constraints on the formation of the martian crustal dichotomy from remnant crustal magnetism the martian crustal dichotomy characterizing the topographic difference between the northern and southern hemispheres is one of the most important features on mars however the formation mechanism for the dichotomy remains controversial with two competing proposals exogenic e g a giant impact and endogenic e g degree 1 mantle convection mechanisms another important observation is the martian crustal remnant magnetism which shows a much stronger field in the southern hemisphere than in the northern hemisphere and also magnetic lineations in this study we examine how exogenic and endogenic mechanisms for the crustal dichotomy are constrained by the crustal remnant magnetism assuming that the dichotomy is caused by a giant impact in the northern hemisphere we estimate that the average thickness of ejecta in the southern hemisphere is 20 25 km while such a giant impact may cause crustal demagnetization in the northern hemisphere we suggest that the impact could also demagnetize the southern hemisphere via ejecta thermal blanketing impact demagnetization and heat transfer from the hot layer of ejecta thus posing a challenge for the giant impact model we explore how the pattern of magnetic lineations relates to endogenic theories of dichotomy formation specifically crustal production via degree 1 mantle convection we observe that the pattern of lineations roughly corresponds to concentric circles about a single pole and determine the pole for the concentric circles at 76 5 e and 84 5 s which nearly overlaps with the centroid of the thickened crust in the southern hemisphere we suggest that the crustal magnetization pattern magnetic lineations and crustal dichotomy i e thickened crust in the highlands can be explained by a simple endogenic process one plume convection causes melting and crustal production above the plume in the southern hemisphere and strong crustal magnetization and magnetic lineations are formed in the southern hemisphere as crustal production fronts spread radially out from the plume center and as the newly created crust cools in the presence of a dynamo with polarity reversals

fractal structure of spatial distribution of microfracturing in rock summary a constant stress fracture experiment of oshima granite was carried out at the confining pressure of 40mpa hypocentres of 2064 acoustic emissions were located during the experiment using the correlation integral we found that the spatial distribution of hypocentres of acoustic emission is a fractal and that the fractal dimension decreases with the evolution of rock fracturing the spatial distribution of earthquake s hypocentres reveals fractals ranging from regional to worldwide distribution if we extrapolate from laboratory measurements it is possible to predict the occurrence of large earthquakes by the decrease in the fractal dimension

rock magnetic and remanence properties of synthetic fe rich basalts implications for mars crustal anomalies abstract we characterized the magnetic mineral assemblage and remanence properties of a set of synthetic samples patterned on the meteorite derived basalt composition a which contains 18 9 total feo basalts were synthesized at conditions that track 4 oxygen fugacity f o 2 buffer curves from 3 4 log units below the quartz fayalite magnetite qfm buffer to 5 log units above qfm and 6 cooling rates from 10 5 to 3 c h the resulting array of samples was characterized using magnetic hysteresis loops temperature dependence of saturation magnetization and saturation remanence 10 to 1000 k and the acquisition and demagnetization of anhysteretic remanent magnetization arm and thermoremanent magnetization trm the magnetic mineral assemblage characteristics are strongly dependent on f o 2 samples synthesized at the iron wustite iw buffer have a very low concentration of remanence carrying grains which are likely near the superparamagnetic stable single domain boundary samples synthesized at the qfm and nickel nickel oxide nno buffers contain a slightly higher concentration of remanence carrying grains which are stable single domain to fine pseudo single domain particles respectively samples synthesized at the manganese oxide mno buffer contain the highest concentration of magnetic grains which are up to 100 m in diameter the dominant fe ti oxide produced is an mg and al bearing titanomagnetite with 2 4 2 7 fe cations per formula unit the curie temperatures of the qfm samples are consistent with their electron microprobe derived compositions those of the nno sample set are very slightly elevated with respect to their electron microprobe derived compositions the curie temperatures of the mno samples are elevated up to 200 c above what they should be for their composition we attribute the curie temperature elevation to high temperature nonstoichiometry of the titanomagnetite the iw sample set acquired very weak trms with intensities of 0 02 to 0 5 a m this intensity of remanence is a factor of 50 500 too low to generate the observed 1000 nt anomalies detected on mars by the mars global surveyor mag er experiment the qfm nno and mno samples acquired trms up to 40 a m in a 10 t applied field and up to 200 a m in a 50 t field with little or no dependence on cooling rate our results suggest that fe rich melts that crystallize extensive titanomagnetite can generate an intensely magnetized layer in the martian crust even if the remanence was acquired in a weak field the qfm sample set can easily account for the observed 1000 nt mars magnetic anomalies even in a magnetized layer as thin as 15 30 km

importance of small earthquakes for stress transfers and earthquake triggering 1 we estimate the relative importance of small and large earthquakes for static stress changes and for earthquake triggering assuming that earthquakes are triggered by static stress changes and that earthquakes are located on a fractal network of dimension d this model predicts that both the number of events triggered by an earthquake of magnitude m and the stress change induced by this earthquake at the location of other earthquakes increase with m as similar to 10 dm 2 the stronger the spatial clustering the larger the influence of small earthquakes on stress changes at the location of a future event as well as earthquake triggering if earthquake magnitudes follow the gutenberg richter law with b d 2 small earthquakes collectively dominate stress transfer and earthquake triggering because their greater frequency overcomes their smaller individual triggering potential using a southern california catalog we observe that the rate of seismicity triggered by an earthquake of magnitude m increases with m as 10 alpha m where alpha 1 05 0 05 we also find that the magnitude distribution of triggered earthquakes is independent of the triggering earthquake s magnitude m when alpha approximate to b small earthquakes are roughly as important to earthquake triggering as larger ones we evaluate the fractal correlation dimension d of hypocenters using two relocated catalogs for southern california the value of d measured for distances 0 1 r 5 km is d 1 54 for the shearer et al catalog and d 1 73 for the hauksson et al catalog the value of d reflects both the structure of the fault network and the nature of earthquake interactions by considering only those earthquake pairs with interevent times larger than 1000 days we can largely remove the effects of short term clustering then d approximate to 2 close to the value d 2 alpha 2 1 predicted by assuming that earthquake triggering is due to static stress the value d approximate to 2b implies that small earthquakes are as important as larger ones for stress transfers between earthquakes and that considering stress changes induced by small earthquakes should improve models of earthquake interactions

the evolution of seismicity at parkfield observation experiment and a fracture mechanical interpretation we compare observations of local seismicity sequences with acoustic emissions during rock fracture and find similarities in the distribution of event magnitudes which lead us to consider a model of seismicity which is characterized by fracture growth laws derived from the field of fracture mechanics and which include both negative and positive feedback processes we find that the model predicts the observed negative correlation between the seismic b value and the fractal dimension of the earthquake epicentres the simple short range interactions described in the model give rise to a system with a long range order similar to current models of critical processes

comparison of the microfracture localization in granite between fracturation and slip of a preexisting macroscopic healed joint by acoustic emission measurements experiments of fracturation and slip of a preexisting macroscopic healed joint have been performed under triaxial deformation on granite from mayet de montagne france this granite shows high grain scale inhomogeneity acoustic emissions have been recorded and hypocenters have been determined during the entire experiments for both rupture experiment and slip experiment precursory localization of microfractures in the final rupture plane has been observed in the early stage of deformation well before the dilatancy it is likely that not only initial closure of favorably oriented cracks but also breaking of partially cemented grains or slipping between grains may occur in the pseudoelastic phase and are already localized on the final rupture plane where the shear stress seems to be concentrate this behavior is observed in both cases where stress heterogeneity and rupture nucleation are controlled by 1 medium scale heterogeneity at the grain scale hs sample or 2 macroscopic heterogeneity in the form of a preexisting healed joint js sample the sample with the healed joint exhibited 1 6 times more acoustic emission events than the intact sample the presence of the healed joint significantly weakened the sample

nontrivial clustering of microseismicity induced by hydraulic fracturing for induced microseismicity associated with hydraulic fracturing the frequency magnitude distribution is typically characterized by a falloff with increasing magnitude that is significantly faster than for seismicity along active fault systems this characteristic may arise from a break in scale invariance possibly due to mechanical layering that typifies fine grained sedimentary rocks in many shale gas and tight oil reservoirs the latter would imply the presence of spatio temporal magnitude correlations using three microseismic catalogs for well stimulations in widely separated locations with varying hydraulic fracturing methods we show that events with similar magnitudes indeed tend to cluster in space and time in addition we show that the inter event time distribution can be described by a universal functional form characterized by two power laws one exponent can be related to the presence of inter event triggering as in aftershock sequences and is a consequence of the omori utsu law the other one is a reflection of the intrinsic spatial variation in the microseismic response rates taken together these features are indicative of non trivial spatio temporal clustering of induced microseismicity and hence of direct relevance for time dependent seismic hazard assessment

