mineral assemblage anomalies in the slip zone of the 1999 taiwan chi chi earthquake ultrafine particles preserved only in the latest slip zone we determined mineral assemblages of samples from the taiwan chelungpu fault and from milling and heating experiments by using x ray diffraction and scanning and transmission electron microscopy the fault system contains three dominant fault zones the shallowest of which slipped during the 1999 chi chi earthquake the quartz and clay mineral contents of the primary slip zone were low and it contained partly amorphous ultrafine particles several tens of nanometers up to 30 weight percent of materials in that zone could not be fit to standard diffraction patterns whereas nearly 100 weight percent of those in surrounding samples could be the unfitted component could be attributed to the observed ultrafine particles produced by comminution during the earthquake because weak diffraction intensities are caused from mineral lattice distortion granulation and amorphous coatings such particles are a potential proxy for identifying the slip zone of the most recent earthquake along a fault

displacement and dynamic weakening processes in smectite rich gouge from the central deforming zone of the san andreas fault the strength of clay rich gouge from the central deforming zone cdz of the san andreas fault saf was measured using a high speed rotary shear apparatus to evaluate the potential for unstable slip along the creeping segment of the saf wet and dry gouge was sheared at 0 1 1 3 m s 0 5 1 5 mpa normal stress and 1 20 m displacement cdz gouge is weaker wet than dry and exhibits displacement strengthening to peak friction followed by weakening to steady state strength that decreases with increasing velocity a clay foliation unit 2 develops from the initial microstructure unit 1 during the first 1 5 m of slip coincident with increasing strength subsequent weakening occurs during shear within unit 2 and subsequently with development of a localized foliated slip zone unit 4 and fluidized material unit 3 displacement and dynamic weakening result from slip along clay foliation assisted by shear heating pressurization of pore fluid in wet gouge and additional grain size reduction and possible clay dehydration in dry gouge peak strength is proportional to normal stress but steady state strength is insensitive to normal stress probably because pore pressure approaches the normal stress as such cdz gouge is weak at coseismic rates relative to interseismic creep strength the potential for sustaining rupture propagation into the cdz from an adjacent seismic segment is sensitive to the relationship used to extrapolate the critical weakening displacement from experimental to in situ conditions rupture propagation from a microseismic patch within the cdz is unlikely but sustained propagation from a large earthquake e g parkfield event may be possible

dynamic rupture processes inferred from laboratory microearthquakes we report macroscopic stick slip events in saw cut westerly granite samples deformed under controlled upper crustal stress conditions in the laboratory experiments were conducted under triaxial loading 1 2 3 at confining pressures 3 ranging from 10 to 100 mpa a high frequency acoustic monitoring array recorded particle acceleration during macroscopic stick slip events allowing us to estimate rupture speed in addition we record the stress drop dynamically and we show that the dynamic stress drop measured locally close to the fault plane is almost total in the breakdown zone for normal stress 75 mpa while the friction f recovers to values of f 0 4 within only a few hundred microseconds enhanced dynamic weakening is observed to be linked to the melting of asperities which can be well explained by flash heating theory in agreement with our post mortem microstructural analysis relationships between initial state of stress rupture velocities stress drop and energy budget suggest that at high normal stress leading to supershear rupture velocities the rupture processes are more dissipative our observations question the current dichotomy between the fracture energy and the frictional energy in terms of rupture processes a power law scaling of the fracture energy with final slip is observed over eight orders of magnitude in slip from a few microns to tens of meters

ammonium partitioning and nitrogen isotope fractionation among coexisting micas during high temperature fluid rock interactions examples from the new england appalachians abstract despite recent advances in the field of n isotope geochemistry our understanding of the behavior of this element in the solid earth remains limited by a lack of fundamental information regarding the partitioning of ammonium and isotopic fractionation of n among coexisting mineral and fluid phases study of n behavior in regionally metamorphosed rocks provides the opportunity to assess intermineral nh 4 partitioning and n isotope fractionation among coexisting micas during metamorphism and affords an application of the n system as a tracer of high t fluid rock interactions analyzed mica samples range in 15 n air from 3 3 to 11 9 and contain 9 to 1820 ppm n the outcrop at townshend dam vermont allows examination of n behavior across strike on a relatively small scale and samples from western maine demonstrate the effect of varying metamorphic conditions on n behavior in metapelites 15 n bt w mica 15 n biotite 15 n white mica ranges from 0 9 to 2 7 for all samples from both suites mean 0 36 with 1 0 79 with samples containing a separate paragonite white mica phase showing the greatest range 0 12 to 1 02 mean 0 58 1 1 03 thirteen samples containing only na poor muscovite six from townshend dam seven from western maine have mean 15 n bt w mica of 0 07 1 0 41 in both suites biotite nearly always contains more n than coexisting white mica but n w mica n bt also shows some significant scatter mean n w mica n bt 0 46 with 1 0 34 the thirteen samples containing only a na poor muscovitic white mica phase have mean n w mica n bt 0 39 with 1 0 26 similar to that reported by others for other metamorphic suites containing only muscovite as the white mica phase there is no obvious suggestion of equilibrium n isotopic fractionation among coexisting micas at epidote amphibolite to amphibolite facies metamorphic conditions although nh 4 appears to partition systematically among coexisting biotite and white mica significant scatter in both 15 n bt w mica and nh 4 partitioning conceivably the result of differential closure to exchange during cooling or of retrograde replacements could however obscure observation of small equilibrium intermica fractionations related to the characteristics of the interlayer sites in which nh 4 resides samples most unlike the mean in both 15 n bt w mica and n w mica n bt contain abundant chlorite some of which is likely retrograde based on petrographic observations chl chl bt 0 3 thus retrograde replacement of biotite by chlorite may have been accompanied by fluid mineral n isotope exchange perhaps involving the production of fine grained retrograde n bearing white mica observed petrographically for some samples it is also possible that sampling at scales greater than those of n isotope equilibrium domains e g across fine interlayers results in some scatter because of varying relative modal proportions of the two micas in adjacent fine interlayers although further investigation of the extent of retrograde reequilibration of mica n systematics is warranted the observed lack of systematic n isotope fractionation among coexisting micas and the reasonably systematic nh 4 partitioning data for these phases provide important preliminary constraints for attempts to model n isotope behavior in fluid rock systems these results and other attempts to calibrate n isotope fractionation through field studies point to the conspicuous lack of experimentally determined mica fluid n isotope fractionation factors

