intensity of the geomagnetic field in western europe over the past 2000 years new data from ancient french pottery 1 we studied 14 groups of french pottery fragments dated between the 4th and 16th centuries the potsherds were analyzed using the thellier and thellier 1959 method revised by coe 1967 intensity values were corrected for thermoremanent magnetization trm anisotropy and cooling rate dependence of trm acquisition we first analyzed modern ceramics produced following ancient techniques and fired in a wood burning kiln inside of which field intensity was measured the recovered mean intensity is within 3 of the expected value which proves the reliability of our experimental procedure thermal experiments carried out at rapid and slow cooling rates clearly indicate that the cooling rate correction is critical in archeointensity studies our data indicate that large variations in intensity occurred in france over the last 2000 years two relative maxima in intensity are observed one between the 8th and 10th centuries and the second between the 14th and 15th centuries similarities are observed between the archeointensity data from france and ukraine yielding some evidence for eastward drift of geomagnetic sources between western and eastern europe from a d 800 to a d 1700 we also show that the dipole moment evolution proposed by mcelhinny and senanayake 1982 and yang et al 2000 for the last two millennia is likely biased toward higher values mainly because of the absence of correction for the cooling rate dependence of trm acquisition in most published archeointensity studies we finally underline a possible relationship valid at least in western europe between changes in direction and intensity of the geomagnetic field

cooling rate correction of paleointensity determination for volcanic glasses by relaxation geospeedometry the influence of the cooling rate on paleointensity estimates was investigated for samples from a vertical profile across a 600 a d obsidian lava flow ramp from lipari italy the natural cooling rates at the glass transition which were previously determined for the seven investigated samples by relaxation geospeedometry vary by a factor of more than 4 rock magnetic investigations indicate a magnetic microlite fraction in the single domain grain size range and strong magnetic anisotropy the thermoremanence anisotropy tensor was determined for each specimen to correct the paleointensity results for this anisotropy the cooling rate dependency of the thermoremanence was determined experimentally extrapolation to natural cooling rates indicate an overestimate of the paleointensity by 13 to 20 during experiments with typical laboratory cooling rates correcting for the different cooling rates and the cooling rate dependencies within the vertical profile significantly reduces the standard deviation of the average flow paleointensity the average paleointensity for the 543 19 a d flow ramp results in 52 4 1 1 t corresponding to a virtual axial dipole moment of 9 2 0 2 am2 uncertainties introduced by anisotropy correction and cooling rate extrapolation are considered by error propagation

large displacement hydrothermal frictional properties of dfdp 1 fault rocks alpine fault new zealand implications for deep rupture propagation the alpine fault new zealand is a major plate bounding fault that accommodates 65 75 of the total relative motion between the australian and pacific plates here we present data on the hydrothermal frictional properties of alpine fault rocks that surround the principal slip zones psz of the alpine fault and those comprising the psz itself the samples were retrieved from relatively shallow depths during phase 1 of the deep fault drilling project dfdp 1 at gaunt creek simulated fault gouges were sheared at temperatures of 25 150 300 450 and 600 c in order to determine the friction coefficient as well as the velocity dependence of friction friction remains more or less constant with changes in temperature but a transition from velocity strengthening behavior to velocity weakening behavior occurs at a temperature of t 150 c the transition depends on the absolute value of sliding velocity as well as temperature with the velocity weakening region restricted to higher velocity for higher temperatures friction was substantially lower for low velocity shearing v 0 3 m s at 600 c but no transition to normal stress independence was observed in the framework of rate and state friction earthquake nucleation is most likely at an intermediate temperature of t 300 c the velocity strengthening nature of the alpine fault rocks at higher temperatures may pose a barrier for rupture propagation to deeper levels limiting the possible depth extent of large earthquakes our results highlight the importance of strain rate in controlling frictional behavior under conditions spanning the classical brittle plastic transition for quartzofeldspathic compositions

mechano chemical interactions in sedimentary rocks in the context of co2 storage weak acid weak effects abstract due to the corrosive nature of dissolved co2 the potential short or long term alteration of rock properties represents a major issue in several sites where natural co2 circulation is observed as well as in reservoirs targeted for storage of anthropogenic co2 to date this has been primarily studied from a transport chemical perspective with laboratory evidence of microstructural modifications together with the consequences for flow properties compared to the transport chemical aspects the mechanical chemical aspects have been less investigated though it is to be expected that mechanical properties e g elastic properties failure parameters and time dependent mechanical behaviour could potentially be affected in a similar manner to hydraulic parameters yet since co2 is a weak acid the ph drop is expected to be moderate with a likely lower limit close to 4 0 the buffering of ph by calcite minerals present in most reservoirs targeted for storage may further limit the ph drop as well as confining it to a localized rock volume around the injection well this leads to the question of the magnitude and time spatial scales of chemically mediated mechanical processes during co2 sequestration the authors propose to address this issue by reviewing recent laboratory based studies restricted to sedimentary rocks namely reservoir rocks carbonate or sandstone intact or fractured caprocks and fault rocks key findings include the following 1 the short term impact on the elastic and inelastic behaviour of intact caprocks remains limited 2 shear strength weakening is likely to be respectively low and low to moderate for shale clay rich and anhydrite rich faults but without modifying slip stability in either case 3 the largest impact is located within carbonate reservoirs but with a broad range of reported responses depending on hydrodynamic conditions closed or open and on dissolution regime uniform or channelling and 4 creep experiments confirm that co2 induced dissolution may enhance long term compaction of carbonate reservoirs but the magnitude of acceleration varying from non significant to 50 times depends to a large extent on site specific conditions grain size ph temperature effective stress state etc which renders any direct extrapolation from laboratory to reservoir scale difficult finally some directions for future research studies are discussed

practical estimation of compression behaviour of dredged clays with three physical parameters understanding the compression behaviour of dredged clays is essential for their effective use with drainage measuring consolidation parameters of dredged clays at high initial water contents is a time consuming and tough work the research team has accumulated the database of 110 one dimensional incremental load consolidation tests for dredged clays based on this database this study proposes a simple method of determining virgin compression of dredged clays using three physical parameters which can be easily measured the compression curves of dredged clays can be divided into the pre yield and the post yield zones basic equations are suggested for describing the compression behaviour of the two zones the associated parameters are correlated with the initial void ratio and the void ratio at liquid limit the predicted compression curves are compared with the measured ones with the accumulated database and the independent data available from literature the error of the void ratios predicted is found to vary within 10 compared with the measured ones in engineering practice the work in this study can be applied instead of the time consuming and tough experiments to simply estimate the compression behaviour of dredged clays

geomagnetic field for 0 3 ka 2 a new series of time varying global models 1 steadily increasing numbers of archeomagnetic and paleomagnetic data for the holocene have allowed development of temporally continuous global spherical harmonic models of the geomagnetic field extending present and historical global descriptions of magnetic field evolution the current work uses various subsets of improved data compilations details of which are given in a companion paper by donadini et al 2009 and minor modifications of standard modeling strategies using temporally and spatially regularized inversion of the data and cubic spline parameterizations for temporal variations to produce five models with enhanced spatial and temporal resolution for 0 3 ka spurious end effects present in earlier models are eliminated by enforcing large scale agreement with the gufm1 historical model for 1650 1990 a d and by extending the model range to accommodate data older than 3 ka age errors are not considered as a contribution to data uncertainties but are included along with data uncertainties in an investigation of statistical uncertainty estimates for the models using parametric bootstrap resampling techniques we find common features but also significant differences among the various models indicating intrinsic uncertainties in global models based on the currently available holocene data model cals3k 3 based on all available archeomagnetic and sediment data without a priori quality selection currently constitutes the best global representation of the past field the new models have slightly higher dipole moments than our previous models virtual axial dipole moments vadms calculated directly from the data are in good agreement with all corresponding model predictions of vadms these are always higher than the spherical harmonic dipole moment indicating the limitations of using vadms as a measure of geomagnetic dipole moments

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

evolution of structural and physical parameters of clays during experimental compaction abstract to provide a better understanding of sedimentary basin geological history it is important to describe correctly the evolution of the various physical mechanical and hydraulic properties of clayey rocks as a function of burial depth as a contribution to this field a programme of experimental studies on reworked clay samples compacted under various load pressures in oedometric conditions has been set up the evolution of samples under compaction was followed with microscopic and macroscopic measurements in a specially designed oedometric cell samples are compacted under different total stresses from 0 1 to 50 mpa in this cell cylindrical cores are submitted to progressive loading from both ends under controlled pore pressure conditions the symmetrical loading allows more even deformation about the midplane of the sample this device allows the evolution of hydraulic pressure radial stress displacement and expelled pore fluid to be followed as a function of time in a first step kaolinite was chosen because it retains a high permeability compared with other clays such as illite or smectite which allows compaction tests to be performed within a few days a complete set of measurements was performed after the tests these measurements are 1 micro structures investigated by means of transmission electron microscopy tem mercury porosimetry water removal under low water vapour pressure granulometry and specific area measured by ethylene glycol adsorption and 2 various physical parameters measured including hydraulic conductivity and thermal conductivity tem gives an understanding of the arrangement of particles it was found that each particle is composed of several crystal units each unit formed by 25 individual kaolinite layers during compaction these particles remain undeformed but are rotated the angular distribution of grain orientation is a function of the applied effective stress this reorientation is in agreement with the observed decrease in porosity and pore size it also explains the occurrence of a strong anisotropy in the thermal conductivity and hydraulic permeability the combination of these experimental results allows a qualitative and quantitative understanding of the behaviour of kaolinite with respect to parameters such as permeability porosity mechanical and thermal properties the knowledge of which are necessary for basin modelling

