicity after a period of production such as i well shut in and ii gas reinjection in the case of well shut in a highly stressed fault zone can still be reactivated several decades after production has ceased although on average the shut in results in a reduction in seismicity in the case of gas reinjection fault reactivation can be avoided if gas is injected directly into the compartment under depletion however gas reinjection into a neighboring compartment does not stop the fault from being reactivated

continental rifting and initial sea floor spreading in the woodlark basin our understanding of the processes by which continents rift and sea floor spreading initiates is derived primarily from studies either of old passive margins and oceanic crust or of young regions of intra continental extension where spreading has not yet started it has been thought that continental rifting ceases when sea floor spreading begins1 2 that oceanic fracture zones develop from transfer or transform faults within continental rifts3 4 and that linear magnetic anomalies correlate with the onset of sea floor spreading during times of magnetic reversals5 6 here we present a marine geophysical survey of one of the few active examples of continental rifting and spreading initiation the western woodlark basin papuan peninsula region of new guinea which shows that in detail these assumptions do not hold the data confirm models of the rifting to spreading transition that invoke both ridge propagation and nucleation of discrete spreading cells7 10 and provide an unambiguous example of a spreading centre reorienting by synchronous jumping rather than propagation

stable isotopes of water show deep seasonal recharge in northern bogs and fens ground water recharge is assumed to occur primarily at raised bog crests in northern peatlands which are globally significant terrestrial carbon reservoirs we synoptically surveyed vertical profiles of peat pore water 18o and 2h from a range of bog and fen landforms across the glacial lake agassiz peatlands northern minnesota contrary to our expectations we find that local scale recharge penetrates to not only the basal peat at topographically high bog crests but also transitional sphagnum lawns and low lying fen water tracks surface landscape characteristics appear to control the isotopic composition of the deeper pore waters depths 0 5 m which are partitioned into discrete ranges of 18o on the basis of landform type mean standard deviation for bog crests 11 9 0 4 lawns 10 6 0 1 fen water tracks 8 8 1 0 fen water tracks have a shallow free water surface that is seasonally enriched by isotope fractionating evaporation fingerprinting recharge to underlying pore waters at depths 3 m isotope mass balance calculations indicate on average 12 of the waters we sampled from the basal peat of the fen water tracks was lost to surface evaporation which occurred prior to advection and dispersion into the underlying formation these new data provide direct support for the hypothesis that methane production in deeper peat strata is fuelled by the downward transport of labile carbon substrates from the surface of northern peat basins copyright 2013 john wiley sons ltd

nucleation and arrest of dynamic rupture induced by reservoir depletion seismic events induced by the depletion of hydrocarbon reservoirs can cause damage to housing and cause societal and economic unrest however the factors controlling the nucleation and size of production induced seismic events are not well understood here we used geomechanical modeling of production induced stresses and dynamic rupture modeling to assess the conditions controlling down dip rupture size a generic model of offset depleting reservoir compartments separated by a fault was modeled in 2 d using the finite element package diana fea linear slip weakening was used to control fault friction behavior fault reactivation was computed in a quasi static analysis simulating stresses during reservoir depletion followed by a fully dynamic analysis simulating seismic rupture the sensitivity of reactivation and rupture size to in situ stress dynamic friction critical slip distance and reservoir offset was evaluated after reactivation a critical fault length was required to slip before seismic instability could occur in a subsequent fully dynamic analysis the propagation and arrest of dynamic rupture was simulated rupture remained mostly confined to the reservoir interval but could also propagate into the overburden and underburden or sometimes transition into a run away rupture propagation outside the reservoir interval was promoted by a critical in situ stress a large stress drop a small fracture energy and no or little reservoir offset with increasing offset up to the reservoir thickness reactivation was promoted but dynamic rupture size decreased 2019 the authors