thermo mechanical pressurization of experimental faults in cohesive rocks during seismic slip earthquakes occur because fault friction weakens with increasing slip and slip rates since the slipping zones of faults are often fluid saturated thermo mechanical pressurization of pore fluids has been invoked as a mechanism responsible for frictional dynamic weakening but experimental evidence is lacking we performed friction experiments normal stress 25 mpa maximal slip rate 3 ms 1 on cohesive basalt and marble under 1 room humidity and 2 immersed in liquid water drained and undrained conditions in both rock types and independently of the presence of fluids up to 80 of frictional weakening was measured in the first 5 cm of slip modest pressurization related weakening appears only at later stages of slip thermo mechanical pressurization weakening of cohesive rocks can be negligible during earthquakes due to the triggering of more efficient fault lubrication mechanisms flash heating frictional melting etc

on the role of phyllosilicates on fault lubrication insight from micro and nanostructural investigations on talc friction experiments 1 exposures of mature faults at the earth s surface show that in the shallow crust some of co seismic slip occurs within a narrow phyllosilicate rich gouge material mechanical behavior of phyllosilicates sheared at seismic slip velocities remains a complex issue as suggested by the experimental and theoretical studies reported so far talc represents a simple case for common phyllosilicates in natural faults and therefore we chose talc as an analogue to better understand the mechanical behavior of clay rich mature crustal faults we present a series of high velocity friction experiments conducted on talc at 1 31 m s 1 and normal stresses of 0 3 1 8 mpa for wet and dry conditions at 1 mpa normal stress both wet and dry experiments show a slip weakening behavior however with a higher decreasing rate in wet conditions dynamic shear stress evolves from a peak value of 0 24 0 52 mpa down to a residual state value of 0 08 0 18 mpa in wet conditions and from a peak value of 0 46 1 17 mpa down to a residual state value of 0 12 0 62 mpa in dry conditions based on a detailed microstructural analysis down to the nanoscale we propose thermal pressurization as a possible slip weakening mechanism for wet conditions on the contrary in dry conditions the long lasting weakening is interpreted to be due to a combination of i progressive disappearance of geometrical incompatibilities ii solid lubrication of talc lamellae and iii powder lubrication by nanometric aggregates we conclude that initial gouge humid conditions and inherited fabric from past sliding may have large influence on the slip weakening for subsequent slip

water vaporization promotes coseismic fluid pressurization and buffers temperature rise we investigated the frictional properties of carbonate rich gouge layers at a slip rate of 1 3 m s under dry and water saturated conditions while monitoring temperature at different locations on one of the gouge host rock interfaces all experiments showed a peak frictional strength of 0 4 0 7 followed by strong slip weakening to steady state values of 0 1 0 3 experiments which used a pore fluid with a constant drainage path to the atmosphere showed the development of a temperature plateau beyond 100 c contemporaneous with the dynamic slip weakening and consistent with thermodynamic considerations of ongoing vaporization of pore water upon pore fluid vaporization the pore pressure increases while the temperature is buffered endothermically such that the pore water moves along the liquid vapor transition curve in a pressure temperature phase diagram pore fluid phase transitions of this kind are expected to occur in natural earthquakes at relatively shallow crustal levels enhancing fluid pressurization while impeding the achievement of high temperatures therefore the operation of vaporization may help explain the low downhole temperature anomalies obtained shortly after large earthquakes

earthquakes produce carbon dioxide in crustal faults abstract fourier transform infrared ftir microanalysis of pseudotachylytes i e friction induced melts produced by seismic slip from the nojima fault japan reveals that earthquakes almost instantaneously expel 99 wt of the wall rock co 2 content carbon is exsolved because it is supersaturated in the friction melts by extrapolation to a crustal scale fault rupture large events such as the m7 2 kobe earthquake 1995 may yield a total production of 1 8 to 3 4 10 3 tons co 2 within a few seconds this extraordinary release of co 2 can cause a flash fluid pressure increase in the fault plane and therefore enhance earthquake slip or trigger aftershocks it may also explain the anomalous discharge of carbon monitored in nearby fault springs after large earthquakes because carbon saturation in silicate melts is pressure dependent ftir can be used as a new tool to constrain the maximum depth of pseudotachylyte formation in exhumed faults