age and origin of the lowlands of mars one of the many significant findings of the mars global surveyor mission is the presence of hundreds of quasi circular depressions qcds observed from high resolution mola topography data their presence has recently been interpreted to reflect a northern lowlands that archive some of the earliest recorded rocks on mars mostly below a veneer of hesperian and amazonian materials here we analyze these data coupled with a recent synthesis of geologic geophysical geomorphic topographic and magnetic information such analysis allows us to suggest a potential plate tectonic phase during the recorded early into middle noachian martian history that transitioned into a monoplate world with episodic magmatic driven activity persisting to present this working hypothesis is based on 1 the observation that the basement of the northern plains is younger than the basement of the southern highlands but older than the material exposures of the cratered highlands suggesting different formational ages for each one of the three geologic time units 2 the observation that parts of the very ancient highland s crust are highly magnetized thus suggesting that most if not all of the formation of the lowlands basement postdates the shut off of the martian dynamo some 4 gyr ago and so allowing hundreds of millions of years for the shaping of the buried lowlands consequently the role of endogenic processes in the earliest geological evolution of mars early perhaps into middle noachian requires reconsideration since mola topographic and mgs magnetic data afford a temporal window sufficient for very early non primordial shaping of the northern lowlands basement

computer modelling of granular material microfracturing abstract microscopic observations indicate that intra and transgranular fracturing are ubiquitous processes in the damage of rock fabrics extensive modelling of intergranular fracturing has been carried out previously using the distinct element approach the current work is aimed at extending these results to include intra and transgranular fracturing numerical experiments have been carried out to simulate these microfractures in granular media using a boundary element computer code digs grains were represented by straight sided polygons generated with a voronoi generator experiments were carried out to simulate experimental microfracture studies of quartzite in triaxial extension tests the results support the experimental observations that the microcracks induced by compressive stress are extensile and sub parallel to the direction of maximum compressive stress various mechanisms of microcrack initiation were identified some cracks were found to be generated from inside the grains in a manner similar to a brazilian test sliding cracks were found to start from grain boundaries as intergranular cracks and propagate as intragranular wing cracks in the direction of maximum compression pores were also modelled as a possible mechanism for microcrack initiation but were found to generate fractures in unexpected directions relative to the direction of applied loading

interpretation of resonance frequencies recorded during hydraulic fracturing treatments hydraulic fracturing treatments are often monitored by strings of geophones deployed in boreholes instead of picking discrete events only we here use time frequency representations of continuous recordings to identify resonances in two case studies this paper outlines an interpretational procedure to identify their cause using a subdivision into source path and receiver side effects for the first case study two main resonances are observed both at depth by the downhole geophones and on the surface by two broadband arrays the two acquisition networks have different receiver and path effects yet recorded the same resonances these resonances are therefore likely generated by source effects the amplitude pattern at the surface arrays indicates that these resonances are probably due to pumping operations in the second case study selective resonances are detected by the downhole geophones resonances coming from receiver effects are either lower or higher frequency and wave propagation modeling shows that path effects are not significant we identify two possible causes within the source area namely eigenvibrations of fractures or non darcian flow within the hydraulic fractures in the first situation 15 10 m long fluid filled cracks could generate the observed resonances an interconnected fracture network would then be required corresponding to mesoscale deformation of the reservoir alternatively systematic patterns in non darcian fluid flow within the hydraulic fracture could also be their leading cause resonances can be used to gain a better understanding of reservoir deformations or dynamic fluid flow perturbations during fluid injection into hydrocarbon and geothermal reservoirs co2 sequestration or volcanic eruptions

assessing the geologic evolution of greater thaumasia mars the greater thaumasia region consists of three chemical provinces that include syria solis and thaumasia planae the corprates rise part of the thaumasia highlands and the transition zone northwest of the argyre basin chemical signatures obtained from the mars odyssey gamma ray spectrometer suggest low abundances of k and th to the west with low h abundances and high si abundances to the east relative to the bulk martian crust at mid latitudes these observations are confirmed and quantified with a modified box and whisker analysis that simultaneously captures the degree of deviation and significance of the regionally anomalous chemistry motivated by regionally unique chemistry as well as its diverse geological history we characterize greater thaumasia in terms of chemistry mineralogy and mapped geology to determine how such complementary data record the evolution of this region our observations are inconsistent with a proposed salt lubricated landslide origin particularly given the lack of chemical or mineralogical signatures to support near surface salt deposits that should arise over geological time scales our observations instead support magmatic processes such as mantle evolution over geological time which may impart the si enriched signature of the eastern portion of greater thaumasia as well as the k and th depletion of the southeastern flank of syria planum while the observed trend of decreasing k and th from noachian to hesperian lavas is inconsistent with previous models of martian mantle evolution we see an increase in ca content at the noachian hesperian boundary consistent with predictions from thermodynamic modeling

geomechanical modeling of rock fracturing and associated microseismicity microseismic monitoring is increasingly being used to assess in real time the effectiveness of hydraulic fracture treatments operators are interested in three key questions 1 where are the microseismic events occurring what is the size of the microseismic cloud 2 what is the failure mechanism are fractures opening closing or shearing 3 why is failure occurring in specific locations but not others why are fractures not always symmetric with respect to the injection well and what is the geomechanical behavior of the reservoir in particular the last question is difficult to answer from the recorded seismicity alone because the geomechanical behavior depends on the in situ stress field the local rock properties lithologies and any existing areas of weakness including faults fractures and joints grob and van der baan 2011 geomechanical modeling can thus play a key role in better understanding both brittle and ductile deformation inside a reservoir because of hydraulic fracturing an

growth of the hemispheric dichotomy and the cessation of plate tectonics on mars 1 although mars is currently not tectonically active it may have experienced plate tectonics early in its history the southern hemisphere of mars possesses a thick crust which probably renders the lithosphere positively buoyant in this paper we present numerical and scaling arguments which show that if the area of positively buoyant lithosphere grows beyond a critical fraction 50 for mars plate tectonics will stop heat transfer through the buoyant lithosphere is inefficient which causes mean mantle temperatures to increase as the surface area of buoyant lithosphere increases the resulting reduction in mantle viscosity reduces shear stresses if these shear stresses drop below the yield strength of the lithosphere plate motions will cease and the planet will behave as a one plate system thus the end of plate tectonics on mars is a natural consequence of the growth of the southern highlands similar arguments for the earth predict that it should operate in the plate tectonic regime now but that it may have experienced stagnant lid convection in the past index terms 5475 planetology solid surface planets tectonics 8149 5430 planetology solid surface planets interiors 8147 5418 planetology solid surface planets heat flow 5455 planetology solid surface planets origin and evolution keywords hemispheric dichotomy mantle convection thermal evolution rheology

internal structure and early thermal evolution of mars from mars global surveyor topography and gravity topography and gravity measured by the mars global surveyor have enabled determination of the global crust and upper mantle structure of mars the planet displays two distinct crustal zones that do not correlate globally with the geologic dichotomy a region of crust that thins progressively from south to north and encompasses much of the southern highlands and tharsis province and a region of approximately uniform crustal thickness that includes the northern lowlands and arabia terra the strength of the lithosphere beneath the ancient southern highlands suggests that the northern hemisphere was a locus of high heat flow early in martian history the thickness of the elastic lithosphere increases with time of loading in the northern plains and tharsis the northern lowlands contain structures interpreted as large buried channels that are consistent with northward transport of water and sediment to the lowlands before the end of northern hemisphere resurfacing