effects of simulated fault gouge on the sliding behavior of tennessee sandstone nonclay gouges stick slip has been considered to be a mechanism of shallow focus earthquakes along pre existing faults and thus laboratory studies relevant to stick slip may lead to a better understanding of the mechanisms of the earthquakes and eventually to an effective prediction and or control of them this paper deals with the effects of various simulated gouges on the sliding behavior of rocks with special emphasis on how fault gouge alters potentially unstable behavior of a brittle rock about 150 specimens of tennessee sandstone with various simulated gouges along a 35 precut are deformed dry in a triaxial apparatus at room temperature with shortening rate of about 2 10 4cm s and at confining pressures to 300 mpa tennessee sandstone is a brittle and strong rock quartz cemented quartzose sandstone which consistently shows stick slip without simulated gouge at confining pressures above 30 40 mpa sliding behavior with a gouge layer depends markedly on the material used to simulate the gouge experimental results from monomineralic gouges show that mohs hardness of a mineral is a useful parameter to predict the general sliding behavior the most unstable sliding is associated with gouge composed of minerals with intermediate hardness dolomite anhydrite and calcite these gouges undergo the transition from stable sliding to stick slip at pressures of 50 80 mpa gouge composed of hard mineral such as quartz or feldspar behave more stably but stick slip occurs at pressures above 250 mpa only stable sliding is observed for gouge composed of soft minerals like halite although anhydrite gouge shows violent stick slip at 100 mpa pressure whereas only stable sliding occurs for quartz at the same pressure when they are mixed the behavior changes systematically with composition the inclusion of only 10 anhydrite permits stick slip and hence even a minor constituent mineral of fault gouge may alter the sliding mode for gouge composed of two distinct layers the more stabilizing member of the two dominates the behavior and the behavior is considerably different from that of homogeneously mixed gouge with the same bulk composition this suggests the significance of internal structures of natural fault zone in understanding the effects of fault zone materials on the fault motion the gouge composed of calcite or a mineral harder than calcite all show high and approximately the same ultimate frictional strength with an exception of fluorite the frictional strength is intermediate for fluorite galena and barite gouges and it is low for halite gouge

a palaeomagnetic secular variation record for new zealand during the past 2500 years abstract a record of the secular variation of the geomagnetic field over new zealand for the past 2500 years has been compiled by measuring and averaging logs of the primary magnetization of five cores of lacustrine sediment from lake pounui a time scale is provided by four radiocarbon age determinations the record is constrained by historical observations for the last 350 years and is in good agreement with instantaneous palaeomagnetic directions from lava flows this is the first continuous record of secular variation from new zealand and one of only a few from the southern hemisphere together with existing australian records the results show significant evidence of westward drift of the geomagnetic field over the past 2500 years it contains new information for modelling the geomagnetic secular variation in the southern hemisphere and will also be valuable for dating applications within new zealand

frictional properties of simulated fault gouges from the seismogenic groningen gas field under in situ p t chemical conditions we investigated the frictional properties of simulated fault gouges derived from the main lithologies present in the seismogenic groningen gas field ne netherlands employing in situ p t conditions and varying pore fluid salinity direct shear experiments were performed on gouges prepared from the carboniferous shale siltstone substrate the upper rotliegend slochteren sandstone reservoir the overlying ten boer claystone and the basal zechstein anhydrite carbonate caprock at 100 c 40 mpa effective normal stress and sliding velocities of 0 1 10 m s as pore fluids we used pure water 0 5 6 2 m nacl solutions and a 6 9 m mixed chloride brine mimicking the formation fluid our results show a marked mechanical stratigraphy with a maximum friction coefficient of 0 66 for the basal zechstein a minimum of 0 37 for the ten boer claystone 0 6 for the reservoir sandstone and 0 5 for the carboniferous mixed gouges showed intermediate values pore fluid salinity had no effect on frictional strength most gouges showed velocity strengthening behavior with little systematic effect of pore fluid salinity or sliding velocity on a b however basal zechstein gouge showed velocity weakening at low salinities and or sliding velocities as did 50 50 mixtures with sandstone gouge tested with the 6 9 m reservoir brine from a rate and state friction viewpoint our results imply that faults incorporating basal zechstein anhydrite carbonate material at the top of the reservoir are the most prone to accelerating slip that is have the highest seismogenic potential the results are equally relevant to other rotliegend fields in the netherlands and n sea region and to similar sequences globally

crystallographic controls on the frictional behavior of dry and water saturated sheet structure minerals 1 we compare the frictional strengths of 17 sheet structure mineral powders measured under dry and water saturated conditions to identify the factors that cause many of them to be relatively weak the dry coefficient of friction ranges upward from 0 2 for graphite leveling off at 0 8 for margarite clintonite gibbsite kaolinite and lizardite the values of dry correlate directly with calculated 001 interlayer bond strengths of the minerals this correlation occurs because shear becomes localized along boundary and riedel shears and the platy minerals in them rotate into alignment with the shear planes for those gouges with dry 0 8 shear occurs by breaking the interlayer bonds to form new cleavage surfaces where dry 0 8 consistent with byerlee s law the interlayer bonds are sufficiently strong that other frictional processes dominate the transition in dry friction mechanisms corresponds to calculated surface energies of 2 3 j m2 adding water causes to decrease for every mineral tested except graphite if the minerals are separated into groups with similar crystal structures wet increases with increasing interlayer bond strength within each group this relationship also holds for the swelling clay montmorillonite whose water saturated strength is consistent with the strengths of nonswelling clays of similar crystal structure water in the saturated gouges forms thin structured films between the plate surfaces the polar water molecules are bonded to the plate surfaces in proportion to the mineral s surface energy and wet reflects the stresses required to shear through the water films

compaction creep of simulated anhydrite fault gouge by pressure solution theory v experiments and implications for fault sealing the sealing and healing behaviour of faults filled with anhydrite gouge by processes such as pressure solution is of interest in relation both to the integrity of faults cutting geological storage systems sealed by anhydrite caprocks and to seismic events that may nucleate in anhydrite bearing sequences such as those present in the seismogenic zone beneath the apennines we have developed a detailed series of kinetic models for pressure solution in anhydrite fault gouge allowing for dissolution diffusion and precipitation control to estimate the time scale on which such sealing and healing effects occur we compare the models obtained with previously reported experimental data on compaction creep rates in simulated anhydrite fault gouge tested under wet upper crustal conditions the results confirm earlier indications that compaction under these conditions likely occurs by diffusion controlled pressure solution applying our most rigorous model for diffusion controlled pressure solution constrained by the fit to the experimental data we infer that anhydrite fault sealing will occur in a few decades at most which is rapid compared with both co2 storage time scales and with the recurrence interval for seismicity in the apennines

effects of temperature and co2 on the frictional behavior of simulated anhydrite fault rock the frictional behavior of anhydrite bearing faults is of interest to a the safety and effectiveness of co2 storage in anhydrite capped reservoirs b seismicity induced by hydrocarbon production and c natural seismicity nucleated in evaporite formations we performed direct shear experiments on simulated anhydrite fault gouges at a range of temperatures 80 150 c and sliding velocities 0 2 10 ms 1 under a fixed effective normal stress of 25 mpa four types of experiments were conducted 1 dry experiments 2 experiments pressurized with water 3 dry experiments pressurized with co2 and 4 wet experiments pressurized with co2 fluid pressures of 15 mpa were used when applied over the temperature range investigated water saturated samples were found to be up to 15 frictionally weaker than dry equivalents wet samples containing co2 were also up to 15 weaker than co2 free equivalents dry sample strength without co2 was independent of temperature whereas wet samples without co2 strengthened 10 from 80 to 150 c samples containing co2 weakened by 4 dry and 10 wet from 80 to 150 c under the p t conditions investigated only dry anhydrite fault gouge showed velocity weakening behavior above 120 c required for faults to potentially generate earthquakes assuming natural fault gouges are wet in situ seismicity is unlikely to nucleate in anhydrite rich faults though the presence of dolomite or reaction produced calcite may change seismic potential co2 penetration into wet anhydrite rich faults may trigger slip on critically stressed faults due to the observed short term co2 weakening effects excluding possible formation of secondary minerals but is not expected to influence slip stability

strength characteristics of the r f and c slip systems in calcite abstract numerical modelling of crystallographic preferred orientation or texture development in plastically deformed materials requires specification of all potential slip and twinning systems and their relative strengths or critical resolved shear stresses crrs it is widely believed that the most important intracrystalline deformation systems in calcite are twinning on e 1 018 40 4 1 slip on r 10 1 4 2 021 and slip on f 1 012 2 2 01 the superscripted signs indicating slip sense it is these systems which have been used in modelling texture development in calcite polycrystals to date however modelling has been limited by insufficient and partly conflicting data on the relative strengths of the various slip and twinning systems in calcite deformation experiments on calcite single crystals performed by us in recent years taken together provide new data for a more complete and consistent quantification of the strength properties of the calcite glide systems in our experiments single crystals of calcite were compressed in both the 4041 and 2243 orientations at temperatures and constant strain rates in the range 300 800 c and 3 10 4 3 10 8 s 1 respectively the stress strain curves obtained mostly show two yield points slip line analysis shows that these yield points are related to the onset of macroscopically observable slip first on an r 2 021 system and then on an f 10 1 1 or c a system the latter two systems dominate at higher temperature 600 c at strain rate 3 10 5 s 1 and have not been taken into account in any texture modelling so far with increasing absolute temperature tabs calculated crss values c for the r f and c systems decrease in a manner which can be described by empirical power law functions of the type c a tabs b where b r 2 021 and f 10 1 1 systems the yield stresses for r 2 021 and f 10 1 1 slip were found to be rather insensitive to strain rate with conventional power law stress exponents 12 comparing the present results with previous data we conclude that two regimes of slip system activity exist namely a low temperature regime involving e twinning slip on r 2 021 and on f 2 2 01 positive direction not reported and a high temperature regime with r 2 021 f 10 1 1 and c a slip this slip system transition occurs at t 400 c at strain rate 10 4 10 5 s 1 and is expected to be associated with one or more texture transitions the results point to a clear need for renewed texture modelling of calcite polycrystals

microwave palaeointensity from the r3 n3 geomagnetic field reversal summary the microwave palaeointensity technique has been used to determine palaeointensity during the r3 n3 geomagnetic field reversal using lavas from the esja region of southwestern iceland the resulting intensity determinations have been compared to two previous studies which used the shaw and thellier techniques both reported low field intensities during the reversal except for four flows that produced high values 20 4 36 8 t using the shaw technique compared to a single maximum intermediate value of 15 9 t using the thellier method in this paper an average microwave transitional palaeointensity of 6 95 2 07 t is found for samples used in the shaw technique study and of 7 80 1 61 t for samples used in the thellier study demonstrating that there is no evidence for strong fields during this reversal

the cooling rate effect on microwave archeointensity estimates 1 new microwave mw paleointensity data on historical bricks from northeast brazil presented a bias toward higher fields when compared to previous cooling rate corrected double heating paleointensity estimates the same relates to the previously reported values for pottery from southwestern pacific islands a simple theoretical approach suggests that the mw bias in both collections is due to a cooling rate effect on mw estimates we then experimentally corrected the mw cooling rate effect on brazilian fragments increasing the degree of consistency between the previous and new results reducing discrepancies from 25 to 8 results indicate similar experimental behavior between microwave and thermal procedures despite the different ways in which the energy is transferred into the spin system finally they allow cooling times of less than 90 s to be empirically estimated in most of these mw experiments highlighting the need for systematic cooling rate corrections to be applied in similar mw paleointensity studies in the future