composition and stable isotope geochemistry of natural gases from kansas midcontinent u s a abstract more than 28 1012 ft 3 79 1010 m3 of natural gas and 5 3 109 bbl 8 4 108 m3 of oil have been produced in kansas u s a from paleozoic carbonate and sandstone reservoirs on structural uplifts and shallow embayments along the northern margin of the anadarko basin a heavily explored geologically well characterized state kansas is an excellent place to study hydrocarbon migration and to test geochemical models for the origin of natural gases immature to marginally mature rocks of eastern kansas cherokee and forest city basins produce mixed microbial and thermogenic gases gases in this region have wetness 0 03 51 methane 13c 65 to 43 and methane d 260 to 150 gases from central and western kansas nemaha uplift to hugoton embayment are entirely thermogenic and have wetness 4 51 methane 13c 48 to 39 and methane d 195 to 140 ethane and propane 13c values throughout kansas vary from 38 to 28 and from 35 to 24 respectively mature thermogenic gas generated from source rocks in southwestern kansas and the anadarko basin with 1 0 ro 1 4 is recognized throughout the state lateral migration into shallow reservoirs on the central kansas and northern nemaha uplifts and in the cherokee basin probably occurred along basal pennsylvanian conglomerates and weathered lower paleozoic carbonates at the regional sub pennsylvanian unconformity early thermogenic gas generated by local source rocks with ro 0 7 is recognized in isolated fields in the salina and forest city basins in ordovician reservoirs beneath the sub pennsylvanian unconformity in the cherokee basin and in reservoirs generally above the unconformity in the cherokee and sedgwick basins the eastern central kansas uplift and the hugoton embayment

mass removal and clay mineral dehydration rehydration in carbonate rich surface exposures of the 2008 wenchuan earthquake fault geochemical evidence and implications for fault zone evolution and coseismic slip 1 we report variations in the mineralogical geochemical and isotopic 13c 18o composition of fault rocks sampled in transects across the zhaojiagou and pingxi exposures of the wenchuan earthquake or longmenshan fault zone where the gouge rich fault core and principal slip surface cuts through carbonate rich strata pervasive fluid infiltration was found to modify the mineralogical and geochemical architecture of the fault zones studied enrichment depletion patterns element partitioning and a very large implied volume loss are quite different from those characterizing faults in granites and clastic sedimentary rocks and can be explained by a mass removal model involving dissolution and advective transport enhanced by pressure solution an increasing enrichment in smectite observed toward the principal slip surface a high abundance of elements such as ba mg and f the deposition of minerals such as barite and fluorapatite as well as the distinct depletion in 13c in vein material consistently suggest reactions involving a hydrothermal fluid originating at depth illitization of black gouges caused by coseismic frictional heating was found to be widespread we propose that coseismic frictional heating along with the action of postseismic hydrothermal fluids controlled the transformation and distribution of smectite and illite within the fault core of the longmenshan fault zone the coseismic dewatering reactions are expected to have been more extensive at depth possibly helping generate excess pore pressure assisting dynamic slip weakening during the wenchuan earthquake

rupture reactivation during the 2011 mw 9 0 tohoku earthquake dynamic rupture and ground motion simulations near source ground motion observations and kinematic source inversions suggest that the rupture process of the 2011 m w 9 0 tohoku earthquake involved rupture reactivation that is repeated rupture nucleation in the same hypocentral area this unusual phenomenon may have provided a second breath to the rupture that enhanced its final size here we propose that rupture reactivation may have been governed by a slip weakening friction model with two sequential strength drops the second one being activated at large slip such frictional behavior has been previously observed in laboratory experiments and attributed to pressurization of fault zone fluids by mineral decomposition reactions activated by shear heating such as dehydration and decarbonation further evidence of this double slip weakening friction model is obtained here from the dynamic stress changes in the hypocentral region derived from a finite source inversion model we incorporate this friction model in a dynamic rupture simulation comprising two main asperities constrained by source inversion models and several deep small asperities constrained by backprojection source imaging studies our simulation produces ground motion patterns along the japanese coast consistent with observations and rupture patterns consistent with a kinematic source model featuring rupture reactivation the deep small asperities serve as a bridge to connect the two main asperities and the rupture reactivation mechanism is needed to reproduce the observed ground motion pattern therefore we argue that rupture reactivation during the 2011 tohoku earthquake is consistent with a second strength drop possibly caused by activation of thermochemical weakening processes at large slip online material movie of simulated rupture and wave propagation and ground velocities recorded at the kik net and k net seismic networks

extreme dynamic weakening of faults during dehydration by coseismic shear heating the dynamic strength of seismogenic faults has a critical effect on earthquake slip instability and seismic energy release high velocity friction experiments on simulated faults in serpentinite at earthquake slip conditions show a decrease in friction coefficient from 0 6 to 0 15 as the slip velocity reaches 1 1 m s at normal stresses up to 24 5 mpa the extraordinary reduction in fault strength is attributed to flash heating at asperity contacts of gouge particles formed during sliding the rapid heating at asperities causes serpentine dehydration in impermeable fault zones in nature serpentine dehydration and subsequent fluid pressurization due to coseismic frictional heating may promote further weakening this dynamic fault weakening mechanism may explain the lack of pronounced heat flow in major crustal faults such as the san andreas