geomechanics of hydraulic fracturing microseismicity part 2 stress state determination we investigate the hydraulic fracturing process by analysis of the associated microseismicity in part 1 we recognized double couple and hybrid microseismic events and their fault plane orientations critical stress instability and stress inversion techniques were used to assess fracture activation conditions in part 2 we apply results from the tensile source model to investigate how activated faults relate to the stress state and geologic setting we assess potential mechanisms for induced microseismicity including leakoff and diffuse pressurized fracture network flow stress shadowing adjacent to large parent hydraulic fractures and crack tip stress perturbations data are from the mississippian barnett shale texas and include microseismic events from sequential pumping stages in two adjacent horizontal wells that were recorded in two downhole monitor wells as well as operations wellbore derived stress and natural fracture data results point to activation of inclined faults whose orientation is dominantly northeast southwest and vertical north south faults the activation stress states for a range of modeling scenarios show stress rotation decreased mean stress and increased deviatoric stress this stress state cannot be explained by sidewall leakoff in the stress shadow region adjacent to hydrofractures but is consistent with hybrid and shear activation obliquely ahead of pressurized fractures information about hydrofracture evolution and operationally related dynamic stress change is obscured by geomechanical heterogeneity that is likely geologic in nature the most compelling observation is that the most highly misoriented microseismic faults occur in the same vicinity as a carbonate dominated submarine fan feature that was previously expected to act as a minor fracture barrier

reconstructing frequency magnitude statistics from detection limited microseismic data microseismic monitoring particularly the monitoring of hydraulic fracturing in gas and oil bearing shales has developed significantly over the last ten years early work focused on the location of microseismic events but more recently there have been attempts to extract more of the information afforded by this rich data source in particular the recovery of the frequency magnitude distribution which is expected to follow a gutenberg richter distribution may provide insights into the prevailing effective stress regime in the vicinity of the events this stress regime varies with distance from the hydraulic fracturing at the propagating fracture one expects conditions for tensile or shear failure away from the fracture one may broadly expect microseismicity associated with pre existing weakness in the rock occurring at effective stress conditions close to the conditions existing prior to the treatment all geophysical experiments are detection limited and the microseismic monitoring case does not differ in this regard in constructing a statistical indicator such as the distribution of moment magnitudes we would like the estimate to be robust and use as much of the data as possible in analysing earthquake catalogues the predominant practise is to determine a magnitude of completeness denoting the detection limit of the catalogue this approach defines a minimum magnitude above which all events are thought to have been reliably recorded in effect this imposes an artificial conservative detection limit to replace the unknown detection limit of the catalogue we present the case of an arbitrary detection limit and introduce an approach from astronomy that is particularly suited to the single well observing geometry most prevalent in hydraulic fracture monitoring we calculate b values for a set of event magnitudes from the barnett shale formation where multiple stimulation treatments were applied in a pair of wells zipper frac followed by a four stage treatment in a third well and find significant variations in the b value between the pumped stages

microseismic properties of a homogeneous sandstone during fault nucleation and frictional sliding summary the formation and evolution of faulting in three initially intact oil saturated specimens of clashach sandstone is examined under conditions of constant strain rate loading at three different confining pressures simulating the effect of tectonic loading at different depths in the earth s upper crust after a fault is formed the specimens are slid for a time and then the initial confining pressure is increased to simulate the long term recovery of strength expected in the earth the differential stress u and natural acoustic emissions ae are measured during the three separate phases of fault nucleation sliding and strengthening at the end of each individual phase the fluid permeability is measured by a pulse decay technique at constant stress the ae are interpreted using a mean field theory for damage evolution which calculates a mean crack length c from the seismic event rate n and the b value and a mean energy release rate g from u and c c and g both increase non linearly in the nucleation stage and are associated with large fluctuations in the scaling exponent b in contrast the b value remains almost constant at a value near unity in the sliding phase even when the fault is mechanically strengthened small fluctuations in g observed during sliding may be associated with the breaking of individual asperities the relative stability of b combined with a relatively constant sliding stress implies that frictional sliding has all the hallmarks of a self organized critical soc phenomenon in contrast the fresh fracture of intact rock shows a continuum of states with power law scaling at a variety of b values where b is negatively correlated to g during the fault nucleation stage the implied subcritical crack growth index n obtained from the ae data shows a distinct break of slope during the failure of the intact specimens from n 2 consistent with a theoretical transition from stable damage to dynamic instability a possible mechanism is the transition from local mechanisms dominated by dilatant hardening negative feedback to microcrack coalescence positive feedback interpreted in this way the results imply that the organized coalescence of microcracks to form a fault occurs before the ultimate strength peak stress is reached this emphasizes the fact that local mechanisms of softening or hardening do not always scale simply to the macroscopic rheology in a heterogeneous medium the fluid permeability reduces steadily throughout all phases of deformation testament to the self sealing nature of deformation in porous sedimentary rocks

microseismic moment tensors a path to understanding frac growth microseismic monitoring is a valuable tool in understanding the efficacy of hydraulic fracture treatments determination of event locations and magnitudes leads to estimations of the geometry of the fracture zone and certain dynamics of the fracturing process with sufficient resolution the hypocenters may even reveal failure planes or other underlying structures controlling the distribution of events and of interest to petroleum engineers to test various hypotheses on fracture growth

an origin for the linear magnetic anomalies on mars through accretion of terranes implications for dynamo timing the proposed existence of magnetic lineations in the terra cimmeria and terra sirenum regions of mars was initially explained by earth like sea floor spreading here we argue instead that these lineations could have been formed at a convergent plate margin through collision and accretion of terranes a similar process produced banded magnetic anomalies similar in geometry and even in size to those in earth s north american cordillera because only sparse and generally weak anomalies have been detected in the martian northern lowlands which could constitute an analog to the terrestrial oceanic crust it is possible that the magnetic field stopped its activity while crustal recycling was still active in mars

marine target craters on mars an assessment study observations of impact craters on earth show that a water column at the target strongly influences lithology and morphology of the resultant crater the degree of influence varies with the target water depth and impactor diameter morphological features detectable in satellite imagery include a concentric shape with an inner crater inset within a shallower outer crater which is cut by gullies excavated by the resurge of water in this study we show that if oceans large seas and lakes existed on mars for periods of time marine target craters must have formed we make an assessment of the minimum and maximum amounts of such craters based on published data on water depths extent and duration of putative oceans within contacts 1 and 2 cratering rate during the different oceanic phases and computer modeling of minimum impactor diameters required to form longlasting craters in the seafloor of the oceans we also discuss the influence of erosion and sedimentation on the preservation and exposure of the craters for an ocean within the smaller contact 2 with a duration of 100 000 yr and the low present crater formation rate only 12 detectable marine target craters would have formed in a maximum estimate with a duration of 0 8 gyr as many as 1400 craters may have formed an ocean within the larger contact 1 meridiani with a duration of 100 000 yr would not have received any seafloor craters despite the higher crater formation rate estimated before 3 5 gyr on the other hand with a maximum duration of 0 8 gyr about 160 seafloor craters may have formed however terrestrial examples show that most marinetarget craters may be covered by thick sediments ground penetrating radar surveys planned for the esa mars express and nasa 2005 missions may reveal buried craters though it is uncertain if the resolution will allow the detection of diagnostic features of marine target craters the implications regarding the discovery of marine target craters on mars is not without significance as such discoveries would help address the ongoing debate of whether large water bodies occupied the northern plains of mars and would help constrain future paleoclimatic reconstructions

review and comparison of models for describing non equilibrium and preferential flow and transport in the vadose zone abstract in this paper we review various approaches for modeling preferential and non equilibrium flow and transport in the vadose zone existing approaches differ in terms of their underlying assumptions and complexity they range from relatively simplistic models to more complex physically based dual porosity dual permeability and multi region type models a relatively simple dual porosity flow model results when the richards equation is combined with composite double hump type equations for the hydraulic properties to account for both soil textural matrix and soil structural fractures macropores peds effects on flow the simplest non equilibrium flow model a single porosity model which distinguishes between actual and equilibrium water contents is based on a formulation by ross and smettem soil sci soc am j 64 2000 1926 that requires only one additional parameter to account for non equilibrium a more complex dual porosity mobile immobile water flow model results when the richards or kinematic wave equations are used for flow in the fractures and immobile water is assumed to exist in the matrix we also discuss various dual permeability models including the formulation of gerke and van genuchten water resour res 29 1993a 305 and the kinematic wave approach as used in the macro model of jarvis technical description and sample simulations department of soil science swedish university of agricultural science uppsala sweden 1994 51 both of these models invoke terms accounting for the exchange of water and solutes between the matrix and the fractures advantages and disadvantages of the different models are discussed and the need for inter code comparison is stressed especially against field data that are sufficiently comprehensive to allow calibration validation of the more complex models and to distinguish between alternative modeling concepts several examples and comparisons of equilibrium and various non equilibrium flow and transport models are also provided