atlantic climate variability and its associated atmospheric circulation cells abstract phenomena important for atlantic climate variability include the atlantic zonal equatorial mode the tropical atlantic meridional gradient mode and the north atlantic oscillation nao these climate phenomena and their associated atmospheric circulation cells are described and discussed using the ncep ncar reanalysis field and the ncep sea surface temperature sst from january 1950 to december 1999 atmospheric divergent wind and vertical motion are used for the identification of atmospheric circulation cells during the peak phase of the atlantic equatorial mode the atlantic walker circulation weakens and extends eastward which results in surface westerly wind anomalies in the equatorial western atlantic these westerly wind anomalies are partly responsible for warming in the equatorial eastern atlantic that occurs in the second half of the year the atlantic equatorial mode involves a positive ocean atmosphere feedback associated with the atlantic walker circulation similar to the pacific

peat porewaters have contrasting geochemical fingerprints for groundwater recharge and discharge due to matrix diffusion in a large northern bog fen complex abstract although northern peatlands represent a globally significant reservoir for carbon considerable uncertainty exists concerning solute transport systems within large 1000 km 2 peat deposits we therefore delineated geochemical gradients linked to groundwater recharge and discharge along a 6 km transect within the 1200 km 2 red lake peatland of northwestern minnesota we used ratios of ca mg and 87 sr 86 sr to distinguish discharge of calcareous groundwater 1 4 and 0 7155 respectively to the peatland from the mineral substratum along a topographic gradient from a bog crest downslope to an internal fen water track and bog islands in contrast the stable isotopes of the porewaters 18 o from 12 8 to 7 8 show that the active pore spaces in these peat profiles have been flushed by recharge from the near surface peat we hypothesize that back diffusion of groundwater derived solutes from the peat matrix to active pore spaces has allowed the geochemical signal from paleo hydrogeologic discharge to persist into the current regime of dilute recharge this effect has not been observed previously on the landform scale and has important implications for carbon cycling in peatlands

coupled multiphase flow and poromechanics a computational model of pore pressure effects on fault slip and earthquake triggering the coupling between subsurface flow and geomechanical deformation is critical in the assessment of the environmental impacts of groundwater use underground liquid waste disposal geologic storage of carbon dioxide and exploitation of shale gas reserves in particular seismicity induced by fluid injection and withdrawal has emerged as a central element of the scientific discussion around subsurface technologies that tap into water and energy resources here we present a new computational approach to model coupled multiphase flow and geomechanics of faulted reservoirs we represent faults as surfaces embedded in a three dimensional medium by using zero thickness interface elements to accurately model fault slip under dynamically evolving fluid pressure and fault strength we incorporate the effect of fluid pressures from multiphase flow in the mechanical stability of faults and employ a rigorous formulation of nonlinear multiphase geomechanics that is capable of handling strong capillary effects we develop a numerical simulation tool by coupling a multiphase flow simulator with a mechanics simulator using the unconditionally stable fixed stress scheme for the sequential solution of two way coupling between flow and geomechanics we validate our modeling approach using several synthetic but realistic test cases that illustrate the onset and evolution of earthquakes from fluid injection and withdrawal

subsidence above a planar reservoir 1 a tabular reservoir circular in plan is located at depth parallel to the surface compaction of the reservoir as fluid is withdrawn causes a change in the stress and displacement fields in the surrounding medium a solution to this problem is obtained by determining the boundary conditions at the interface between the reservoir and the half space using eshelby s 1957 cut and weld technique the displacement field in the reservoir and the surrounding medium is then calculated using the reciprocal theorem the displacement field is found to be the same as that calculated by geertsma 1973a 1973b and by segall 1992 segall et al 1994 using another technique