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

mineralogical characterization of protolith and fault rocks from the safod main hole 1 washed cuttings provide a continuous record of the rocks encountered during drilling of the main hole of the san andreas fault observatory at depth safod both protolith and fault rocks exhibit a wide variety of mineral assemblages that reflect variations in some combination of lithology p t conditions deformation mechanisms and fluid composition and abundance regions of distinct neomineralization bounded by faults may record alteration associated with fluid reservoirs confined by faults in addition both smectites occurring as mixed layer phases and serpentine minerals are found in association with active strands of the san andreas fault that were intersected during drilling although their rheological influence is not yet fully known faults containing these mineralogical phases are prime candidates for continuous coring during phase 3 of safod drilling in the summer of 2007

noble gases in crude oils from the paris basin france implications for the origin of fluids and constraints on oil water gas interactions in order to investigate the potential of noble gases to trace the dynamics of oil reservoirs we have analysed the abundance and isotopic composition of all noble gases he ne ar kr and xe in crude oils from the paris basin france using a new extraction and purification procedure the main oil reservoirs are presently located in the jurassic dogger limestone and in the triassic keuper sandstone but hydrocarbons originated from a common source rock formation located in the interbedded liassic sequence despite this common origin the abundance and isotopic ratios of the noble gases differ between the dogger and the keuper the isotopic compositions of kr and xe are indistinguishable from that of air 3he 4he ratios higher than those predicted from radiogenic production in the sediments or in the crust are attributed to the occurrence of mantle derived 3he in the basin each sedimentary sequence is characterised by well defined and homogeneous 21ne 22ne and 40ar 36ar ratios which average 0 0306 0 0008 and 312 10 for the dogger and 0 0367 0 0012 and 664 30 for the keuper respectively the main source of radiogenic noble gases appears to be the continental crust underlying the basin with possible regional contributions of noble gas isotopes produced in the sediments the helium and argon isotopic ratios of the dogger oils are very similar to those observed in geothermal waters flowing in the dogger aquifer throughout the basin demonstrating that noble gases in oils derive from associated groundwaters oil reservoirs in the paris basin therefore accumulate noble gases from wide regions of the continental crust through cross formational flow of groundwaters and subsequent partitioning into oil this observation implies that noble gases cannot be directly used to date oils but can provide time constraints if 1 water oil interactions are quantified and 2 the residence time as well as the noble gas characteristics of associated groundwaters are known oil water gas partitioning processes are well recorded in the fractionation of noble gas elemental abundance two distinct processes have been identified 1 accumulation of atmosphere derived ang and radiogenic noble gases both initially dissolved in groundwaters resulting in a positive correlation between absolute amount of ang and the extent of heavy noble gas fractionation and 2 subsequent fractional degassing resulting in a negative correlation between ang abundance and heavy noble gas fractionation degassing is particularly evident for the keuper oils and might have occurred in the reservoirs following hydrodynamic gas stripping the ang abundance in the dogger reservoirs requires that about one order of magnitude more water than presently observed has interacted with the oil given current estimates of the residence time for groundwaters in the dogger aquifer the duration of oil water interaction is in qualitative agreement with a palaeocene oligocene age for the major episode of secondary oil migration in the paris basin high xenon contents in keuper reservoirs suggest that they have experienced hydrodynamic interactions between flowing waters and oils for long time and that the trias might have reached hydrostatic condition only recently

origin of natural gas in sacramento basin california the remnant of a late mesozoic early cenozoic fore arch basin the sacramento valley is principally a dry gas province complex stratigraphic relations widespread deformation and faulting and the presence of igneous intrusives testify to an active tectonic history sediments are organically immature down to an average of 10 000 ft 3 000 m and are geopressured throughout much of the basin small fields with nitrogen concentrations as high as 88 are found along the shallow eastern margin gases were sampled from 94 producing wells methane dgr13c and dgrd values range from 61 to 15 pmil and 200 to 96 pmil respectively indicating a wide range of source rock maturities wet gases in the western delta area sgrc2 c5 sgrc1 c5 12 5 have ethane dgr13c values between 31 and 20 pmil and propane dgr13c values between 27 and 12 pmil dry gases elsewhere in the basin sgrc2 c5 sgrc1 c5 1 6 have ethane dgr13c values as light as 58 pmil the chemical and stable isotope data are consistent with four principal sources gases in the western delta area were generated from mature source rocks within the oil and condensate window gases in the northern and southeastern basin are mixtures of indigenous gas mostly microbial methane with small quantities of diagenetic ethane dry thermogenic gas migrated from post mature source rocks in the basin deep and nitrogen rich gases thought to originate in metasedimentary rocks deep beneath the sediment these results place logical constraints on the location of productive traps with high btu gas and suggest caution in the interpretation of stable isotope data from other tectonically active basins due to dynamic migration and mixing processes

frictional melting of clayey gouge during seismic fault slip experimental observation and implications clayey gouges are common in fault slip zones at shallow depths thus the fault zone processes and frictional behaviors of the gouges are critical to understanding seismic slip at these depths we conducted rotary shear tests on clayey gouge 41 wt clay minerals at a seismic slip rate of 1 3 m s here we report that the gouge was melted at 5 mpa of normal stress and room humidity conditions the initial local melting was followed by melt layer formation clay minerals e g smectite and illite and plagioclase were melted and quenched to glass with numerous vesicles both flash heating and bulk temperature increases appear to be responsible for the melting this observation of clayey gouge melting is comparable to that of natural faults e g chelungpu fault taiwan due to heterogeneous fault zone properties e g permeability frictional melting may be one of the important processes in clayey slip zones at shallow depths

the mechanics of lubricated faults insights from 3 d numerical models 1 the weakening mechanisms occurring during an earthquake failure are of prominent importance in determining the resulting energy release and the seismic waves excitation in this paper we consider the fully dynamic response of a seismogenic structure where lubrication processes take place in particular we numerically model the spontaneous propagation of a 3 d rupture in a fault zone where the frictional resistance is controlled by the properties of a low viscosity slurry formed by gouge particles and fluids this model allows for the description of the fault motion in the extreme case of vanishing effective normal stress by considering a viscous fault response and therefore without the need to invoke in the framework of coulomb friction the generation of the tensile mode of fracture we explore the effects of the parameters controlling the resulting governing law for such a lubricated fault the viscosity of the slurry the roughness of the fault surfaces and the thickness of the slurry film our results indicate that lubricated faults produce a nearly complete stress drop i e a very low residual friction coefficient 0 01 a high fracture energy density eg few 10s of mj m2 and significant slip velocities vpeak few 10s of m s the resulting values of the equivalent characteristic slip weakening distance d0eq 0 1 0 8 m depending on the adopted parameters are compatible with the seismological inferences moreover in the framework of our model we found that supershear ruptures are highly favored in the case of enlarging gap height we can have the healing of slip or even the inhibition of the rupture quantitative comparisons with different weakening mechanisms previously proposed in the literature such as the exponential weakening and the frictional melting are also discussed