faulting induced by forced fluid injection and fluid flow forced by faulting an interpretation of hydraulic fracture microseismicity carthage cotton valley gas field texas we analyzed precisely located microearthquake data detected during five hydraulic fracture treatments in the carthage gas field of east texas the treat ments were conducted in two adjacent boreholes within interbedded sands and shales of the upper cotton valley formation the microearthquakes were induced within narrow horizontal bands that correspond to the targeted sandstone layers events throughout all the treatments show strike slip faulting occurring uniformly along vertical fractures trending close to maximum horizontal stress direction these events are consistent with the reservoir s prevalent natural fractures known to be isolated within the sands and trending subparallel to the expected hydraulic fracture orien tation when this uniform fracture system was activated exclusively the detected shear deformation measured as the moment release per unit volume of fluid injected was constant independent of various fluid viscosities and flow rates used within the base of the upper cotton valley formation anomalous event counts and moment release occurred within dense clusters that delineate bends or jogs in the fracture zones the mechanisms are also strike slip but the fault planes are more favorably oriented for failure the dense clusters show location patterns diverging in time suggesting the expulsion of fluids from compressive fault jogs fluid flow forced by the adjacent slip induced loading appears to initially extend the treatments but the loading also tends to lock up and concentrate stress at the jogs as evident by fewer but larger events populating the structures as treatments progress as a result effec tive drainage lengths from the boreholes may be shorter than would be inferred from the seismicity extending past the jogs these high moment asperities are similar to dense patches of seismicity observed along creeping sections of the san andreas fault where they have been attributed to localized zones of strength or stress con centration surrounded by larger regions undergoing stable aseismic slip this sim ilarity plus large moment deficits in terms of volume injected suggests a large component of aseismic slip is induced by the cotton valley treatments

magnetic lineations in the ancient crust of mars the mars global surveyor spacecraft in a highly elliptical polar orbit obtained vector magnetic field measurements above the surface of mars altitudes 100 kilometers crustal magnetization mainly confined to the most ancient heavily cratered martian highlands is frequently organized in east west trending linear features the longest extending over 2000 kilometers crustal remanent magnetization exceeds that of terrestrial crust by more than an order of magnitude groups of quasi parallel linear features of alternating magnetic polarity were found they are reminiscent of similar magnetic features associated with sea floor spreading and crustal genesis on earth but with a much larger spatial scale they may be a relic of an era of plate tectonics on mars

dike intrusion as a possible cause of linear martian magnetic anomalies a model in which the ancient 100 200 km wide magnetic lineations in the southern hemisphere of mars are formed by repeated dike intrusion is proposed on the basis of magnetizations observed at terrestrial mid ocean ridges a likely upper bound on the martian crustal magnetization is about 15 a m 1 this upper bound gives a minimum magnetized layer thickness of 35 60 km because the magnetic material is likely to be magnetite the long term stability of the anomalies probably requires ilmenite lamellae to have exsolved from the magnetite this exsolution process provides an upper bound on the cooling time available of 102 105 yr these bounds can be satisfied if the magnetic anomalies are created by successive intrusion of dikes from 20 to 2000 m wide production of a martian crust 35 60 km thick by rifting requires either mantle potential temperatures in excess of 1430 c or stretching factors greater than 5

variations in earthquake size distribution across different stress regimes the earthquake size distribution follows in most instances a power law1 2 with the slope of this power law the b value commonly used to describe the relative occurrence of large and small events a high b value indicates a larger proportion of small earthquakes and vice versa statistically significant variations of b values have been measured in laboratory experiments mines and various tectonic regimes such as subducting slabs near magma chambers along fault zones and in aftershock zones3 however it has remained uncertain whether these differences are due to differing stress regimes as it was questionable that samples in small volumes such as in laboratory specimens mines and the shallow earth s crust are representative of earthquakes in general given the lack of physical understanding of these differences the observation that b values approach the constant 1 if large volumes are sampled4 was interpreted to indicate that b 1 is a universal constant for earthquakes in general5 here we show that the b value varies systematically for different styles of faulting we find that normal faulting events have the highest b values thrust events the lowest and strike slip events intermediate values given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress

a thick crustal block revealed by reconstructions of early mars highlands the global scale crustal structure of mars is shaped by impact basins volcanic provinces and a hemispheric dichotomy with a thin crust beneath the northern lowlands and a thick crust beneath the southern highlands the southern highlands are commonly treated as a coherent terrain of ancient crust with a common origin and shared geologic history plausibly originating from a giant impact s or a hemispheric scale mantle upwelling previous studies have quantified the contribution of volcanism to this crustal structure however the influence of large impacts remains unclear here we present reconstructions of the past crustal thickness of mars about 4 2 gyr ago where the four largest impact basins hellas argyre isidis and utopia are removed assuming mass conservation as well as the main volcanic provinces of tharsis and elysium our reconstruction shows more subdued crustal thickness variations than at present although the crustal dichotomy persists however our reconstruction reveals a region of discontinuous patches of thick crust in the southern highlands associated with magnetic and geochemical anomalies this region corresponding to terra cimmeria sirenum is interpreted as a discrete crustal block our findings suggest that the southern highlands are composed of several crustal blocks with different geological histories such a complex architecture of the southern highlands is not explained by existing scenarios for crustal formation and evolution a discrete block of thick ancient crust revealed by a crustal reconstruction suggests a complex geologic history for the southern highlands of mars

high resolution local magnetic field models for the martian south pole from mars global surveyor data we present two high resolution local models for the crustal magnetic field of the martian south polar region models sp130 and sp130m were derived from three component measurements made by mars global surveyor at nighttime and at low altitude 200 km the availability area for these data covers the annulus between latitudes 76 and 87 and contains a strongly magnetized region southern parts of terra sirenum adjacent to weakly magnetized terrains such as prometheus planum our localized field inversions take into account the region of data availability a finite spectral bandlimit spherical harmonic degree l 130 and the varying satellite altitude at each observation point we downward continue the local field solutions to a sphere of martian polar radius 3376 km while weakly magnetized areas in model sp130 contain inversion artifacts caused by strongly magnetized crust nearby these artifacts are largely avoided in model sp130m a mosaic of inversion results obtained by independently solving for the fields over individual subregions robust features of both models are magnetic stripes of alternating polarity in southern terra sirenum that end abruptly at the rim of prometheus planum an impact crater with a weak or undetectable magnetic field from a prominent and isolated dipole like magnetic feature close to australe montes we estimate a paleopole with a best fit location at longitude 207 and latitude 48 from the abruptly ending magnetic field stripes we estimate average magnetization values of up to 15 a m

dynamics of fault interaction parallel strike slip faults we use a two dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures our results indicate that a strike slip earthquake is unlikely to jump a fault step wider than 5 km in correlation with field observations of moderate to great sized earthquakes we also find that dynamically propagating ruptures can jump both compressional and dilational fault steps although wider dilational fault steps can be jumped dilational steps tend to delay the rupture for a longer time than compressional steps do this delay leads to a slower apparent rupture velocity in the vicinity of dilational steps these dry cases assumed hydrostatic or greater pore pressures but did not include the effects of changing pore pressures in an additional study we simulated the dynamic effects of a fault rupture on undrained pore fluids to test sibson s 1985 1986 suggestion that wet dilational steps are a barrier to rupture propagation our numerical results validate sibson s hypothesis by demonstrating that the effect of the rupture on the undrained pore fluids is to inhibit the rupture from jumping dilational stepovers the basis of our result differs from sibson s hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments

the formation of the south tharsis ridge belt basin and range style extension on early mars the south tharsis ridge belt strb is located along the northeastern edge of terra sirenum and partially surrounds the southwestern part of tharsis in an arc it consists of 29 large ridges separated by distances 130 to 260 km with average relief of 1 5 km above the surrounding plains because the strb is among the oldest tectonic features associated both spatially and developmentally with tharsis it may provide key information on the early evolution of tharsis and possibly pre tharsis processes earlier studies concluded that the ridges formed through compressional tectonism however the shape size and separation of the ridges support the interpretation that the strb resembles the extensional basin and range province on earth both regions are characterized by series of parallel mountain ranges separated by broad valleys in this study we evaluate both extensional and compressional hypotheses for the origin of the ridges using evidence from topography deformed craters crustal thickness models and strain modeling though neither interpretation explains all aspects of the ridges the topography of the ridges and crustal thinning associated with the western part of the ridge belt support an extensional origin strain models predict that basin and range style wide rifting would be expected for early martian conditions this extension may have been initiated by plume induced uplift in the early stages of tharsis formation but the large amount of extensional strain inferred in the western strb must have been accommodated by compression elsewhere possibly in the heavily deformed craters of western terra sirenum