frictional properties of sedimentary rocks and natural fault gouge from the longmen shan fault zone sichuan china in this paper we report friction experiments performed on samples collected from the region hit by the 2008 wenchuan earthquake in the longmen shan fault zone lfz of sichuan southwestern china the materials tested consisted of simulated gouges prepared from intact clay rich mudstone and sandstone a calcite limestone plus a natural fault gouge from a trenched surface rupture cutting the mudstone and sandstone the clay rich samples including the natural gouge were dominated by illite and quartz in our experiments we sheared 1 mm thick gouge layers between saw cut driver blocks using a triaxial testing machine at conditions corresponding to 2 km depth in the lfz temperature was varied from 25 c to 150 c and to investigate the velocity dependence of friction we stepped the shear displacement rate between 1 22 and 0 122 m s our results show that the natural gouge was more illite rich and much weaker than the protolith mudstone and sandstone and showed a steady state friction coefficient of 0 4 compared with 0 6 for the latter the limestone gouge displayed values of 0 6 0 7 all samples except the limestone showed stable velocity strengthening slip the limestone showed velocity strengthening at 25 c 50 c but quasi static oscillations at 100 c 150 c along with velocity weakening behavior at 150 c we apply our results to discuss the role of the sedimentary rocks studied during events such as the wenchuan earthquake and argue that the clay rich sediments of the region may have a damping effect upon ruptures propagating from depth whereas the limestone may accelerate propagation producing significant stress drops

velocity dependence of strength and healing behaviour in simulated phyllosilicate bearing fault gouge abstract despite the fact that phyllosilicates are widespread in fault zones little is known about the strength of phyllosilicate bearing fault rocks under brittle ductile transitional conditions in this study we explored the steady state strength and healing behaviour of a simulated phyllosilicate bearing fault rock i e muscovite plus halite and brine at room temperature normal stresses of 1 9 mpa atmospheric fluid pressure and sliding velocities of 0 001 13 m s using a rotary shear apparatus while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behaviour the strength of mixtures containing 10 50 muscovite is both normal stress and sliding velocity dependent at low velocities 1 m s velocity weakening frictional behaviour occurs along with the development of a structureless intermixed cataclastic microstructure the steady state porosity of samples deformed in this regime increases with increasing sliding velocity we propose that this behaviour involves competition between dilatation due to granular flow and compaction due to pressure solution towards higher sliding velocities dilatation increasingly dominates over pressure solution compaction so that porosity increases and frictional strength decreases during periods of zero slip pressure solution compaction occurs causing a significant strength increase on reshearing our results imply that cataclastic overprinting of mylonitic rocks in natural fault zones does not require any changes in temperature or effective pressure conditions but can simply result from oscillating fault motion rates our healing data suggest that foliated aseismically creeping fault segments will remain weak and aseismic whereas segments that have slipped seismically will rapidly re strengthen and remain in the unstable velocity weakening regime

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

the effects of lateral variations in rock composition and texture on anhydrite caprock integrity of co2 storage systems abstract we investigated the effect of rock texture and composition on the mechanical strength and volumetric behaviour of anhydrite rich caprock conventional triaxial experiments were performed at 80 c confining pressures of 1 5 35 mpa and strain rates of 10 5 s 1 both dry and in the presence of fluids we determined the failure envelope and the effect of fluids upon it for anhydrite displaying an equigranular texture consisting of euhedral grains we observed a general pattern of increasing peak compressive strength with increasing confining pressure consistent with a previous study on anhydrite from the same formation which showed an irregular texture consisting of interlocking acicular grains embedded in a fine grained matrix the peak strength of euhedral anhydrite was found to be 15 65 lower than that of acicular anhydrite no chemical effects were observed for the fluids tested hydraulic fracturing experiments showed the tensile strength to be approximately 4 mpa for euhedral and 6 mpa for acicular anhydrite we combined the failure envelopes with simple analytical calculations on the effect of cooling on in situ stress showing that tensile caprock failure due to cooling would be unlikely for both types of anhydrite assuming a hypothetical reservoir at a depth of 3800 m however it should be noted that tensile failure may pose an issue for shallower reservoirs or weaker caprock formations

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

permeability control on transient slip weakening during gypsum dehydration implications for earthquakes in subduction zones a conflict has emerged from recent laboratory experiments regarding the question of whether or not dehydration reactions can promote unstable slip in subduction zones leading to earthquakes although reactions produce mechanical weakening due to pore fluid pressure increase this weakening has been associated with both stable and unstable slip here new results monitoring strength permeability pore fluid pressure reaction progress and microstructural evolution during dehydration reactions are presented to identify the conditions necessary for mechanical instability triaxial experiments are conducted using gypsum and a direct shear sample assembly with constant normal stress that allows the measurement of permeability during sliding tests are conducted with temperature ramp from 70 to 150 c and with different effective confining pressures 50 100 and 150 mpa and velocities 0 1 and 0 4 m s 1 results show that gypsum dehydration to bassanite induces transient stable slip weakening that is controlled by pore fluid pressure and permeability evolution at the onset of dehydration the low permeability promoted by pore compaction induces pore fluid pressure build up and stable slip weakening the increase of bassanite content during the reaction shows clear evidence of dehydration related with the development of r1 riedel shears and p foliation planes where bassanite is preferentially localized along these structures the continued production of bassanite which is stronger than gypsum provides a supporting framework for newly formed pores thus resulting in permeability increase pore fluid pressure drop and fault strength increase after dehydration reaction deformation is characterized by unstable slip on the fully dehydrated reaction product controlled by the transition from velocity strengthening to velocity weakening behaviour of bassanite at temperature above 140 c and the localization of deformation along narrow y shear planes this study highlights the generic conditions required to trigger instabilities during dehydration reactions it shows that pore fluid pressure build up during dehydration reactions associated with the localization of a velocity weakening reacting or dehydrated phase along shear planes is necessary for earthquake triggering

frictional properties of simulated anhydrite dolomite fault gouge and implications for seismogenic potential the frictional properties of anhydrite dolomite fault gouges and the effects of co2 upon them are of key importance in assessing the risks associated with co2 storage in reservoir formations capped by anhydrite dolomite sequences and in understanding seismicity occurring in such formations such as the italian apennines therefore we performed velocity stepping direct shear experiments on simulated dolomite anhydrite and 50 50 anhydrite dolomite gouges at representative in situ conditions 120 c and ne 25 mpa they were conducted under vacuum or else using water or co2 saturated water as pore fluid pf 15 mpa friction coefficients varied between 0 55 and 0 7 all dry samples exhibited velocity weakening behavior whereas all wet samples exhibited velocity strengthening behavior without or with co2 this is consistent with trends previously reported for such gouges a compilation of literature data shows that the transition from velocity strengthening to velocity weakening occurs in these materials between 80 and 120 c when dry and between 100 and 150 c when wet this implies little seismogenic potential for wet dolomite anhydrite and mixed gouges under co2 storage conditions at 2 4 km depth seismic slip in the italian apennines at depths of 6 km and beyond may be explained by the velocity weakening behavior expected in anhydrite and especially dolomite at temperatures above 150 c

fault zone architecture and deformation processes within evaporitic rocks in the upper crust 1 recently in the northern apennines geophysical data have identified the triassic evaporites te anhydrites and dolomites as the source region of the major extensional earthquakes of the area m 6 in order to characterize fault zone architecture and deformation processes within the te we have studied exhumed evaporite bearing normal faults within the upper crust the structure of large displacement 100 m normal faults is given by 1 a zoned fault core with a wider portion of fault parallel foliated ca sulphates ductile deformation overprinted by an inner fault core ifc of localized brittle deformation and 2 wide dolostones to absent ca sulphates damage zones of fault fracture patterns fault rock assemblage within the ifc is characterized by fault breccia gouge and cataclasites of different grain size most of the deformation within the ifc is localized along thin and fault parallel principal slip surfaces pss made of dolomite rich fine grained cataclasite sem analyses show an evolution from ca to st to gypsum rich mineralization due to episodic fluid flow events channeled along the fault zones during different stages of fault exhumation the development of the observed fault geometry can be explained by a mechanical fault evolution model where initial faulting occurs along broad and ductile shear zones within the anhydrites and causes fracturing within the dolostones progressive deformation within the fault core leads to the development of fault parallel dolomite rich cataclastic layers their reactivation coupled with transient fluid overpressures can produce embrittlement and localization of brittle deformation within the ifc

a new three axis vibrating sample magnetometer for continuous high temperature magnetization measurements applications to paleo and archeo intensity determinations we have developed a new three axis vibrating sample magnetometer which allows continuous high temperature magnetization measurements of individual cylindrical 0 75 cm3 samples up to 650 c and the acquisition of thermoremanent magnetization trm in any direction and field intensity up to 200 t we propose a fast less than 2 5 h automated experimental procedure adapted from boyd s nature 319 1986 208 209 modified version of the thellier and thellier ann geophys 15 1959 285 376 method which provides continuous intensity determinations over a large typically 300 c temperature interval for each sample this procedure allows one to take into account both the cooling rate dependence of trm acquisition and anisotropy of trm several examples of analyses of ancient magnetization demonstrate the quality and reliability of the data and illustrate the promising potential of this new instrument in paleo and archeomagnetism

deformation experiments on anhydrite rocks of different grain sizes rheology and microfabric miiller w h schmid s m and briegel u 1981 deformation experiments on anhydrite rocks of different grain sizes rheology and microfabric in g s lister h j behr k weber and h zwart editors the effect of deformation on rocks tectonophysics 78 527 543 three anhydrite rocks of different grain size were experimentally deformed in the 20 to 45o c temperature range the strength of the finegrained varieties becomes strongly temperature and strain rate dependent above 3oo c and a flow law of the form 6 a exp h rt sinh o a with an activation energy h of 27 36 kcal mole and a very low value for the exponent n of 1 5 2 was found the course grained rock remains relatively insensitive to strain rate and temperature even at 45o c microstructural and x ray texture analyses suggest that intracrystalline glide and twinning are the major deformation mechanisms the transition from strong work hardening observed at high stresses into steady state flow at low stresses was found to correlate with the onset of dynamic recrystallization by grain boundary migration