thermal decomposition of carbonates in fault zones slip weakening and temperature limiting effects 1 during an earthquake the heat generated by fault friction may be large enough to activate the devolatilization of minerals forming the fault rocks in this paper we model the mechanical effects of calcite thermal decomposition on the slip behavior of a fault zone during an earthquake to do so we introduce the coupled effects of calcite volume loss heat consumption and co2 production in the theoretical analysis of shear heating and thermal pressurization of pore fluids we consider a rapidly deforming shear band consisting of a fluid saturated carbonate rock the equations that govern the evolution of pore pressure and temperature inside the band and the mass of emitted co2 are deduced from the mass and energy balance of the multiphase saturated medium and from the kinetics of the chemical decomposition of calcite numerical simulation of seismic slip at depths of 5 to 8 km show that decarbonation has two critical consequences on fault slip first the endothermic reaction of calcite decomposition limits the coseismic temperature increase to less than 800 c corresponding to the initiation of the chemical reaction inside the shear band second the rapid emission of co2 by decarbonation significantly increases the slip weakening effect of thermal pressurization the pore pressure reaches a maximum and then decreases due to the reduction of solid volume causing a restrengthening of shear stress our theoretical study shows on the example of decarbonation that the thermal decomposition of minerals is an important slip weakening process and that a large part of the frictional heat of earthquakes may go into endothermic devolatilization reactions

vaporization of fault water during seismic slip laboratory and numerical studies as well as field observations indicate that phase transitions of pore water might be an important process in large earthquakes we present a model of the thermo hydro chemo mechanical processes including a two phase mixture model to incorporate the phase transitions of pore water occurring during fast slip i e a natural earthquake in order to investigate the effects of vaporization on the coseismic slip using parameters from typical natural faults our modeling shows that vaporization can indeed occur at the shallow depths of an earthquake irrespective of the wide variability of the parameters involved sliding velocity friction coefficient gouge permeability and porosity and shear induced dilatancy due to the fast kinetics water vaporization can cause a rapid slip weakening even when the hydrological conditions of the fault zone are not favorable for thermal pressurization e g when permeability is high at the same time the latent heat associated with the phase transition causes the temperature rise in the slip zone to be buffered our parametric analyses reveal that the amount of frictional work is the principal factor controlling the onset and activity of vaporization and that it can easily be achieved in earthquakes our study shows that coseismic pore fluid vaporization might have played important roles at shallow depths of large earthquakes by enhancing slip weakening and buffering the temperature rise the combined effects may provide an alternative explanation for the fact that low temperature anomalies were measured in the slip zones at shallow depths of large earthquakes

helium accumulation in groundwater ii a model for the accumulation of the crustal 4he degassing flux abstract a model of 4 he accumulation in a confined aquifer is presented which includes in situ production and a crustal degassing flux of 4 he into the bottom of a confined aquifer the model is consistent with the measurements of 4 he concentration in the great artesian basin australia torgersen and clarke 1985 and also with the measurements of 4 he concentration in the auob sandstone of namibia heaton 1981 the measured value of the crustal degassing flux of 4 he from these two areas is comparable with the 4 he degassing flux calculated from the atmospheric budget of 4 he wasserburg et al 1963 ozima and podosek 1983 a literature review suggests that a crustal degassing 4 he source may account for the 4 he accumulation in many other groundwater systems it is concluded from the literature review the detailed modelling of 4 he accumulation in gab and the auob sandstone and the atmospheric budget that crustal degassing of 4 he may be a common and widespread phenomena

regional groundwater focusing of nitrogen and noble gases into the hugoton panhandle giant gas field usa the hugoton panhandle giant gas field located across sw kansas and the texas and oklahoma panhandles in the usa is the case type example of high nitrogen concentrations in a natural gas being linked with high helium concentrations we collected 31 samples from producing wells in a north south traverse of the 350 km long field the samples reflect the previously observed north south change in 4he n2 with values changing from 0 020 to 0 049 respectively 3he 4he 21ne 22ne and 40ar 36ar vary between 0 14 0 25 ra 0 0373 0 0508 and 818 1156 respectively and are caused by quantifiable contributions from mantle crustal and atmosphere derived sources the atmosphere derived 20ne 36ar ratios are indistinguishable from groundwater values the crustal 4he 21ne and 4he 40ar ratios show a 60 excess of 4he compared to predicted production ratios in the crust and are typical of noble gases released from the shallow crust the mantle 3he n2 and groundwater recharge 36ar n2 ratios enable us to rule out significant magmatic or atmosphere contributions to the gas field n2 which is dominantly crustal in origin correlated 20ne n2 and 4he n2 shows mixing between two distinct crustal n2 components one n2 component n2 is associated with the crustal 4he and groundwater derived 20ne and the other with no resolvable noble gas contribution measured 15nn2 values vary from 2 7 to 9 4 the n2 and non he associated n2 endmembers are inferred to have 15nn2 3 and 13 and contribute from between 25 60 and 75 40 of the nitrogen respectively the non he associated nitrogen is probably derived from relatively mature organic matter in the sedimentary column the 15nn2 value is not compatible with a crystalline or high grade metamorphic source and similar to the 4he is inferred to be from a shallow or low metamorphic grade source rock 4he mass balance requires a regional crustal source its association with significant magmatic 3he pointing to a tectonically active source to the west of the hugoton system the volume of groundwater required to source the 20ne in the gas field demonstrates the viability of the groundwater system in providing the collection transport and focusing mechanism for the 4he and n2 the n2 20ne ratio shows that the n2 transport must be in the aqueous phase and that the degassing mechanism is probably contact between the regional groundwater system and the preexisting reservoir hydrocarbon gas phase