micromechanical modeling of cracking and failure in brittle rocks dynamic micromechanical models are used to analyze crack nucleation and propagation in brittle rock models of rock are created by bonding together thousands of individual particles at points of contact the feasibility of using these bonded particle models to reproduce rock mechanical behavior is explored by comparing model behavior to results from actual laboratory tests on different rock types the behavior of two granite models are examined in detail to study cracking and failure patterns that occur during compressional loading because discontinuum models are being used the rock models are free to crack and break apart under stress such that the micromechanics of cracking can be examined stress waves are allowed to propagate outward from each crack and it is shown that these dynamic waves significantly affect the rock behavior as the peak stress in the modeled rock is approached and many of the bonds are close to breaking a passing wave from a nearby crack is sufficient to break more bonds this causes clusters of cracks to be created and then eventual macroscopic shear failure occurs as these clusters connect to bisect the sample the failure patterns observed in the granite models are similar to those observed in actual laboratory tests

modelling natural attenuation of heavy metal groundwater contamination in the selebi phikwe mining area botswana seepage from a tailings dam is the major source of groundwater pollution in the selebi phikwe area where mining of sulphidic nickel copper cobalt ore started in 1973 and will continue until 2014 the seepage water has a ph in the range of 1 7 2 8 and is strongly enriched in so4 2 5 680 g l and heavy metals 6 230 g l ni 1 860 g l cu and 410 g l co the fracture aquifer affected by pollution from the dam exhibits a remarkable capacity of heavy metal sorption most of the ni cu and co is scavenged at less than 500 m distance downgradient from the polluting source whereas so4 2 is not immobilized significantly the heavy metal sorption process is assumed to be due to surface complexation which is supported by a relatively high groundwater ph in the range of 6 2 7 8 at 200 m distance from the tailings dam the objective of this study is to demonstrate that the sorption process can be incorporated into a realistic three dimensional reactive transport groundwater model that is implicitly charge balanced the simulations are performed with the phast1 2 program which is based on the hst3d flow and transport code and the hydrochemical phreeqc2 12 code

timing of the martian dynamo on mars the strong magnetization in the highland crust of the southern hemisphere and the absence of magnetic anomalies at the hellas and argyre impact basins have been taken as signs that the core dynamo that once drove the planet s magnetic field turned off more than 4 billion years gyr ago here we argue instead that the martian dynamo turned on less than 4 gyr ago and turned off at an unknown time since then high spatial resolution magnetometry in both martian hemispheres is needed to reveal the true history of the martian dynamo

a 50 degree spherical harmonic model of the magnetic field of mars this paper presents a 50 degree and order spherical harmonic model of the magnetic field of mars derived at 120 km altitude using the three orthogonal components of the vector magnetic field data acquired by the mars global surveyor within the 80 to 200 km altitude range the downward continued vector magnetic field components to the surface of mars delineate details of the martian magnetic field the strong magnetic anomalies in the southern hemisphere do not show compelling evidence for magnetic lineations similar to those associated with the seafloor spreading on earth the anomalies are more or less equidimensional the magnetic signatures surrounding the impact basins hellas argyre and isidis show that the prominent magnetic anomalies are older than the impact events an average magnetization of 2 a m is estimated for the upper crust surrounding hellas basin four isolated magnetic anomalies are modeled by vertical prisms with circular cross sections of 170 to 190 km radius and 20 to 30 km thickness the magnetization of the model prisms is 5 a m more than that of the surroundings

episodic flood inundations of the northern plains of mars throughout the recorded history of mars liquid water has distinctly shaped its landscape including the prominent circum chryse and the northwestern slope valleys outflow channel systems and the extremely flat northern plains topography at the distal reaches of these outflow channel systems paleotopographic reconstructions of the tharsis magmatic complex reveal the existence of an europe sized noachian drainage basin and subsequent aquifer system in eastern tharsis this basin is proposed to have sourced outburst floodwaters that sculpted the outflow channels and ponded to form various hypothesized oceans seas and lakes episodically through time these floodwaters decreased in volume with time due to inadequate groundwater recharge of the tharsis aquifer system martian topography as observed from the mars orbiter laser altimeter corresponds well to these ancient flood inundations including the approximated shorelines that have been proposed for the northern plains stratigraphy geomorphology and topography record at least one great noachian early hesperian northern plains ocean a late hesperian sea inset within the margin of the high water marks of the previous ocean and a number of widely distributed minor lakes that may represent a reduced late hesperian sea or ponded waters in the deepest reaches of the northern plains related to minor tharsis and elysium induced amazonian flooding

an altitude normalized magnetic map of mars and its interpretation techniques developed for the reduction and analysisofterrestrialsatellitemagneticelddataareusedto better understand the magnetic eld observations made by mars global surveyor a global distribution of radial br magnetic eld observations and associated uncertainties is invertedfor an equivalentsource magnetization distribution and then used to generate an altitude normalized map of br at 200 km the observations are well represented by a potential function of crustal origin consistent with a rema nent origin for the martian magnetic features the correla tion between the 40546 br observations andbr calculated from the magnetization solution at observation locations is 0 978 for a magnetization distribution connedto a 50 km layer calculatedmagnetizationsrangefrom 22to 17a m we see correlations with tectonics that were only hinted at in earlier maps magnetic features appear to be truncated against valles marineris and ganges chasma suggestive of a major change in crustal properties associated with fault ing

late noachian to hesperian climate change on mars evidence of episodic warming from transient crater lakes near ares vallis 1 the ares vallis region is surrounded by highland terrain containing both degraded and pristine large impact craters that suggest a change in climate during the late noachian early hesperian from warmer wetter conditions to colder dryer conditions however the regional occurrence of hesperian age crater outlet channels indicates that this period on mars was characterized by episodic climate fluctuations that caused transient warming facilitating the stability of liquid water at the surface an extensive survey of the morphology and topography of 75 impact basins in the region indicates that of the largest degraded craters 4 were identified with single outlet channels that suggest the former presence of water infill these basins lack inlets indicating that water influx was likely derived from sapping of groundwater a comparison of measured crater rim heights to modeled rim heights suggests that the bulk of the depth diameter reduction on these craters was the result of infilling possibly by sediments crater statistics indicate that crater degradation and infill occurred during a short 200 ma interval in the late noachian from 3 8 ga to 3 6 ga craters that formed after 3 6 ga exhibit a near pristine morphology our results support the hypothesis of rapid climate change at the end of the noachian period however geologic relationships between the crater outlet channels and ares vallis indicate that drainage occurred only after the period of intense crater modification during the hesperian 3 5 2 9 ga this suggests a delay between the time of infill of the craters and the time of drainage

violations of gutenberg richter relation in anthropogenic seismicity anthropogenic seismicity as is the undesired dynamic rockmass response to technological processes as environments are shallow hence their heterogeneities have important impact on as moreover as is controlled by complex and changeable technological factors this complicated origin of as explains why models used in tectonic seismicity may be not suitable for as we study here four cases of as testing statistically whether the magnitudes follow the gutenberg richter relation or not the considered cases include the data from mponeng gold mine in south africa the data observed during stimulation of geothermal well basel 1 in switzerland the data from acu water reservoir region in brazil and the data from song tranh 2 hydropower plant region in vietnam the cases differ in inducing technologies in the duration of periods in which they were recorded and in the ranges of magnitudes in all four cases the observed frequency magnitude distributions statistically significantly differ from the gutenberg richter relation although in all cases the gutenberg richter b value changed in time this factor turns out to be not responsible for the discovered deviations from the gutenberg richter born exponential distribution model though the deviations from gutenberg richter law are not big they substantially diminish the accuracy of assessment of seismic hazard parameters it is demonstrated that the use of non parametric kernel estimators of magnitude distribution functions improves significantly the accuracy of hazard estimates and therefore these estimators are recommended to be used in probabilistic analyses of seismic hazard caused by as