ensembles of low degree archeomagnetic field models for the past three millennia abstract we introduce ensembles of time varying archeomagnetic field models consisting of a reference model a mean model and a thousand individual models we present a set of three such ensembles built from archeomagnetic volcanic and sedimentary data sets that cover the past three millennia these ensembles can be used to describe the field at any location from the core surface to the magnetosphere and assess the way uncertainties due to the limited distribution and quality of the data affect any of its component or parameter such as individual gauss coefficients they provide alternative and we argue more complete descriptions of the archeomagnetic field to those provided by previously published archeomagnetic field models being better suited to existing and emerging needs such as those of geomagnetic data assimilation we present the data sets we rely on essentially the same as those used by other recent archeomagnetic field models and describe how errors affecting the data and errors due to non modelled small spatial scales of the field are taken into account we next explain our modeling strategy and motivation for building low degree spherical harmonic degree ensembles of models we carry on a number of end to end simulations to both illustrate the usefulness of such ensembles and point at the type of errors one should expect practical illustrations of what can be done with these three ensembles of models with examples of geomagnetic inferences are also described northern high latitude flux patches for instance appear to be the most robust features of all these patches tend to fluctuate but clearly have some favored locations resulting in the same clear signature with three tongues over northern america europe and asia in the time averaged field at the core mantle boundary similar to what had been found in earlier models inferences about the field behavior in the southern hemisphere are more difficult to draw still some suggestions that the well known present south atlantic reversed patch could have arisen as early as in 1500 a d are found in some of the ensembles we otherwise confirm that most of the current archeomagnetic field model limitations are related to a number of sediment cores identified as producing frequent outliers in the modeling process we provide evidence that such cores are likely affected by timing errors and timing delays between magnetization lock in and sediment deposition that future more advanced treatment should be able to handle all source files for the three ensembles of models together with appropriate matlab applications can be downloaded from http geomag ipgp fr download archeo fm zip

interseismic fault strengthening and earthquake slip instability friction or cohesion the slip instability of an earthquake and its abrupt energy release depend primarily on the intensity of strength drop during accelerated fault slip this process is typically attributed to changes of frictional resistance between two sliding blocks here we show that friction changes alone cannot explain observed strength variations of artificial fault zones sandstone samples with saw cut faults and gypsum gouge zones were subjected to many cycles of hold slide loading samples with water saturated gouge display 1 systematic time dependent increase of gouge strength 2 unstable failure of gouge with large stress drops and 3 lithification of gouge by crack sealing recrystallization porosity reduction and grain bonding all these features are absent in identical tests with dry gouge these observations indicate that gouge particles are cemented by chemical processes during hold periods and suggest that the cyclical strength variations are controlled by cohesion strengthening rather than by friction changes we further hypothesize that crustal fault zones could be lithified during the interseismic stage and this lithification would control earthquake slip instability

correlation between hydrocarbon reservoir properties and induced seismicity in the netherlands earthquakes induced by gas production are a social concern in the netherlands over the last two decades a total of about 350 such earthquakes have been recorded with magnitudes ranging up to 3 5 on richter s scale the new dutch mining law prescribes the operators to give a quantitative estimation of the likelihood of future seismic activity hazard and the associated damage risk this estimation has to be given for every onshore field producing or to be produced a traditional probabilistic seismic hazard analysis psha can not give an estimation of the hazard for a field before the occurrence of seismic activity we have therefore investigated the correlation between parameters related to reservoir and production properties and the occurrence of induced seismicity in a hydrocarbon field statistically using bayes theorem and the rule of succession three key parameters have been identified that show a good correlation with the occurrence of earthquakes pressure drop fault density of the reservoir and stiffness ratio between seal and reservoir rock based on the observed correlation a probability for the occurrence of earthquakes in fields that have no historical earthquake record has been calculated this has resulted in the definition of four groups of hydrocarbon fields having all a different probability 2006

a low angle normal fault in the umbria region central italy a mechanical model for the related microseismicity abstract in the northern apennines the upper crust is thinned by a set of east dipping low angle normal faults lanfs the easternmost and more recent of these lanfs is the altotiberina fault atf located in northern umbria the geometry of the atf has been reconstructed matching surface geology with seismic reflection profiles and borehole data the fault whose average dip is 20 borders the upper pliocene quaternary tiber basin and has a displacement of about 8 km the deeper portion of the atf is located below the axial zone of the northern apennines where the strongest instrumental and historical seismicity is recorded the microseismicity of the region m 1 to evaluate the boundary conditions for the brittle activity of the atf the fault can be reactivated for low values of differential stress 1 3 t 10 mpa and tensile fluid overpressure p f 3 e g v 0 93 in the peculiar situation of the northern apennines the deep emissions of large amounts of co 2 documented in the area can be entrapped in their ascent by structural seals e g atf favouring localised fluid overpressures the impossibility of sustaining p f 3 for wide fault portions counteracted by hydraulic fracturing increased permeability under low effective stress and load weakening behaviour for normal faulting would prevent the nucleation of moderate ruptures along the fault the short lived attainment of p f 3 along small fault portions can account for the microseismic activity located along the atf which occurs on rupture surfaces in the range of 10 1 10 3 km 2

eastern australian geomagnetic field intensity over the past 14000 yr summary two north eastern australian volcanic crater lake cores have been used to obtain relative intensity estimates for the geomagnetic field arm imparted in a low dc bias field has been used as a normalizing parameter the intensity fluctuations in the two lakes are in excellent agreement with each other and with south eastern australian archaeointensity data over their coeval time spans this strongly suggests that the same sources are influencing the geomagnetic secular variation throughout eastern australia at this time the relative intensity records go back to about 14000yr bp thereby extending currently available recent australian intensity records by some 7000 yr

archaeomagnetic secular variation in germany during the past 2500 years summary the german archaeomagnetic data set was supplemented with 35 new directions from ger man sites mainly dating from the past 3000 yr together with archaeomagnetic data from the neighbouring countries of germany a database of 166 reliable archaeomagnetic directions has been compiled for the reference area which is defined by a 500 km radius circle around gottingen the retrieved directions come from well dated archaeological structures and about 40 per cent of the dating relay on natural science methods such as radiocarbon thermolumines cence dendrochronology dating or historical documents from this data set a secular variation sv reference curve has been calculated using a bivariate algorithm which fits a natural cubic spline based on roughness penalty to declination inclination and time simultaneously the error tube surrounding this curve was obtained from bayesian modelling of the experimental errors which can also take stratigraphic information into account the obtained sv reference curve for the past 2500 yr is similar to that from france but also significant differences are seen comparison of the curves does not show a simple westward drift of the sv pattern the german reference curve allows archaeomagnetic dating in the reference area and extends this dating technique to sites situated in middle europe

advances in archaeomagnetic dating in britain new data new approaches and a new calibration curve abstract archaeomagnetic dating offers a valuable chronological tool for archaeological investigations particularly for dating fired material the method depends on the establishment of a dated record of secular variation of the earth s magnetic field and this paper presents new and updated archaeomagnetic directional data from the uk and geomagnetic secular variation curves arising from them the data are taken from publications from the 1950 s to the present day 422 dated entries derived from existing archaeo and geomagnetic databases are re evaluated and 487 new directions added resulting in 909 entries with corresponding dates the largest collection of dated archaeomagnetic directions from a single country an approach to improving the largest source of uncertainty the independent dating is proposed and applied to the british iron age resulting in 145 directions from currently available databases being updated with revised ages and or uncertainties and a large scale reassessment of age assignments prior to inclusion into the magnetic moments of the past and geomagia50 databases from the significantly improved dataset a new archaeomagnetic dating curve for the uk is derived through the development of a temporally continuous geomagnetic field model and is compared with previous uk archaeomagnetic dating curves and global field models the new model arch uk 1 allows model predictions for any location in the uk with associated uncertainties it is shown to improve precision and accuracy in archaeomagnetic dating and to provide new insight into past geomagnetic field changes

geomagnetic secular variation in the cretaceous normal superchron and in the jurassic abstract it is now widely thought that geomagnetic polarity reversals occur spontaneously as a result of normal dynamo action rather than being externally triggered if this is the case then it may well be that periods of time in which the geomagnetic reversal frequency was dramatically different were characterised by different styles of secular variation two such periods were the cretaceous normal superchron cns 84 125 ma when the field was dominantly of a single polarity for 40 myr and the jurassic period 145 200 ma when reversals occurred at an average rate of as much as 4 6 myr 1 here we analyse a database of new and published palaeomagnetic directions from lavas emplaced during these periods in order to obtain first order descriptions of the palaeosecular variation psv during these times we then compare these records with one another and with that produced for the period 0 5 ma with average reversal frequency of 4 0 myr 1 our results are more equivocal than those obtained in a previous similar study mcfadden p l merrill r t mcelhinny m w lee s h 1991 reversals of the earths magnetic field and temporal variations of the dynamo families journal of geophysical research solid earth and planets 96 3923 3933 we demonstrate that this is probably a result of the previous study being affected by an artefact of their correction for within site scatter the usefulness of our jurassic record is severely limited by the restricted palaeolatitudinal span of the available data however our record for the cns is sufficient to allow us to conclude that it was likely that secular variation then was different from that in the 0 5 ma period this supports the hypothesis of a link between psv and reversal frequency and therefore endorses psv analysis as a first order tool for determining geomagnetic stability in the past

healing and sliding stability of simulated anhydrite fault gouge effects of water temperature and co2 abstract anhydrite bearing faults are currently of interest to 1 co2 storage sites capped by anhydrite caprocks such as those found in the north sea and 2 seismically active faults in evaporite formations such as the italian apennines in order to assess the likelihood of fault reactivation the mode of fault slip and or fault leakage it is important to understand the evolution of frictional strength during periods of no slip and upon reloading healing and relaxation behavior and of the velocity dependence of friction of anhydrite fault gouge therefore we performed slide hold slide experiments combined with a velocity stepping sequence using simulated anhydrite fault gouge 95 wt caso4 vacuum dry and water wet experiments were performed at temperatures ranging from 20 to 150 c and at an effective normal stress of 25 mpa we also performed tests using dry co2 water wetted co2 and co2 saturated water as pore fluid but only at 120 c if pore fluid was present a fluid pressure of 15 mpa was present vacuum dry samples exhibit similar frictional healing to samples containing lab air but healing is significantly enhanced in wet samples dry samples exhibit velocity weakening behavior at t 120 c and wet samples exhibit velocity strengthening behavior over the full temperature range the presence of co2 does not influence the healing behavior or the velocity dependence of friction samples containing water wetted co2 exhibit behavior similar to wet samples we infer that the healing in dry samples is controlled by plastic asperity creep dieterich type possibly through dislocation creep and or twinning in wet samples healing is inferred to be controlled by increases in contact area and cohesion by pressure solution using a pressure solution rate model to extrapolate healing by contact area growth indicates that the maximum re strengthening through such a mechanism will only take days to tens of days