natural gas association with water and oil as depicted by atmospheric noble gases case studies from the southeastern mediterranean coastal plain a model for the equilibrium distribution of atmospheric noble gases ang between natural gas and water phases was extended to include the gas oil relationship the basic assumption is that the ang are introduced into hydrocarbon systems via recharge or seawater theoretical ang patterns of natural gas at equilibrium with water fall between the air pattern and that of air saturated water theoretical ang patterns of natural gas degassed from oil which had previously equilibrated with a water phase cover a wide range including the range calculated for gas water equilibration ang patterns in natural gas which fall outside the overlapping range may thus indicate association of the gas with oil the model was tested in the case of hydrocarbon systems of the southeastern mediterranean coastal plain other geochemical evidence indicates that the ashdod sadot shallow gas wells are not related to oil the relative ang abundances in gas samples from these wells lie within the range predicted for gas water association the relative ang abundances in samples degassed from oils of the helez kokhav oilfield are similar to specific patterns predicted for the degassing of oil which was previously associated with water published and new analyses of methane dominated natural gas samples from california japan and northern israel also reveal ang patterns of the type predicted by the model the results of the case studies support our hypothesis that water is responsible for transport of atmospheric noble gases into subsurface hydrocarbon systems this forms the basis for identification of oil associated natural gases via ang study

u pb ages of plutonism wollastonite formation and deformation in the central part of the lac saint jean anorthosite suite the lac saint jean anorthosite suite lsjas is the largest ensemble of plutons in the grenville province zircons from a pluton in the central part of the lsjas yield an age of 1140 10 5 ma which accords with existing 1157 1142 ma ages determined from the southern part of the lsjas the du bras granite is a linear pluton emplaced into the dominantly dextral pipmuacan shear zone psz the u pb zircon age of 1148 2 ma for the du bras granite confirms earlier proposals that it is a granophyre formed at greater depths by partial melting of paragneiss by the heat of the crystallizing anorthosite parts of the pluton are strongly deformed by the shear zone and other parts cut the shear zone this suggests that the pluton dates one phase of movement of the psz the canton st onge wollastonite deposit was developed in paragneisses preserved within the psz titanite from this deposit gives an age of 1163 18 ma identical within error to that of the du bras granite indicating that this pluton may have pro

dual megathrust slip behaviors of the 2014 iquique earthquake sequence the transition between seismic rupture and aseismic creep is of central interest to better understand the mechanics of subduction processes a mw 8 2 earthquake occurred on april 1st 2014 in the iquique seismic gap of northern chile this event was preceded by a long foreshock sequence including a 2 week long migration of seismicity initiated by a mw 6 7 earthquake repeating earthquakes were found among the foreshock sequence that migrated towards the mainshock hypocenter suggesting a large scale slow slip event on the megathrust preceding the mainshock the variations of the recurrence times of the repeating earthquakes highlight the diverse seismic and aseismic slip behaviors on different megathrust segments the repeaters that were active only before the mainshock recurred more often and were distributed in areas of substantial coseismic slip while repeaters that occurred both before and after the mainshock were in the area complementary to the mainshock rupture the spatiotemporal distribution of the repeating earthquakes illustrates the essential role of propagating aseismic slip leading up to the mainshock and illuminates the distribution of postseismic afterslip various finite fault models indicate that the largest coseismic slip generally occurred down dip from the foreshock activity and the mainshock hypocenter source imaging by teleseismic back projection indicates an initial down dip propagation stage followed by a rupture expansion stage in the first stage the finite fault models show an emergent onset of moment rate at low frequency 0 5 hz 0 5 hz this indicates frequency dependent manifestations of seismic radiation in the low stress foreshock region in the second stage the rupture expands in rich bursts along the rim of a semi elliptical region with episodes of re ruptures suggesting delayed failure of asperities the high frequency rupture remains within an area of local high trench parallel gravity anomaly tpga suggesting the presence of subducting seamounts that promote high frequency generation our results highlight the complexity of the interactions between large scale aseismic slow slip and dynamic ruptures of megathrust earthquakes

helium accumulation in groundwater iii limits on helium transfer across the mantle crust boundary beneath australia and the magnitude of mantle degassing abstract the groundwaters of the great artesian basin australia have been previously shown to be accumulating in situ production helium for groundwaters ages 100 kyr 1 2 new helium isotope measurements show that the observed in situ production helium 3he 4he 1 6 10 8 is isotopically distinct from the crustal degassing helium flux 3he 4he 6 6 10 8 furthermore the crustal degassing helium isotope ratio is marginally in excess of the whole crustal production ratio 3he 4he 3 5 10 8 and the production ratio in a variety of continental rock types this suggests that the upper limit on volatile transport across the mantle crust boundary beneath the relatively stable and complacent australian continent can be characterized by a conductive diffusive helium heat flux ratio of 2 6 1064he atoms mw 1 s 1 which is two orders of magnitude less than the intrusive volcanic ratio of 2 9 1084he atoms mw 1 s 1 measured at the galapagos 16 these results constrain the transcrustal mantle degassing fluxes of4he and40ar to be much less than the mid ocean ridge degassing fluxes which are much less than the degassing of4he and40ar from continental crust thus the degassing of the earth s interior is dominated by magmatic processes but the dominant fluxes of4he and40ar to the atmosphere must come from the continental crust