east west trending magnetic anomalies in the southern hemisphere of mars modeling analysis and interpretation abstract maps of the vector components of the martian crustal magnetic field over the strongly magnetized terra cimmeria sirenum region are constructed using mars global surveyor magnetometer data although pronounced east west trending anomalies are present on the radial and north field component maps at the mapping altitude 360 380 km these trends are much less prominent at the lower aerobraking altitude 90 150 km comparisons with similar maps produced using artificial data at the aerobraking altitude indicate that elongated sources in this region may have maximum lengths along the martian surface of 500 km and maximum aspect ratios of 2 iterative forward modeling of several relatively isolated anomalies in the mapped region yields paleomagnetic pole positions consistent with those estimated in previous studies of other anomalies using mapping phase and science phasing orbit data on this basis it is inferred that sources in the studied region are most probably magnetized primarily in northward or southward directions using this additional constraint iterative forward modeling is then applied to determine a magnetization distribution that is consistent with data at both the aerobraking altitude and the mapping altitude the model magnetization distribution which includes 41 discrete sources again indicates no highly elongated sources an examination of surface geology in the region as well as a consideration of the global distribution of anomalies suggests that magmatic intrusions e g subsurface dike swarms cooling in the presence of water are the most likely sources of the magnetic anomalies

martian plate tectonics the northern lowlands of mars may have been produced by plate tectonics preexisting old thick highland crust was subducted while seafloor spreading produced thin lowland crust during late noachian and early hesperian time in the preferred reconstruction a breakup margin extended north of cimmeria terra between daedalia planum and isidis planitia where the highland lowland transition is relatively simple south dipping subduction occurred beneath arabia terra and east dipping subduction beneath thatsis montes and tempe terra lineations associated with gordii dotsum are attributed to ridge parallel structures while phelegra montes and scandia colles are interpreted as transform parallel structures or ridge fault fault triple junction tracks other than for these few features there is little topographic roughness in the lowlands seafloor spreading if it occurred must have been relatively rapid quantitative estimates of spreading rate are obtained by considering the physics of seafloor spreading in the lower c 0 4 g gravity of mars the absence of vertical scarps from age differences across fracture zones and the smooth axial topography to the first order the height of vertical scarps across fracture zones does not involve gravity crustal thickness at a given potential temperature in the mantle source region scales inversely with gravity thus the velocity of the rough smooth transition for axial topography also scales inversely with gravity plate reorganizations where young crust becomes difficult to subduct are another constraint on spreading age possible plate reorganizations for example the end of spreading through alba patera occur when the ridge axis is far from the trench that is rapid plate motions are inferred to have placed young oceanic crust far from the ridge axis the preferred full spreading rate 900 from the plate pole is 80 mm yr 1 plate tectonics if it occurred dominated the thermal and stress history of the planet a geochemical implication is that the lower gravity of mars allows deeper hydrothermal circulation through cracks and hence more hydration of oceanic crust so that more water is easily subducted than on the earth age and structural relationships from photogeology as well as median wavelength gravity anomalies across the now dead breakup and subduction margins are the data most likely to test and modify hypotheses about mars plate tectonics

the matter of size on the moment magnitude of microseismic events abstractwe investigated the method of estimating seismic moment and moment magnitude for microseismic events we determined that the m0t defined by bowers and hudson is the proper scalar moment to be used in microseismic studies for characterizing the size of an event and calculating its moment magnitude for non double couple sources the proportional relationship between body wave amplitude and seismic moment in the brune model breaks down so under such situations the brune model is not an appropriate way to estimate the seismic moment and magnitude moreover the s wave alone is not sufficient for determining the total seismic moment instead the p wave must be analyzed an example barnett shale data set was studied and the results concluded that the magnitudes estimated with the brune model could be off by as much as 1 92 with an absolute average of 0 35 the moment magnitudes based on the scalar moment m0t also gave a significantly different event size distribution and b value estimation finall

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

control of impact crater fracture systems on subsurface hydrology ground subsidence and collapse mars 1 noachian layered materials are pervasively exposed throughout the highlands of mars the layered deposits in places many kilometers thick exhibit impact craters of diverse morphologic characteristics ranging from highly degraded to pristine most of which formed during the period of heavy bombardment in addition exhumed impact craters ancient channels and fluvial and alluvial fans are visible in the layered deposits through moc imagery these features are more abundant in noachian terrains which indicates relatively high erosion rates during ancient mars that competed with heavy meteoritic bombardment the noachian layered materials are thus expected to contain numerous buried impact craters in various states of preservation here we propose that impact craters buried and exposed and associated fracture systems dominate the basement structural fabric of the ancient highlands and that they have significantly influenced the hydrogeology diversity in the occurrence of high and low densities of impact craters and associated fracture systems controls the magnitude of the local effects of magmatic driven hydrothermal activity in and surrounding the tharsis region for example the formation of chaotic terrains the source regions of the circum chryse outflow channel system and a large diversity of collapse structures including impact crater moats and pit chains appear to be the result of enhanced hydrothermal activity

evidences for a noachian hesperian orogeny in mars a number of tectonic structures have been located at the thaumasia plateau daedalia planum and aonia terra mars they include isolated folds with axial traces up to 200 km long trains of tightly folded structures tens of km long and thrusts their size and geometry are similar to those on earth and the direction of compression seems to have varied amply with time suggesting a complex tectonic evolution crater counts on the deformed terrain point to noachian to early hesperian ages on the basis of the geometry and geological relations of these structures we propose that they form part of an old martian orogen the thaumasia aonia orogen which embraced not only the thaumasia plateau but areas of daedalia planum aonia terra and nereidum montes as well a regional coherent layering is previous to the deformation and could represent the trace of even older stresses on the martian lithosphere

information entropy of earthquake populations in northeastern italy and western slovenia abstract the spatio temporal evolution of eight seismicity populations preceding and following moderate earthquake sequences occurred in ne italy and w slovenia are investigated by means of the normalized shannon entropy and the fractal dimension three phases are recognized in the temporal seismic series the period preceding the mainshock is characterized by oscillations of the shannon entropy around a nearly constant level and by fluctuations of the fractal dimension the phase of mainshock and aftershock sequences is characterized by a significant decrease of the shannon entropy a simultaneous marked decrease of the fractal dimension is observed in five cases after the sequence the entropy recovers the nearly constant trend before the mainshock and the fractal dimension is characterized by fluctuations we interpreted the fluctuations of the normalized shannon entropy and the fractal dimension caused by the coupling between the stress field and the mechanical heterogeneities of the crust that results in spatial and temporal fluctuations of the strain energy

magnetization paleomagnetic poles and polar wander on mars abstract using the geometry of an areocentric dipole field we have remodeled the anomalies observed in the terra sirenum and terra cimmeria regions by the mars global surveyor mgs for the origin of the anomalies we used both magnetic stripe and magnetic shell hypotheses for magnetic stripe models we assume that sea floor spreading formed the anomalies in the presence of a reversing axial dipole field we found that a simple magnetic stripe model with constant magnetization and crustal thickness results in poor agreement with the observed anomalies however by allowing the intensity of magnetization to vary freely between and along the stripes a reasonable fit to the observed data can be found while maintaining the observed polarity along the stripes applying the simple magnetic stripe model to the terra sirenum and terra cimmeria anomalies results in best fit spreading centers located well to the north of the surviving magnetic stripes this could possibly explain the apparent lack of symmetry that has been noted in the anomalies for the magnetic shell hypothesis we assume that a monopolarity dipole field magnetized the early crust of mars the anomalies that persist today would then be the result of partial demagnetization of the original magnetic shell by more recent processes the magnetic anomalies in terra sirenum and terra cimmeria are consistent with this model especially if the magnetization of the original magnetic shell was laterally inhomogeneous if the monopolarity magnetic shell hypothesis is correct best fit magnetic paleopole locations for the original magnetization lie in a sub equatorial belt centered near the prime meridian if the ancient magnetic pole of mars coincided with the spin pole this result implies that either apparent or true polar wander has occurred on mars apparent polar wander would be consistent with plate tectonics having occurred after the formation of the magnetic anomalies true polar wander has previously been proposed to explain certain morphologic features found on mars

comparison of aquifer vulnerability assessment techniques application to the neblon river basin belgium five different methods for assessing intrinsic aquifer vulnerability were tested in a case study and their results compared the test area was a slightly karstified district in the condroz region of belgium the basin covers about 65 km2 and the karst aquifer provides a water supply of about 28 000 m3d 1 the methods tested were epik doerfliger et al 1999 drastic aller et al 1987 german method von hoyer and sofner 1998 god foster 1987 and isis civita and de regibus 1995 the results are compared and critically examined from the analysis it seems that reducing the number of parameters is unsatisfactory due to the variety of geological conditions the various methods produce very different results at any given site as only physically based methods can be checked for their reliability it is clear that future vulnerability mapping techniques must incorporate such methods