ultra thick triassic dolomites control the rupture behavior of the central apennine seismicity evidence from magnetic modeling of the l aquila fault zone high resolution tomography from the 2009 l aquila extensional seismic sequence has shown that the mw 6 1 main shock and most of the aftershocks occurred within a high velocity body 6 6 vp 6 8 km s located between depths of 3 and 12 km the nature of the high vp body has remained speculative although exhumed mafic deep crustal and upper mantle rocks serpentinites have been favored we used 3d magnetic anomaly modeling to investigate the plausibility of these favored sources for the l aquila body the modeling does not support the presence of high velocity serpentinites with a 30 50 serpentinization degree and gabbros accordingly we conclude that the high vp body may represent non magnetic upper triassic and possibly lower liassic dolomites that have been drilled in neighboring wells for 2 4 km this conclusion is also consistent with the lack of a coherent gravity anomaly for the body we speculate that ultra thick triassic dolomites reaching a thickness of 8 km may have been deposited in syntectonic wedges formed at the northern margin of the ionian sea where oceanic spreading occurred in mid late triassic times

slip weakening as a mechanism for slow earthquakes slow earthquakes form part of a spectrum of fault behaviour between steady creep and fast rupture during a normal earthquake laboratory simulations of slow slip in rock samples taken from the nankai subduction zone japan reveal similar characteristics to fast earthquakes implying that some slow slip events could be prematurely arrested earthquakes

establishing a first archaeointensity record for the sw pacific abstract the dearth of archaeomagnetic intensity data from the southern hemisphere is a limiting factor in evaluating models of global geomagnetic field evolution during the holocene here we present high quality microwave archaeointensity data obtained from 34 ceramic fragments 21 archaeological contexts from the duke of york islands fiji and vanuatu sw pacific complementary thellier type experiments corrected for anisotropy give good agreement with the microwave results the majority of the new data prior to 250 ad exhibit significantly lower intensity than predicted by current global field models cals3k 3 and arch3k for the region with an apparent intensity minimum at 250 bc reaching as low as 50 of the present day field strength between 400 ad and 1500 ad the data are broadly consistent with the global field models but with a 20 higher field between 1200 and 1400 ad these new results therefore imply that the geomagnetic field has a greater range of variability than predicted and that further data from the region are essential to better constrain the global field models the results also demonstrate that establishing an archaeomagnetic reference curve for dating sw pacific ceramic artefacts is feasible which has potentially significant implications for pacific archaeological research

a comparison of a quasi perpendicular method of absolute palaeointensity determination with other thermal and microwave techniques abstract we describe test and explain the relative effectiveness of six thermal and microwave techniques of absolute palaeointensity determination applied to lava samples from mount etna sicily that acquired their thermoremanence trm in a known geomagnetic field the application of a single alternating field demagnetisation treatment prior to each measurement in a standard thellier experiment is demonstrated to be beneficial increasing the quality of the results and making the analysis simpler a new quasi perpendicular qp method dramatically improves the linearity of arai plots by reducing or altogether eliminating non ideal effects due to multidomain md grains two broad types of sample were encountered by this study and the qp method is the only one capable of producing accurate measurements from them both its ease of use speed and increased reliability should allow it to replace standard double heating methods in studies such as these where laboratory alteration can be detected by other means than ptrm checks the results of experiments performed using a microwave palaeointensity system provide further proof that the microwave de magnetisation process is equivalent to conventional thermal excitation they also demonstrate that microwave experiments are subject to the same risks as conventional experiments when performed on samples containing md grains the behaviour of the samples in both the thermal and microwave experiments is in good agreement with a recently proposed model of md trm which is based on first order symmetry of the remagnetisation process

diagenetic compaction experiments on simulated anhydrite fault gouge under static conditions faults that crosscut subsurface co2 storage systems offer potential leakage pathways especially if fault reactivation and dilation occur after reactivation however newly formed fault gouge is expected to gradually compact and seal as a function of time to estimate the time scale on which this occurs the processes that control compaction must be understood we performed uniaxial compaction experiments on simulated anhydrite fault gouge to investigate the deformation mechanisms that operate under postslip conditions in faulted anhydrite caprocks this involved constant stress 5 12 mpa and stress stepping experiments 5 7 5 10 mpa performed at 80 c under dry and wet conditions on fault gouge samples prepared from crushed natural anhydrite sieved into different grain size fractions in the range 20 500 m dry samples showed little to no compaction creep whereas wet samples i e flooded with presaturated caso4 solution showed rapid compaction our mechanical data and microstructural observations on wet samples suggest that for fine grain sizes 50 m and low stresses gouge compaction is controlled by diffusion controlled pressure solution with increasing grain size and stress fluid assisted subcritical microcracking becomes dominant pressurizing solution flooded samples with co2 15 mpa led to no significant effect on compaction rates in fine grained material but it decreased compaction rates in coarse samples since fine grain sizes are expected in reactivated faults we infer that pressure solution will dominate in anhydrite cap rocks with extrapolation of our lab data to reservoir conditions suggesting sealing time scales of a few decades

frictional slip of granite at hydrothermal conditions sliding on faults in much of the continental crust likely occurs at hydrothermal conditions i e at elevated temperature and elevated pressure of aqueous pore fluids yet there have been few relevant laboratory studies to measure the strength sliding behavior and friction constitutive properties of faults at hydrothermal conditions we slid laboratory granite faults containing a layer of granite powder simulated gouge velocity stepping experiments were performed at temperatures of 23 to 600 c pore fluid pressures ph2o of 0 dry and 100 mpa wet effective normal stress of 400 mpa and sliding velocities v of 0 01 to 1 m s 0 32 to 32 m yr conditions were similar to those in earlier tests on dry granite to 845 c by lockner et al 1986 the mechanical results define two regimes the first regime includes dry granite up to at least 845 and wet granite below 250 c in this regime the coefficient of friction is high 0 7 to 0 8 and depends only modestly on temperature slip rate and ph2o the second regime includes wet granite above 350 c in this regime friction decreases considerably with increasing temperature temperature weakening and with decreasing slip rate velocity strengthening these regimes correspond well to those identified in sliding tests on ultrafine quartz we infer that one or more fluid assisted deformation mechanisms are activated in the second hydrothermal regime and operate concurrently with cataclastic flow slip in the first cool and or dry regime is characterized by pervasive shearing and particle size reduction slip in the second hot and wet regime is localized primarily onto narrow shear bands adjacent to the gouge rock interfaces weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide or from a dependence of strength on gouge particle size major features of the granite data set can be fit reasonably well by a rate and temperature dependent three regime friction constitutive model chester this issue we extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural slipping faults for sliding rates as low as 31 mm yr we do this for two end member cases in the first case pore pressure is assumed hydrostatic at all depths shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data while at depths below about 9 13 km depending on slip rate strength becomes less sensitive to depth but sensitive to slip rate in the second case pore pressure is assumed to be near lithostatic at depths below 5 km strength is low at all depths in this case 20 mpa in agreement with observations of weak faults such as the san andreas the predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm yr in rough agreement with the seismic aseismic transition depth observed on mature continental faults these results highlight the importance of fluid assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry large displacements higher concentrations of phyllosilicates and time dependent fault healing

geomagnetic field for 0 3 ka 1 new data sets for global modeling 1 paleomagnetic and archeomagnetic records are used in both regional and global studies of earth s magnetic field we present a description and assessment of five newly compiled data sets also used in the companion paper by korte et al 2009 to produce a series of time varying spherical harmonic models of the geomagnetic field for the last 3000 years data are drawn from our compilation of lake sediment records and from the online database geomagia50v2 the five selections are available from the earthref digital archive at http earthref org cgi bin erda cgi n 944 data are grouped according to the source of material and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows the arch3k dat data set and for sediments sed3k dat the overall number of data is 55 greater than in previous compilations constrained data sets were selected using different criteria for each group winnowing of archeological data was based on uncertainties supplied by the original data providers the lake sediment data assessment relied on preassigned age uncertainties and one or more of the following comparisons with archeomagnetic data from the same region regional consistency among several lakes and consistency with global archeomagnetic models we discuss relative merits of a larger unconstrained data set or a smaller possibly more reliable one the constrained data sets eliminate a priori up to 35 of the available data in each case and rely on potentially subjective assessments of data quality given the limited data available our analyses indicate that iterative rejection of a small number 1 1 5 of outlying data during global field modeling is a preferable approach specific regional comparisons among the models and data support the conclusion that korte et al s outlier free cals3k 3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0 3 ka field the arch3k 1 model provides a better fit to the denser european archeomagnetic data and may be better in that region

archaeomagnetic dating of archaeological sites from switzerland and bulgaria baked materials from three archaeological sites in switzerland and one in bulgaria have been examined and dated archaeomagnetically the detailed rock magnetic properties of the burnt clay collected have already been studied therefore and have been published elsewhere geophysical journal international 153 2003 146 the main aim of this paper is to determine the terminus ante quem for the last firing of the clay collected from pottery kilns or burnt layers the directional and intensity results of the ancient geomagnetic field corresponding to the time of the structure s last firing are given for each site these geomagnetic parameters are then compared with reference curves to provide archaeomagnetic dates for the last use of the kilns application of archaeomagnetism as a dating tool is only possible for those regions that already have reference curves bulgaria is fortunate in having established reference curves covering almost the last 8000 years two swiss sites were dated using only directional results on the basis of existing palaeodirectional results for switzerland as well as the french directional reference curves the archaeomagnetic dates of three of the sites were then compared to archaeological evidence and available radiocarbon dates the palaeointensity data obtained from this study will enrich the existing archaeomagnetic database for western europe

on partial thermoremanent magnetization tail checks in thellier paleointensity determination 1 coarse magnetite grains of multidomain md or large pseudo single domain psd size 10 m approximately cause nonlinearity and other problems in paleomagnetic field intensity determination and can lead to spurious paleointensity values one suggested method of detecting their presence is to carry out an additional zero field heating step following the acquisition of partial thermoremanent magnetization ptrm to check for complete demagnetization of the ptrm residual undemagnetized ptrms ptrm tails are characteristic of md and large psd grains and cause nonideal behavior in thellier type paleointensity experiments we have measured ptrm tails in synthetic and natural magnetites of many sizes and domain states including psd and md starting from two different initial states first a thermally demagnetized sample was given a ptrm by cooling in a field h from the blocking temperature tb the ptrm tail is the remanence mptr remaining after zero field reheating to tb mptr is most prominent for large grain sizes and when tb 500 c we next measured ptrm tails mptr produced in a thellier paleointensity experiment the initial state in this case is a total thermoremanent magnetization gradually reduced by double heatings zero field followed by in field to a set of increasing tb for all synthetic and natural samples containing large psd and md magnetites regardless of grain size or lithologic differences mptr is significant but mptr is negligible that is the ptrm tail checks are always zero in our model thellier experiments however in practical paleointensity studies the ptrm tail check method has been shown to be successful in detecting md grains the reason for these contrasting results is that our experiments were carried out with the field hlab that produced ptrm equal in intensity and parallel to the field h that produced natural remanent magnetization nrm in this special situation the ptrm tail is completely masked for the ptrm tail check method to be most effective either hlab should be applied at a large angle to the nrm direction or hlab should be larger than twice or more the paleofield h this will highlight the ptrm tail in the first case by deflecting the apparent nrm direction after the tail check step and in the second case by increasing the apparent nrm intensity

nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation crystal plasticity at sub seismic slip rates at 18 150 c a central aim in fault mechanics is to understand the microphysical mechanisms controlling aseismic seismic transitions in fault gouges and to identify microstructural indicators for such transitions we present new data on the slip stability of calcite fault gouges and on microstructural development down to the nanometer scale our experiments consisted of direct shear tests performed dry at slip rates of 0 1 10 m s at a constant normal stress of 50 mpa at 18 150 c the results show a transition from stable to potentially unstable slip above 80 c all samples recovered showed an optical microstructure characterized by narrow 30 40 m wide riedel and boundary shear bands marked by extreme grain comminution and a crystallographic preferred orientation cpo boundary shear bands sectioned using fib sem focused ion beam scanning electron microscopy revealed angular grain fragments decreasing from 10 to 20 m at the outer margins to 0 3 m in the shear band core where dense aggregates of nanograins also occurred transmission electron microscopy applied to foils extracted from boundary shears using fib sem combined with the optical cpo showed that these aggregates consist of calcite nanocrystals 5 20 nm in size with the 104 201 dislocation glide system oriented parallel to the shear plane and direction our results suggest that the mechanisms controlling slip include cataclasis and localized crystal plasticity because crystal plasticity is strongly thermally activated we infer that the transition to velocity weakening slip is likely due to enhanced crystal plasticity at 80 c this implies that tectonically active limestone terrains will tend to be particularly prone to shallow focus seismicity

experimental postseismic recovery of fractured rocks assisted by calcite sealing postseismic recovery within fault damage zones involves slow healing of coseismic fractures leading to permeability reduction and strength increase with time to better understand this process experiments were performed by long term fluid percolation with calcite precipitation through predamaged quartz monzonite samples subjected to upper crustal conditions of stress and temperature this resulted in a p wave velocity recovery of 50 of its initial drop after 64 days in contrast the permeability remained more or less constant for the duration of the experiment microstructures fluid chemistry and x ray microtomography demonstrate that incipient calcite sealing and asperity dissolution are responsible for the p wave velocity recovery the permeability is unaffected because calcite precipitates outside of the main flow channels the highly nonparallel evolution of strength recovery and permeability suggests that fluid conduits within fault damage zones can remain open fluid conduits after an earthquake for much longer durations than suggested by the seismic monitoring of fault healing

frictional properties and microstructure of calcite rich fault gouges sheared at sub seismic sliding velocities we report an experimental and microstructural study of the frictional properties of simulated fault gouges prepared from natural limestone 96 caco3 and pure calcite our experiments consisted of direct shear tests performed under dry and wet conditions at an effective normal stress of 50 mpa at 18 150 c and sliding velocities of 0 1 10 m s wet experiments used a pore water pressure of 10 mpa wet gouges typically showed a lower steady state frictional strength 0 6 than dry gouges 0 7 0 8 particularly in the case of the pure calcite samples all runs showed a transition from stable velocity strengthening to potentially unstable velocity weakening slip above 80 100 c all recovered samples showed patchy mirror like surfaces marking boundary shear planes optical study of sections cut normal to the shear plane and parallel to the shear direction showed both boundary and inclined shear bands characterized by extreme grain comminution and a crystallographic preferred orientation cross sections of boundary shears cut normal to the shear direction using focused ion beam sem from pure calcite gouges sheared at 18 and 150 c revealed dense arrays of rounded 0 3 m sized particles in the shear band core transmission electron microscopy showed that these particles consist of 5 20 nm sized calcite nanocrystals all samples showed evidence for cataclasis and crystal plasticity comparing our results with previous models for gouge friction we suggest that frictional behaviour was controlled by competition between crystal plastic and granular flow processes active in the shear bands with water facilitating pressure solution subcritical cracking and intergranular lubrication our data have important implications for the depth of the seismogenic zone in tectonically active limestone terrains contrary to recent claims our data also demonstrate that nanocrystalline mirror like slip surfaces in calcite rich faults are not necessarily indicative of seismic slip rates

absolute paleointensities and paleodirections of miocene and pliocene lavas from fernando de noronha brazil abstract forty four volcanic units were sampled on the equatorial atlantic island of fernando de noronha brazil twenty seven sites are from the 1 8 3 3 myr old quixaba formation and the others are from the approximately 8 12 myr old remedios formation analyses of paleodirections yielded normal and reverse polarities in both formations with virtual geomagnetic poles vgps and vgp dispersion s f of vgp lat 86 9 vgp long 136 3 s f 11 2 for the quixaba formation and vgp lat 86 8 vgp long 196 7 s f 15 5 for the remedios formation successful paleointensity determinations were obtained for 9 of the 27 sampled units in the quixaba formation a modified thellier method was used that includes checks to monitor alterations and repetition of demagnetization steps to detect multidomain tails of partial thermoremanent magnetizations ptrm and magnetization stabilization processes furthermore the additivity of ptrms was monitored during this modified technique of the thellier experiment these additivity checks provide a quality test for correcting the paleointensity determination for magnetomineralogical changes which occur during laboratory treatment such correction methods were used for analyzing approximately 50 of the determinations and lead to a reduction of the within site scatter the mean paleointensity result of 29 3 t corresponding to a virtual axial dipole moment vadm of 7 5 10 22 a m 2 yields approximately the present day field strength mean values of the paleodirections for volcanic units of reversed and normal polarity are closer to a dipolar field configuration than previously observed today s geomagnetic field in the equatorial atlantic shows a significant dipole anomaly our results indicate that this anomaly was far less pronounced in the geological past

holocene geomagnetic secular variation records from north eastern australian lake sediments summary secular variation records have been obtained from cores from lakes barrine and eacham two north eastern australian volcanic crater lakes the results from several cores have been stratigraphically correlated and then stacked and smoothed the chronology provided by radiocarbon dating indicates that the lake eacham sequence spans the last 5700 calendar years the time scale for the lake barrine record is less weil constrained but it

improving our knowledge of rapid geomagnetic field intensity changes observed in europe between 200 and 1400 ad vailable archaeomagnetic data indicate that during the past 2500 yr there have been periods of rapid geomagnetic field intensity fluctuations interspersed with periods of almost constant field strength despite europe being the most widely covered region in terms of archaeomagnetic data the occurrence and the behaviour of these rapid geomagnetic field intensity changes is under discussion and the challenge now is to precisely describe them the aim of this study is to obtain an improved description of the sharp intensity change that took place in western europe around 800 ad as well as to investigate if this peak is observed at the continental scale for this purpose 13 precisely dated early medieval spanish pottery fragments four archaeological french kilns and three collections of bricks used for the construction of different french historical buildings with ages ranging between 335 and 1260 ad have been studied classical thellier experiments performed on 164 specimens and including anisotropy of thermoremanent magnetisation and cooling rate corrections gave 119 reliable results the 10 new high quality mean archaeointensities obtained confirm the existence of an intensity maximum of 85 t at the latitude of paris centred at 800 ad and suggest that a previous abrupt intensity change occurred around 600 ad together with previously published data from western europe that we deem to be the most reliable the new data also suggest the existence of two other abrupt geomagnetic field intensity variations during the 12th century and around the second half of the 13th century ad high quality archaeointensities available from eastern europe indicate that very similar geomagnetic field intensity changes occurred in this region european data indicate that very rapid intensity changes of at least 20 t century took place in the recent history of the earth s magnetic field the results call for additional high quality archaeointensities obtained from precisely dated samples and for a selection of the previously published data if a refined description of geomagnetic field intensity changes at regional scales is to be obtained

the geomagnetic field intensity in new zealand palaeointensities from holocene lava flows of the tongariro volcanic centre very few absolute palaeointensity data exist from holocene aged rocks in new zealand here we present a new suite of high quality palaeointensities supported by detailed rock magnetic investigations samples from 23 sites representing 10 distinct eruptive units of the tongariro volcanic centre taupo volcanic zone new zealand were studied both traditional double heating and microwave palaeointensity methods were employed the reliability of the palaeointensity data varies with rock magnetic properties of the samples corresponding in particular to their positions within the lava flows the highest success rates are from samples obtained from near the flow tops where a significant proportion of the remanence unblocked at intermediate temperatures 200 350 c by contrast samples from flow centres particularly the parts showing platey fracturing have the lowest success rates reliable high quality palaeointensity results ranging from 32 4 5 1 t to 72 1 4 7 t were obtained from six flows with ages between c 12 800 yr bp and the present these correspond to virtual dipole moments that increase from 52 10 zam2 in the early holocene and peak at 112 14 zam2 about 300 yr ago the data agree well with calibrated relative palaeointensities from new zealand lake sediments the volcanic and sedimentary data together yield a holocene virtual axial dipole moment curve that fits the global average variation well in the early holocene but which differs significantly in recent millennia this difference is associated with recent migration of the southern high latitude core mantle boundary flux lobe towards new zealand as is seen in global field models less

temperature and gas brine content affect seismogenic potential of simulated fault gouges derived from groningen gas field caprock we investigated the rate and state frictional properties of simulated anhydrite carbonate fault gouge derived from the basal zechstein caprock overlying the seismogenic groningen gas reservoir in the ne netherlands direct shear experiments were performed at in situ conditions of 50 150 c and 40 mpa effective normal stress using sliding velocities of 0 1 10 m s reservoir pore fluid compositions were simulated using 4 4 molar nacl brine as well as methane air and brine gas mixtures brine saturated samples showed friction coefficients of 0 60 0 69 with little dependence on temperature along with velocity strengthening at 50 100 c transitioning to velocity weakening at 120 c and above by contrast gas filled evacuated and partially brine saturated samples showed values of 0 72 0 02 plus strongly velocity weakening behavior accompanied by stick slip at 100 c the only temperature investigated for gas bearing and dry samples a microphysical model for gouge friction assuming competition between dilatant granular flow and thermally activated compaction creep captures the main trends seen in our brine saturated samples but offers only a qualitative explanation for our gas bearing and dry samples since the reservoir temperature is 100 c our results imply high potential for seismogenic slip nucleation on faults that cross cut and juxtapose the basal zechstein anhydrite carbonate caprock against the groningen reservoir sandstone specifically in the gas filled upper portion of the reservoir system