influence of dilatancy on the frictional constitutive behavior of a saturated fault zone under a variety of drainage conditions we use numerical simulations to investigate how fault zone dilatancy and pore fluid decompression influence fault strength and friction constitutive behavior dilatant hardening can change the frictional response and the effective critical stiffness kcr which determines the transition from stable to unstable sliding in velocity weakening fault zones we study the frictional shear strength response to numerical velocity stepping experiments and show that when the duration of pore fluid decompression is long compared to the time necessary for frictional evolution as dictated by rate and state friction both the effective critical slip distance dc and the effective shear strength direct effect a are increased we investigate the role of fault zone permeability k dilatancy coefficient varepsilon and the magnitude of shearing velocity of the fault zone vlp and compare results using the dieterich and ruina state evolution laws over the range from k 10 15 to 10 21 m2 dc increases from 25 m to 2 mm and a increases from 0 15 to 5 mpa we vary varepsilon from 10 5 to 10 3 and the size of the velocity perturbation from 3x to 1000x and find large increases in the values of dc and a which may lead to inhibition of unstable sliding our results indicate that spatial variations with either depth or lateral extent in varepsilon and k could result in significant changes in the drainage state in fault zones such variation may lead to spatial variation of the nucleation and propagation of earthquakes based upon the drainage capabilities of the fault zone

clay mineral reactions caused by frictional heating during an earthquake an example from the taiwan chelungpu fault 1 to understand the chemical reactions of clay minerals in a fault zone during an earthquake we analyzed the clay minerals in the chelungpu fault which slipped during the 1999 chi chi earthquake x ray diffraction spectroscopy showed that kaolinite and smectite contents were lower in the black gouge zone than in the surrounding gray gouge breccia or fracture damaged zones we applied a chemical kinetics approach to examine whether dehydroxylation of kaolinite and dehydration of interlayer water dehydroxylation and illitization of smectite occurred during coseismic frictional heating and found that the first two reactions could complete under the temperature time profile of the chi chi earthquake reconstructed by a previous study because dehydration of smectite interlayer water and dehydroxylation of kaolinite would have completed 3 6 and 8 2 s after the beginning of slip the resulting release of water might have affected the frictional mechanism during the earthquake

growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting 1 to understand how frictional melting affects fault instability we performed a series of high velocity friction experiments on gabbro at slip rates of 0 85 1 49 m s 1 at normal stresses of 1 2 2 4 mpa and with displacements up to 124 m experiments have revealed two stages of slip weakening one following the initial slip and the other immediately after the second peak friction the first weakening is associated with flash heating and the second weakening is due to the formation and growth of a molten layer along a simulated fault the two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin continuous molten layer at the second peak friction the frictional coefficient decays exponentially from 0 8 1 1 to 0 6 during the second weakening the host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer melt viscosity increases notably soon after a molten layer forms however a fault weakens despite the increase in melt viscosity and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip weakening distance effect of frictional melting on fault motion can be predicted by solving a stefan problem dealing with moving host rock molten zone boundaries

distribution of ammonium in minerals of metamorphic and granitic rocks abstract sixty nine analyses are given for nh4 in minerals of metamorphic and granitic rocks mostly from the ryoke belt japan the distribution of nh4 in coexisting minerals is quite systematic suggesting that nh4 is one of the stable geochemical components in high temperature processes biotite has the highest content of nh4 followed by muscovite k feldspar and plagioclase pure quartz is almost free from nh4 calcic plagioclase contains less nh4 than does sodic plagioclase the partition coefficients d pl bi d kf bi and d kf bi are on the average 0 11 0 38 and 0 43 respectively the fractionation of nh4 in these minerals is quite similar to that of rb but much smaller than that of cs distribution of nh4 as well as those of rb and cs appears to be explained by its ionic radius and the shortest cation o distances in alkali positions of minerals

effects of permeability and porosity evolution on simulated earthquakes numerical simulations are a fundamental tool to access the typical conditions attained during earthquake instabilities and to simulate the large number of dissipative processes taking places during faulting in this study we consider a single degree of freedom spring slider system a simplified fault model which can describe the whole seismic cycle and the dynamics of a fault with spatially homogeneous properties we assume a rate and state dependent friction in which we incorporate the effects of pore fluid pressure thermally pressurized as a consequence of the frictional heat produced during sliding we explore in a single framework the role of the time variations of the porosity permeability or both ultimately leading to changes in hydraulic diffusivity which has been recognized as one of the key parameters in thermally pressurized faults our synthetic ruptures show that the changes in the hydraulic diffusivity only due to porosity variations do not markedly affect the earthquake recurrence cycle time the traction evolution and the thermal history of the fault on the contrary when the evolutions of both the porosity and the permeability are accounted for the cycle time is significantly reduced this result has a clear implication in the context of the hazard assessment