in situ imaging of fracture development during maturation of an organic rich shale effects of heating rate and confinement abstract the statistics and dynamics of fractures formed during the accelerated maturation of kerogen rich shale was investigated by heating green river shale r 8 unit piceance basin northwestern colorado core samples while 3d x ray microtomographic images were acquired previous studies have shown that when there was no confining stress fractures formed while the kerogen contained in the shale matured and the produced hydrocarbon was expelled through these fractures in the present study x ray tomographic scans at multiple voxel sizes were conducted on similar samples during heating in one experiment the shale sample was tightly fitted in a non porous ceramic tube to confine it while it was heated three unconfined samples were heated and then held at different final temperatures to investigate the effects of the gas production rate on fracturing 3d image processing was used to survey fracture network development and time lapse 2d digital image correlation analysis was used to monitor the development of the displacement and strain fields the results revealed that fracturing is strongly dependent on the heating rate and the final heating temperature while most of the fractures were oriented more or less parallel to the bedding plane some were strongly inclined relative to the bedding plane the formation of inclined fractures is attributed primarily to the shape and orientation of a minority of the flake like kerogen patches and also to the effective stress field and fractured zones of weakness a conceptual model is proposed to explain the dynamics of fluid expulsion and the associate fracturing behavior

geological and hydrological histories of the argyre province mars the geologic history of the multi ringed argyre impact basin and surroundings has been reconstructed on the basis of geologic mapping and relative age dating of rock materials and structures the impact formed a primary basin rim materials and a complex basement structural fabric including faults and valleys that are radial and concentric about the primary basin as well as structurally controlled local basins since its formation the basin has been a regional catchment for volatiles and sedimentary materials as well as a dominant influence on the flow of surface ice debris flows and groundwater through and over its basement structures the basin is interpreted to have been occupied by lakes including a possible mediterranean sized sea that formed in the aftermath of the argyre impact event the hypothesized lakes froze and diminished through time though liquid water may have remained beneath the ice cover and sedimentation may have continued for some time at its deepest the main argyre lake may have taken more than a hundred thousand years to freeze to the bottom even absent any heat source besides the sun but with impact induced hydrothermal heat geothermal heat flow due to long lived radioactivities in early martian history and concentration of solutes in sub ice brine liquid water may have persisted beneath thick ice for many millions of years existence of an ice covered sea perhaps was long enough for life to originate and evolve with gradually colder and more hypersaline conditions the argyre rock materials diverse in origin and emplacement mechanisms have been modified by impact magmatic eolian fluvial lacustrine glacial periglacial alluvial colluvial and tectonic processes post impact adjustment of part of the impact generated basement structural fabric such as concentric faults is apparent distinct basin stratigraphic units are interpreted to be linked to large scale geologic activity far from the basin including growth of the tharsis magmatic tectonic complex and the growth into southern middle latitudes of south polar ice sheets along with the migration of surface and sub surface volatiles towards the central part of the primary basin the substantial difference in elevation with respect to the surrounding highlands and tharsis and the thaumasia highlands result in the trapping of atmospheric volatiles within the basin in the form of fog and regional or local precipitation even today in addition the impact event caused long term millions of years hydrothermal activity as well as deep seated basement structures that have tapped the internal heat of mars as conduits for far greater time possibly even today this possibility is raised by the observation of putative open system pingos and nearby gullies that occur in linear depressions with accompanying systems of faults and fractures long term water and heat energy enrichment complemented by the interaction of the nutrient enriched primordial crustal and mantle materials favorable to life excavated to the surface and near surface environs through the argyre impact event has not only resulted in distinct geomorphology but also makes the argyre basin a potential site of exceptional astrobiological significance

polygonal impact craters in argyre region mars implications for geology and cratering mechanics impact craters are not always circular sometimes their rims are composed of several straight segments such polygonal impact craters pics are controlled by pre existing target structures mainly faults or other similar planes of weakness in the argyre region mars pics comprise 17 of the total impact crater population 7 km in diameter and pics are relatively more common in older geologic units their formation is mainly controlled by radial fractures induced by the argyre and ladon impact basins and to a lesser extent by the basin concentric fractures also basin induced conjugate shear fractures may play a role unlike the pics ridges and graben in the argyre region are mostly controlled by tharsis induced tectonism with the ridges being concentric and graben radial to tharsis therefore the pics primarily reflect an old impact basin centered tectonic pattern whereas tharsis centered tectonism responsible for the graben and the ridges has only minor influence on the pic rim orientations according to current models of pic formation complex pics should form through a different mechanism than simple pics leading to different orientations of straight rim segments however when simple and complex pics from same areas are studied no statistically significant difference can be observed hence in addition to enhanced excavation parallel to the strike of fractures simple craters and slumping along the fracture planes complex craters we propose a third mechanism involving thrusting along the fracture planes this model is applicable to both simple and small complex craters in targets with some dominating orientations of structural weakness

vadose zone lag time and potential 21st century climate change effects on spatially distributed groundwater recharge in the semi arid nebraska sand hills summary deep drainage of water below plant root zones potential groundwater recharge will become groundwater recharge gr after a delay or lag time in which soil moisture traverses the vadose zone before reaching the water table depending on the thickness of the vadose zone the magnitude of deep drainage and soil hydraulic properties lag times will vary broadly exceeding decades to centuries in semi arid and arid environments yet studies of future climate change impacts to gr have typically avoided focusing on impacts beyond 100 years and often neglect to consider lag effects caused by the vadose zone we investigate the effects of vadose zone lag time and potential 21st century climate change on the spatial distribution and timing of gr throughout the semi arid nebraska sand hills nsh region 50 000 km2 we propose a simple and rapid quantitative approach for assessment of the groundwater system response time to changes in climate understanding of such effects is needed for groundwater modeling analysis of climate change impacts on groundwater and the effective management and sustainability of future water resources lag time estimates are made using the pressure based vertical velocity of soil moisture changes equivalent to a kinematic wave approximation of richards equation the underlying assumptions unit hydraulic gradient and relatively slow changes in climate are supported by observations in the high plains aquifer region encompassing the nsh the analysis relies on four sources of input data including spatially distributed high resolution 1 1 km gr rates as the difference in 2000 2009 mean precipitation and evapotranspiration estimated using a novel modis based approach thickness of the vadose zone based on 30 m digital elevation models of the land surface and water table elevations statistically downscaled estimates of future 2010 2099 potential gr rates from two hydroclimate model projections wrcp cmip3 with opposing gr trends and estimates of hydraulic conductivity and vertical velocity of the vadose zone pressure changes using the van genuchten mualem equation we found that vadose zone lag times with a mean gr rate of 78 mm yr for 2000 2009 have an estimated spatial mean of 5 28 yrs vertical velocity of 5 68 m yr end of century lag times with mean potential gr rates of 128 and 61 mm yr for 2090 2099 have estimated spatial means of 3 78 and 6 55 yrs vertical velocities of 8 13 and 4 69 m yr for wet and dry scenarios respectively findings suggest that the highly conductive materials primarily sand deposits and high potential gr rates in the nsh lead to relatively quick drainage of vadose zone water on the order of several years less than a decade for the entire 21st century however considerable spatial variations of lag times exist due to gradients in potential gr rates and the highly variable vadose zone thickness in the nsh

numerical investigation of induced cracking and seismic velocity changes in brittle rock 1 a numerical technique is presented to investigate seismic velocity changes in brittle rock due to stress and damage a rock sample is simulated by an assembly of bonded particles which is then subjected to hydrostatic and deviatoric loading regimes during hydrostatic loading cracks close through formation of new contacts and velocities increase during deviatoric loading new cracks form by the breaking of bonds and velocities decrease in directions perpendicular to the opening cracks the nature of the velocity changes mimics closely what is observed in the laboratory the model offers the unique ability to examine directly the microprocesses leading to observed velocity changes