healing behavior of simulated fault gouges from the groningen gas field and implications for induced fault reactivation we investigated the frictional strength recovery healing and subsequent reactivation and slip weakening behavior of simulated fault gouges derived from key stratigraphic units in the seismogenic groningen gas field n e netherlands direct shear slide hold slide shs experiments were performed at in situ conditions of 100 c 40 mpa effective normal stress and 10 15 mpa pore fluid pressure synthetic formation brine sheared gouges were allowed to heal for periods up to 100 days before subsequent reshearing the initial coefficient of steady sliding friction was highest in the basal zechstein caprock 0 65 0 02 and slochteren sandstone reservoir 0 61 0 02 gouges and the lowest in the ten boer claystone at the reservoir top 0 38 0 01 and in the carboniferous shale substrate 0 45 healing and subsequent reactivation led to a marked increase in static friction coefficient of up to 0 16 in basal zechstein and 0 07 in slochteren sandstone gouges for the longest hold periods investigated followed by a sharp strength drop up to 25 and slip weakening trajectory by contrast the ten boer and carboniferous gouges showed virtually no healing or strength drop healing rates in the basal zechstein and slochteren sandstone gouges were significantly affected by the stiffness of different machines used in line with the ruina slip law and with a microphysical model for gouge healing our results point to marked stratigraphic variation in healed frictional strength and healing rate of faults in the groningen system and high seismogenic potential of healed faults cutting the reservoir and basal zechstein caprock units upon reactivation

geomagnetic field variations in western europe from 1500 bc to 200 ad part ii new intensity secular variation curve abstract in order to extend the secular variation curve svc of archaeointensity in western europe to the first millennium bc we studied 24 kilns and hearths in place two displaced hearths and six sets of pottery sherds from french archaeological sites archaeological artefacts radiocarbon and dendrochronology dated the acquisition of the thermoremanent magnetization trm carried by the studied objects rock magnetism experiments suggest that the main carrier of the magnetization is a ti poor titanomagnetite archaeointensity was determined by the thellier thellier classical protocol with ptrm checks a strict criteria set was applied to select only the most reliable results with linear nrm trm diagrams 55 of total specimens this study demonstrates that pottery sherds with two trms give reliable archaeointensities in the low temperature interval if the nrm trm diagram is adequately adjusted eighteen new mean archaeointensities 14 corrected from the anisotropy of trm and 16 from cooling rate were computed the comparison with previously published western europe paleointensities show a strong dispersion between data primarily due to their variable quality western europe data were weighted following the archaeointensity protocol the number of specimens per site and the type of studied materials in order to better highlight the secular variation of archaeointensity during the first millennium bc the svc built with sliding windows of 160 years shifted every 50 years presents at paris a maximum of 90 t around 800 bc and a minimum of 60 t around 250 bc these archaeointensity maximum and minimum correspond to cusps of the geomagnetic field direction in western europe this new curve is consistent with mesopotamian and eastern europe data the archaeointensity secular variation in western europe predicted by global geomagnetic models cals3k 4 arch3k 1 and arch3k cst 1 is smoother than our svc we used our directional dataset herve et al 2013 to build a new western europe vgps and vdms mean curves comparison with the predictions given by the global models points out a possible persistent non dipole fields effect over europe between 1000 bc and 600 500 bc finally we note that the strong variations of intensity of the geomagnetic field with a mean decrease rate per century close to 6 t will be useful for archaeomagnetic dating purposes

effects of dissolution precipitation processes on the strength and mechanical behavior of quartz gouge at high temperature hydrothermal conditions laboratory experiments on simulated quartz gouges at a temperature of 927 c a confining pressure of 300 mpa and a pore water pressure of 200 mpa demonstrate the interactions between cataclastic and dissolution precipitation processes and their effects on the strength and mechanical behavior significant porosity reduction and microstructures including grain interlocking grain interpenetration and widespread growth of euhedral shaped grains indicate the operation of dissolution precipitation processes under conditions which favor the activity of dissolution precipitation processes i e for small grain size or at slow displacement rates deformation is distributed across the whole gouge layer and strength increases continuously with increasing displacement so that coefficients of friction reach to 0 7 in contrast for specimens with large grain size sheared at fast displacement rates deformation is localized along a gouge forcing block interface after a small amount of displacement and results in slip softening and subsequent quasi stable sliding with coefficients of friction as low as 0 45 analysis of our results indicates that where the kinetics of dissolution precipitation is sufficiently fast to partly accommodate shear strain the mechanical behavior and strength of quartz gouge can be characterized by slip hardening and high strength with velocity strengthening in contrast where cataclastic processes predominantly accommodate shear strain the mechanical behavior and strength can be characterized by slip softening and subsequent localized sliding with low strength and velocity strengthening

a comparison of archaeointensity results from chinese ceramics using microwave and conventional thellier s and shaw s methods palaeo and archaeointensity determinations of the geomagnetic field are limited by mineralogical alteration that may occur when samples are heated in the laboratory to produce a trm thellier thellier 1959 shaw 1974 by using microwave excitation of the magnetic grains we have been able to create a trm without significantly heating the bulk samples thereby avoiding thermal alteration walton et al 1993 shaw et al 1996 when applied to peruvian ceramics in a manner analogous to the standard thellier method the microwave technique was found to reduce the scatter dramatically in the archaeointensity results shaw et al 1996 in the present study we apply this microwave technique to a collection of chinese ceramics covering the time interval 2700 7500 yr bp these ceramics have already been investigated using adaptations of both thellier s coe 1967 and shaw s rolph shaw 1985 palaeointensity methods yang et al 1993a although an acceptable agreement was found between those two methods the equivalent virtual axial dipole moments vadms were significantly lower than for the global model of mcelhinny senanayake 1982 the present study using the microwave technique with cooling rate correction has provided more consistent vadms and is much closer to the global model

a microphysical model for strong velocity weakening in phyllosilicate bearing fault gouges 1 previous rotary shear experiments performed on a halite muscovite fault gouge analogue system have shown that the presence of phyllosilicates under conditions favoring the operation of cataclasis and pressure solution in the matrix phase can have major effects on the frictional behavior of gouges while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behavior the strength of mixtures containing 10 30 muscovite is both normal stress and sliding velocity dependent at high sliding velocities 1 m s 1 such mixtures show unusually marked velocity weakening along with the development of a structureless cataclastic microstructure in the present paper a micromechanical model is developed in an attempt to explain this behavior the model assumes a granular flow process involving competition between intergranular dilatation and compaction by pressure solution the predictions of the model agree favorably with the experimental results extension of the model to quartz mica systems implies that the presence of phyllosilicates plus the operation of pressure solution can strongly promote unstable velocity weakening behavior at rapid slip rates on natural faults under midcrustal conditions static stress drop predictions based on the model agree reasonably well with estimates from seismic observations our results may help explain the discrepancy between laboratory derived rate and state friction parameter values obtained for dry low strain and or single phase rock systems and the values for natural fault rocks inferred from seismological data

dynamical weakening by fluidization under oscillatory viscous flows dynamical weakening of granular materials plays a critical role on diverse geological events such as seismic faulting and landslides a common feature in the dynamics of these processes is the development of fluid solid relative flows which could lead to fluidization by hydrodynamic viscous stresses this work is focused on analyzing hydrodynamic fluidization under oscillatory viscous flows as a possible driving mechanism for dynamical weakening the theoretical estimations and experimental observations presented and reviewed suggest that fluidization can be greatly promoted by oscillatory viscous flows which are usually expected in geological events involving vibration of granular materials in viscous fluids fluidization under oscillatory viscous flows may occur at not excessively large vibration velocities of fine particles in gases or relatively larger particles in liquids or supercritical fluids in particular the enhancement of fluidization by high frequency vibrations would be a powerful mechanism to promote dynamical weakening of fine powders in dry fault gouges failure of liquid or supercritical fluid saturated beds and sustained fluidization of pyroclastic flows and lahars

rate of diffuse carbon dioxide earth degassing estimated from carbon balance of regional aquifers the case of central apennine italy central italy is characterized by an anomalous flux of deeply derived co2 in the western tyrrhenian sector of central italy co2 degassing occurs mainly from focused emissions vents and strong diffuse degassing and thermal springs whereas in the eastern apennine area deep co2 is dissolved in cold groundwaters of regional aquifers hosted by mesozoic carbonate evaporite formations influx of deep co2 into 12 carbonate aquifers 12 500 km2 of the central apennine is computed through a carbon mass balance that couples aquifer geochemistry with isotopic and hydrogeological data mass balance calculations estimate that 6 5 1010 mol yr 1 of inorganic carbon are dissolved in the studied aquifers approximately 23 of this amount derives from biological sources active during the infiltration of the recharge waters 36 comes from carbonate dissolution while 41 is representative of deep carbon sources characterized by a common isotopic signature 13c 3 the calculated deep co2 influx rate ranges from 105 to 107 mol yr 1 km 2 increasing regionally from east to west in the study area

frictional properties and slip stability of active faults within carbonate evaporite sequences the role of dolomite and anhydrite seismological observations show that many destructive earthquakes nucleate within or propagate through thick sequences of carbonates and evaporites for example along the apennines range italy carbonate and evaporite sequences are present at hypocentral depths for recent major earthquakes 5 0 mw 6 3 however little is known about the frictional properties of these rocks which are dominated by dolomite and anhydrite and differ from typical quartzofeldspathic rocks here we report on laboratory experiments designed to illuminate friction constitutive properties and associated microstructures of anhydrite and dolomite fault rocks we performed frictional sliding experiments on powdered samples for a range of normal stresses n from 10 to 150 mpa and sliding velocities from 1 to 300 m s experiments employed the double direct shear configuration and were conducted both dry at room temperature and water saturated at 75 c under true triaxial stresses at 25 c sliding friction is 0 65 for all samples and conditions studied at 75 c samples develop cohesive strength and sliding friction ranges from 0 4 to 0 5 for n from 10 to 35 mpa velocity step tests were used to assess rate state frictional behavior at 25 c the slip behavior is velocity strengthening neutral for all samples microstructural observations indicate localized deformation on r1 riedel and incipient y shear planes with marked grain size reduction at 75 c and larger strain we find that rate state friction behavior correlates with velocity dependent layer dilation and compaction and this is associated with the development of through going y shear planes we conclude that elevated temperature strain and the presence of pore fluids enhance shear localization into continuous y shear planes which could promote frictional instability at seismogenic depth within dolomite anhydrite sequences