geophysical and hydrological evaluation of two bog complexes in a northern peatland implications for the distribution of biogenic gases at the basin scale 1 ground penetrating radar gpr was used to determine peat basin geometry and the spatial distribution of free phase biogenic gasses in two separate units of a northern peatland central and southern unit of caribou bog maine the central unit is characterized by a deep basin structure 15 m maximum depth and a raised eccentric bog topographic profile up to 2 m topographic variation here numerous regions of electromagnetic em wave scattering are considered diagnostic of the presence of extensive free phase biogenic gas in contrast the southern unit is shallower 8 m maximum depth and has a slightly convex upwards bog profile less than 1 m topographic variation and areas of em wave scattering are notably absent the biogenic gas zones interpreted from gpr in the central unit are associated with 1 wooded heath vegetation at the surface 2 open pools at the surface 3 high water table elevations near the center of the basin and 4 a region of overpressure at approximately 5 m depth immediately below the zone of free phase gas accumulation the latter suggests 1 a transient pressure head associated with low hydraulic conductivity resulting from the biogenic gasses themselves or confining layers in the peat that restrict both gas release and groundwater flow and or 2 overpressure in the peat column as a result of the gas buildup itself in contrast the southern unit where zones of em scattering are absent is characterized by 1 predominantly shrub vegetation 2 a lack of open pools 3 only minor variations less than 1 m in water table elevation throughout the entire unit and 4 generally upward groundwater flow throughout the basin the results illustrate the nonuniformity of free phase biogenic gas distribution at the peat basin scale and provide insights into the processes and controls associated with ch4 and co2 accumulation in peatlands

dynamics of the extratropical response to a tropical atlantic sst anomaly abstract previous atmospheric general circulation model agcm experiments revealed that atmospheric responses to a tropical atlantic sea surface temperature anomaly ssta were asymmetric with respect to the sign of the ssta a positive ssta produced a south north dipole in geopotential heights much like the north atlantic oscillation nao while a negative ssta yielded an eastward propagating wave train with the northern lobe of the nao absent here these height responses are decomposed into components that are symmetric or antisymmetric with respect to the sign of the ssta the symmetric or notionally linear component is a nearly south north dipole projecting on the nao while the antisymmetric or notionally nonlinear component is a different dipole experiments with a diagnostic linear baroclinic model lbm suggest that both components are maintained primarily by transient eddy forcing dynamical mechanisms for the formation of the two components are explored using the lbm and a nonlinear baro

87sr 86sr as a tracer of groundwater discharge and precipitation recharge in the glacial lake agassiz peatlands northern minnesota a contrast in the 87sr 86sr ratio of atmospherically derived sr 0 710 and sr from regional bedrock 0 730 allows quantification of the proportions of precipitation recharge and groundwater discharge into peat columns of bogs and fens in the glacial lake agassiz peatlands two contrasting bog fen pairs were studied the lost river peatland located over a regional groundwater discharge zone and the red lake peatland situated atop a hydrologic divide where recharge is expected to occur bog pore waters consist of 90 100 precipitation in the upper 2 m below which they mix with groundwater adjacent fens consist mostly of groundwater ranging from 30 near the surface to 100 with depth at the red lake peatland mixing relations suggest an additional sr source that may be peat mineralization distinct 87sr 86sr and ca sr ratios for groundwater discharging into bog and fen sites are indicative of distinct groundwater sources and are likely indicative of flow paths at two different length scales

role of drainage conditions in deformation and fracture of porous rocks under triaxial compression in the laboratory 1 in order to investigate the role of drainage conditions in deformation and fracture behaviors of porous rocks the authors carried out a series of rock fracture tests under triaxial compression in the laboratory the detailed space time distribution of acoustic emission due to microcracking was used to examine pre failure damage and failure behavior in berea sandstone which has a porosity of 20 and a permeability of 100 md the pore pressures or flow rates at the ends of the test sample were precisely controlled to simulate different drainage conditions experimental results indicate that drainage conditions play a governing role in deformation and fracture the well established dilatancy hardening effect can be greatly suppressed by dilatancy driven fluid flowing under good drainage conditions fast diffusion of pore pressure leads to a significant reduction in rock strength and stabilization of the dynamic rupture process furthermore good drainage conditions have the potential to enlarge the nucleation dimension and duration thereby improving the predictability of the final catastrophic failure in addition compaction bands which were observed in porous rocks under higher confining pressure were also observed at low confining pressure corresponding to a depth of 1 km in undrained tests these results are particularly important for research fields in which fluid migration or pore pressure diffusion is expected to play a role such as hydrocarbon reservoirs enhanced geothermal systems geological storage of co2