possible ancient giant basin and related water enrichment in the arabia terra province mars a circular albedo feature in the arabia terra province was first hypothesized as an ancient impact basin using viking era information to test this unpublished hypothesis we have analyzed the viking era information together with layers of new data derived from the mars global surveyor mgs and mars odyssey mo missions our analysis indicates that arabia terra is an ancient geologic province of mars with many distinct characteristics including predominantly noachian materials a unique part of the highland lowland boundary a prominent paleotectonic history the largest region of fretted terrain on the planet outflow channels with no obvious origins extensive exposures of eroded layered sedimentary deposits and notable structural albedo thermal inertia gravity magnetic and elemental signatures the province also is marked by special impact crater morphologies which suggest a persistent volatile rich substrate no one characteristic provides definitive answers to the dominant event s that shaped this unique province collectively the characteristics reported here support the following hypothesized sequence of events in arabia terra 1 an enormous basin possibly of impact origin formed early in martian history when the magnetic dynamo was active and the lithosphere was relatively thin 2 sediments and other materials were deposited in the basin during high erosion rates while maintaining isostatic equilibrium 3 sediments became water enriched during the noachian period and 4 basin materials were uplifted in response to the growth of the tharsis bulge resulting in differential erosion exposing ancient stratigraphic sequences parts of the ancient basin remain water enriched to the present day

brittle ductile transition and associated seismicity experimental and numerical studies and relationship with the b value the acoustic emission ae and the mechanical behavior of granite samples during triaxial compression tests have been analyzed the size of ae events displays power law distributions conforming to the gutenberg richter law observed for earthquakes which is characterized by the b value as the confining pressure increases the macroscopic behavior becomes more ductile for all different stages of the rock mechanical behavior linear nonlinear prepeak nonlinear postpeak shearing there is a systematic decrease of the b value with increasing confining pressure a numerical model based on progressive elastic damage and the finite element method allows simulations of the main experimental observations on ae and of a wide range of macroscopic behaviors from brittleness to ductility the model reproduces a decrease in the b value that appears to be related to the type of macroscopic behavior brittle ductile rather than to the confining pressure both experimental and numerical results suggest a relationship between the b value and the brittle ductile transition moreover these results are consistent with recent earthquake observations and give new insight into the behavior of the earth s crust

numerical modelling of groundwater vulnerability the example namibia the unsat h help3 and macro4 3 computer codes which have been developed for simulating the water balance of the unsaturated zone of soils and unconsolidated sediments are also capable simulating water flow in low porosity media such as fractured rock the codes can be used to model the ability of rocks and overlying soils in the vadose zone to protect the groundwater in the uppermost aquifer the net infiltration rates simulated by the different codes are compared against recharge determined by the chloride balance method in namibia the dual permeability code macro4 3 was found to produce more realistic estimates of net infiltration than the unsat h and help3 codes which are based on a single permeability or effective continuum method the net infiltration rate together with the water storage in the unsaturated zone and the groundwater depth are used to calculate the residence time of pore water in the unsaturated zone this parameter determines the intrinsic vulnerability of the aquifer

numerical simulation of earthquake faults with gouge toward a comprehensive explanation for the heat flow paradox the particle based lattice solid model is used to simulate transform faults with and without fault gouge stick slip frictional behavior is observed in two dimensional numerical experiments of model faults both with and without gouge when no gouge is present the fault is strong and the heat generation and stress drops are correspondingly high in disaccord with observations surrounding the heat flow paradox in contrast when a gouge is specified the fault is weak and the heat generation as well as stress drops are low in quantitative agreement with observational constraints the heat flow is low on average and during short periods of aseismic creep seismic efficiencies are compatible with observationally based bounds counter intuitively the fault strength decreases as the intrinsic friction between particles is increased beyond a given threshold the mechanism for low fault strength and heat is rolling and jostling of fault gouge grains during slip this allows macroscopic movement of the fault with only minimal slip between surfaces of the gouge grains as this dynamical mechanism operates during seismic and aseismic slip it provides an explanation for the lack of a heat flow anomaly in both the seismic and creeping parts of the san andreas fault the simulation results provide the first comprehensive and quantitative possible explanation of the heat flow paradox and suggest that fault gouge plays a fundamental role on the dynamics of earthquake faults whether rolling and jostling of fault gouge particles provides the explanation for the heat flow paradox in nature remains to be validated by observation evidence

breakdown of the gutenberg richter relation for microearthquakes induced by hydraulic fracturing influence of stratabound fractures hydraulic fracturing a powerful completion technique used to enhance oil or gas production from impermeable strata may trigger unintended earthquake activity the primary basis for assessment of triggered and natural seismic hazard is the classic gutenberg richter g r relation which expresses scale independent behaviour of earthquake magnitudes here we use a stochastic approach to simulate and test magnitude distance trends expressed by microseismic catalogues derived from three hydraulic fracture monitoring programmes in north america we show that a widely observed rapid fall off in large magnitude events almost universally quantified using the g r b value may in our case be an artefact of the strongly laminated character of the stimulated oil and gas reservoirs we also show that for the three reservoirs considered mechanical bed thickness can be approximated by a lognormal distribution for a stratabound fracture network this leads asymptotically to a gaussian decay for induced magnitudes we show that the stratabound model provides a more significant correspondence with our observations if applicable in general this result has important implications for determining the energy balance of hydraulic fracture systems i e radiated seismic energy versus injected energy as well as hazard assessments based on the probability of occurrence of anomalous seismic events key words monitoring passive method reservoir geophysics

integrated microseismic and geomechanical study in the barnett shale formation abstractwe have developed a careful chronological review of multistage stimulation treatments in which microseismic interpretation was supported by fracture simulation and finite element geomechanical simulation the interpretation of a zipper frac stimulation treatment conducted in the barnett shale formation in 2011 progressed from microseismic interpretation from well site observation to a self consistent understanding across multiple stages and multiple wells through this process we have developed a tutorial on practical interpretation of microseismic data in the context of the hydraulic fracture treatment information pump data and regional structural information from interpretation of 3d seismic sections analysis of microseismic data included summarizing groups of events in the form of b value d value and extracted planar geometries we advocate the development of a chronological interpretation the consistency of which is tested using forward modeling the process is iterative and relies o

spatio temporal evolution of frequency magnitude distribution and seismogenic index during initiation of induced seismicity at guy greenbrier arkansas abstract in this study we carry out detailed analyses of the spatio temporal variations of seismic b value during the onset of the potentially induced earthquake sequence between guy and greenbrier arkansas to investigate correlations with pore pressure change the range of b values suggests that the seismicity in the guy greenbrier area is mostly a result of the activation of pre existing faults the spatial distribution of b value correlates with modeled pore pressure changes in the northern segment of the fault b value increases with depth due to large pore pressure changes and opening of new fractures in the deeper part and stress relaxation in the shallower parts whereas in the southern segment the shallower part shows higher b values due to higher pore pressure fluctuations but the deeper part in the crystalline basement has low b value due to higher confining stress the correlation between the temporal variation of b value and hypocentral depth explains a previously observed temporal drop of b value this suggests that temporal variations should be interpreted along with the spatial variations estimation of the seismogenic indices for the guy greenbrier fault is provided our analysis suggests that monitoring changes in b value and seismogenic indices during an injection period might be used to avoid the occurrence of significant events if injection volume is reduced at critical times

inferring in situ stress changes by statistical analysis of microseismic event characteristics two commonly estimated fractal dimensions called b and d values represent statistical characteristics in the distribution of magnitude sizes b and spatial hypocenter locations d of microseismic events respectively we establish that the values of these two dimensions are related to specific stress regimes through the case study of a heavy oil field drained using cyclic steam stimulation we infer that the measured temporal variation in fractal dimension b is most likely due to significant changes in the local stress regime over a seven month period ranging from extensional faulting fractures opening via a strike slip regime to finally compressive faulting fractures closing the fracture dimension d indicates predominantly planar to spherical hypocenter spatial distributions in the first and last stages but changes to a more linear to planar spatial pattern in the intermediate strike slip regime when the vertical stress is anticipated to be in between the maximum and minimum horizontal stre

claritas rise mars pre tharsis magmatism claritas rise is a prominent ancient noachian center of tectonism identified through investigation of comprehensive paleotectonic information of the western hemisphere of mars this center is interpreted to be the result of magmatic driven activity including uplift and associated tectonism as well as possible hydrothermal activity coupled with its ancient stratigraphy high density of impact craters and complex structure a possible magnetic signature may indicate that it formed during an ancient period of mars evolution such as when the dynamo was in operation as tharsis lacks magnetic signatures claritas rise may pre date the development of tharsis or mark incipient development since some of the crustal materials underlying tharsis and older parts of the magmatic complex respectively could have been highly resurfaced destroying any remanent magnetism here we detail the significant characteristics of the claritas rise and present a case for why it should be targeted by the mars odyssey mars reconnaissance orbiter and mars express spacecrafts as well as be considered as a prime target for future tier scalable robotic reconnaissance