archeoint an upgraded compilation of geomagnetic field intensity data for the past ten millennia and its application to the recovery of the past dipole moment 1 this paper presents a compilation of intensity data covering the past 10 millennia archeoint this compilation which upgrades the one of korte et al 2005 contains 3648 data and incorporates additional intensity and directional data sets a large majority of these data 87 were acquired on archeological artifacts and the remaining 13 correspond to data obtained from volcanic products the present compilation also includes important metadata for evaluating the intensity data quality and providing a foundation to guide improved selection criteria we show that 50 of the data set fulfill reasonable reliability standards which take into account the anisotropic nature of most studied objects potsherds the stability of the magnetization and the data dispersion the temporal and geographical distributions of this sub data set are similar to those of the main data set with 72 of the data dated from the past three millennia and 76 obtained from western eurasia approximately half of the selected intensity data are associated with at least an inclination value to constrain the axial and full dipole evolution over the past three millennia requires that we avoid any overrepresentation of the western eurasian data we introduce a first order regional weighting scheme based on the definition of eight widely distributed regions of 30 width within which the selected data are numerous enough the regional curves of virtual axial dipole moments vadm and of mixed vadm virtual dipole moments vdm averaged over sliding windows of 200 years and 500 years testify for strong contributions from either equatorial dipole or nondipole components the computation of global vadm and mixed vadm vdm variation curves assuming an equal weight for each region yields a dipole evolution marked by a distinct minimum around 0 b c a d followed by a maximum around the third fourth century a d a second minimum is present around the eighth century a d this variation pattern is compatible with the one deduced from earlier more

microwave archaeointensities from peruvian ceramics summary accurate determination of the strength of the ancient geomagnetic field is limited by mineralogical alteration that may occur when samples are heated to produce a thermoremanent magnetization thellier thellier 1959 shaw 1974 by using direct microwave excitation of the magnetic grains we have been able to form a thermoremanent magnetization without significantly heating the bulk sample thus avoiding thermal alteration walton et al 1993 incorporation of this new microwave heating technique with the conventional thellier palaeointensity method has dramatically reduced the scatter of archaeointensity results from peruvian ceramics and clearly defined how the field strength has varied in peru over the past 2000 years the new microwave technique will therefore enhance the feasibility of archaeointensity dating and provide accurate field strength data for geomagnetic modelling

chemical mechanical coupling observed for depleted oil reservoirs subjected to long term co2 exposure a case study of the werkendam natural co2 analogue field geological storage of co2 is one of the most promising technologies to rapidly reduce anthropogenic emissions of carbon dioxide in order to ensure storage integrity it is important to understand the effect of long term co2 brine rock interactions on the mechanical behaviour of a storage complex as most of these reactions are too slow to reproduce on laboratory timescales we studied a natural co2 analogue reservoir the rot fringe sandstone werkendam field the netherlands 125 135 ma of co2 exposure and its unreacted counterpart we focused on co2 induced mineralogical and porosity permeability changes and their effect on mechanical behaviour of both intact rock and simulated fault gouge overall co2 exposure did not lead to drastic mineralogical changes the co2 exposed material shows a stronger dependence of permeability on porosity which is attributed to differences in diagenesis closed system diagenesis and hydrocarbon emplacement taking place before co2 charging the limited extent of reaction was in part the result of bitumen coatings protecting specific mineral phases from reaction in local mm sized zones displaying significant anhydrite cement dissolution enhanced porosity was observed for most of the reservoir the long term mechanical behaviour after co2 exposure could be described by the behaviour of the unreacted sandstone while these more porous zones had a lower rock strength in addition co2 exposure did not affect the fault friction behaviour and slip is expected to result in stable sliding simple stress path calculations predict that reservoir failure due to depletion and injection is unlikely even for the porous zones nor will fault reactivation occur for realistic injection scenarios

time series analysis of the 10 000 yr geomagnetic secular variation record from se australia summary a 10 000 yr continuous secular variation record from intensively dated lake sediments in se australia has been subjected to periodogram and maximum entropy method analysis tests on synthetic data reveal some of the limitations of the latter method particularly when applied to complex number series anticlockwise precession of the magnetic vector at a period of 5000 1000 yr is tentatively ascribed to dipole precession and clockwise precession at a period of about 2800 yr is probably due to westward drift of features of the non dipole field the effect of calibrating the radiocarbon time scale is important and results in periodicity shifts of up to 25 per cent even for well dated lacustrine sequences power spectra are poorly constrained it is thus possible that the geomagnetic secular variation on a time scale of thousands of years is more uniform than often supposed mismatches between declination and inclination spectra can arise as a natural consequence of certain types of source mechanism and should not be simply attributed to figments of the analysis employed

slip localization and fault weakening as a consequence of fault gouge strengthening insights from laboratory experiments abstract a laboratory study of simulated quartz gouges was conducted to investigate how solution transfer processes influence the mechanical behaviour of fault wear products at high temperature hydrothermal conditions experiments were performed under nominally dry conditions as well as in the presence of an aqueous pore fluid at elevated temperatures 500 to 927 c and at effective confining pressure conditions 2 3 100 mpa to simulate on a laboratory timescale processes that may be important in fluid active fault zones at depth in the continental crust the mechanical data and microstructural analysis indicate that the kinetics of solution transfer processes can exert a fundamental control on the mechanical behaviour of fault wear products it is found that at nominally dry conditions gouges deform by cataclastic creep and distributed shear with strength and microstructures being relatively unaffected by temperature at moderately chemically reactive hydrothermal conditions 500 600 c coarse grain size or fast deformation rate the presence of a reactive pore fluid slightly reduces the shear strength with respect to dry conditions however at highly chemically reactive hydrothermal conditions 600 927 c small grain size and slow deformation rate rapid porosity reduction is accommodated by dissolution precipitation processes deformation under such conditions results in a fast increase of grain contact area and the development of cohesive bonds between adjacent particles which in turn inhibits cataclastic granular flow with increasing displacement and compaction of the quartz gouge there is a sudden transition from distributed cataclastic flow to slip localization at the interface between the gouge and one of the forcing blocks this deformation mode switch is associated with dramatic weakening up to 50 drop in shear resistance and changes in the apparent coefficient of friction from 0 7 to 0 4 stress drop occurs over many minutes in the laboratory it is speculated that solution assisted gouge compaction and consequent slip localization with associated slow yet dramatic stress drop could provide a mechanism for the occurrence of slow earthquakes

collapse of reacted fracture surface decreases permeability and frictional strength five 3d x ray computed tomography images of eagle ford shale rock coupon pairs one for each condition shown in figure 3 in the related publication details of the data in this part are presented in readme txt for more information about the experiments and methodology please refer to related publication

evidence for rapid geomagnetic field intensity variations in western europe over the past 800 years from new french archeointensity data abstract the number of reliable archeointensity determinations obtained from western europe for the past millennium remains limited moreover the large scatter between different datasets available is puzzling the present study analyzed 31 new groups of baked clay ceramic or brick fragments sampled in france 29 groups and in belgium 2 groups these groups contain several fragments collected from different artefacts and are precisely dated principally from historical constraints between the xiiith and the xixth centuries additionally we re evaluated 14 intensity values that we previously obtained from the same time period the fragments were analyzed using two different thermal methods 1 the in field zero field iz or the izzi version of the classical thellier and thellier method and 2 the triaxe protocol that involves high temperature magnetization measurements data were corrected for the anisotropy of thermoremanent magnetization trm and the dependence of trm acquisition on the cooling rate was taken into account in the different protocols archeointensity data obtained on twin specimens sampled from the same fragment and using both experimental techniques generally show a good agreement i e within 5 at the fragment and at the site level all retained site level averaged intensity results 43 of 45 groups have standard deviations of less than 5 t furthermore groups of approximately the same age have very consistent archeointensity altogether the data presented herein recover a detailed and smoothed geomagnetic field intensity variation curve characterized by two peaks in intensity the first during the second half of the xivth century and the second around ad 1600 followed by a significant decreasing trend in intensity during most the xviith and xviiith centuries this evolution does not satisfactorily fit with the expected intensity values for france derived from geomagnetic field models relying on a different evolution of the axial dipole moment our results lead us to propose that the axial dipole moment decreased from ad 1600 to the end of the xviiith century then slightly increased up to ad 1850 before decreasing again to present day

fluid assisted healing processes in gouge bearing faults insights from experiments on a rock analogue system on the basis of both experimental and field studies solution transfer processes are expected to be important in contributing to fault strength recovery healing under the hydrothermal conditions prevailing in large parts of the seismogenic zone however most experimental work on healing effects in faults has been done using quartzo feldspathic sliding surfaces or fault gouges under conditions in which solution transfer processes are very slow mechanisms of fluid assisted fault healing are accordingly rather poorly understood we have performed slide hold slide fault healing experiments on simulated faults containing brine saturated granular halite as a fault rock analogue halite was used because solution transfer processes are known to be rapid in this system under easily accessible conditions we studied the effects of hold time duration shear stress during hold pre hold sliding velocity and pore fluid composition on strength evolution the results show rapid fluid assisted compaction and stress relaxation during hold periods and a significant hold time dependent strengthening upon re shear the data reveal that healing resulted from both a packing density increase and a contact area strength increase during hold periods the general type of behaviour observed is similar to that observed in quartzo feldspathic gouges at room temperature although important differences were observed as well notably the time dependence of healing deviates from the log linear trend observed in gouges where solution transfer processes are absent this means that if pressure solution is an important healing mechanism in natural faults applying a log linear trend will underestimate natural fault healing rates

radiography of a normal fault system by 64 000 high precision earthquake locations the 2009 l aquila central italy case study 1 we studied the anatomy of the fault system where the 2009 l aquila earthquake mw 6 1 nucleated by means of 64 k high precision earthquake locations spanning 1 year data were analyzed by combining an automatic picking procedure for p and s waves together with cross correlation and double difference location methods reaching a completeness magnitude for the catalogue equal to 0 7 including 425 clusters of similar earthquakes the fault system is composed by two major faults the high angle l aquila fault and the listric campotosto fault both located in the first 10 km of the upper crust we detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops we observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays the width of the l aquila fault zone varies along strike from 0 3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution up to 1 5 km at the fault tips with an increase in the geometrical complexity these characteristics similar to damage zone properties of natural faults underline the key role of aftershocks in fault growth and co seismic rupture propagation processes additionally we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological i e creeping discontinuity explaining how normal faults detach at depth