high velocity frictional behavior and microstructure evolution of fault gouge obtained from nojima fault southwest japan abstract high velocity experiments on fault gouge taken from the nojima fault that slipped during the 1995 kobe earthquake were conducted to investigate physical mechanism associated with the slip weakening behavior with increasing slip the friction values of the gouge sheared at 0 62 mpa normal stress and 1 03 m s slip velocity decrease exponentially from a peak value of more than 0 6 to a steady state value of 0 2 the textures of the gouge are characterized by grain comminution oblique and parallel shear planes and localized deformation zone with strongly preferred orientation in the friction weakening stage and folding and fluttering structures at the steady state friction stage numerical modeling based on the temperature measurements close to the gouge layer shows that the temperature inside the gauge layer did not exceed 400 c during the experiments in a slide hold slide test a full strength recovery of the fault gouge was observed only after 12 s slip pause and the slip weakening curves are the same between the two successive slips the steady state coefficient of friction decreased from 0 8 to about 0 2 when the slip velocity increased from 0 006 m s to 1 03 m s this high velocity weakening feature was observed in a synthetic quartz gouge as well as in the nojima gouge although it is unclear which mechanism causes the weakening among thermal pressurization silica gel lubrication flash heating moisture draining and so on the present experimental results suggest that the high velocity weakening is related to the high heat production rate finally the flow structures observed in the samples deformed up to the final steady state stages have never been reported in previous slow rate experiments and could be a key structure characteristic of high velocity frictional sliding

permo carboniferous hydrocarbon accumulations mid continent u s a approximately 19 4 billion bbl of oil and 119 tcf of nonassociated gas have been discovered in the mid continent as of january 1 1978 although these volumes of hydrocarbons were trapped in thousands of fields throughout the mid continent the bulk of these resources were emplaced in a relatively few fields about 14 2 billion bbl of oil have been found in 111 significant and giant oil fields and 103 tcf of nonassociated gas have been discovered in 57 significant and giant gas fields permo carboniferous reservoirs are important in 101 of the large oil fields and 55 of the large gas fields these fields contained 9 5 billion bbl of oil and 99 tcf of gas respectively our calculations of the total oil and gas accumulations in permo carboniferous reservoirs are extrapola ed from these data about 2 1 billion bbl of oil and 5 1 tcf of nonassociated gas accumulated in lower carboniferous mississippian reservoirs most of this oil and gas was stratigraphically trapped in upper mississippian sandstones and carbonates which are truncated at the pre pennsylvanian unconformity surface approximately 8 8 billion bbl of oil and about 31 9 tcf of nonassociated gas have been found in upper carboniferous pennsylvanian reservoirs in the mid continent most of these oil and gas accumulations were stratigraphically trapped in lenticular sandstone bodies the environments in which the majority of these sandstones were deposited range from fluvial deltaic to shallow marine about 2 billion bbl of oil and 77 7 tcf of nonassociated gas accumulated in permian reservoirs most of these hydrocarbons accumulated in rocks of lower permian age in the hugoton panhandle panoma fields the oil and gas in these fields were trapped by regional changes in lithofacies with associated hydrodynamic gradients significant stratigraphic accumulations of oil and gas will continue to be discovered in the mid continent even though much of this region is now in a late mature stage of exploration and development

thermal pressurization and slip weakening distance of a fault an example of the hanaore fault southwest japan a controversy over the last decade is the large difference in the slip weakening distance d c between laboratory measured and seismically estimated values our numerical analyses of the thermal pressurization process based on measured transport properties of the hanaore fault zone in japan reveal that the process is effective for this fault and that d c becomes of the order of several decimeters to about 1 m when the width of the deformation zone is less than 10 20 mm this d c value is of exactly the same order as that determined seismically so thermal pressurization can be a mechanism for solving the d c paradox the thermal pressurization process effectively reduces the heat production rate and the temperature rise expected for this fault zone is at most about 600 c which is not high enough a temperature to melt rocks

importance of mechanochemical effects on fault slip behavior during earthquakes 1 recent analyses of natural faults and laboratory rock friction experiments have revealed that physical transformations and chemical reactions of minerals and fluids occur in faults during earthquakes however exactly how these physicochemical processes affect fault weakening and earthquake energetics is not well understood we present quantitative experimental evidence of mechanochemical changes in a clay mineral illite during frictional slip and evaluate the effect of those changes on slip behavior friction distorts the crystal structure of illite and dramatically decreases the activation energy ea of the dehydroxylation reaction numerical modeling shows that the lower ea allows the dehydroxylation to occur at relatively low temperatures 200 300 c which in turn drastically reduces fault strength by pressurization of released water from the mineral thus dynamic thermo chemo mechanical processes can strongly affect earthquake instability especially along clay rich faults such as those in the shallow part of subduction zone plate boundaries

fast slip with inhibited temperature rise due to mineral dehydration evidence from experiments on gypsum anomalously low heat flow around active faults has been a recurrent subject of debate over past decades we present a series of high velocity friction experiments on gypsum rock cylinders showing that the temperature of the simulated fault plane is efficiently buffered due to large scale endothermic dehydration reaction the tests were performed at 1 mpa normal stress and a velocity of 1 3 m s 1 while measuring the temperature close to the sliding surface and the relative humidity around the sample the temperature close to the sliding surface is remarkably stable at 100 c during the dehydration reaction of gypsum microstructural and x ray diffraction investigations show that dehydration occurs at the very beginning of the test and progresses into the bulk as slip increases in the hottest parts of the sample anhydrite crystal growth is observed the half thickness of the dehydrated layer ranges from 160 m at 2 m slip to 5 mm at 68 m slip thermodynamic estimates of the energy needed for the dehydration to occur yield values ranging from 10 to 50 of the total mechanical work input the temperature plateau is thus well explained by the energy sink due to the dehydration reaction and the phase change from liquid water into steam we suggest that similar endothermic reactions can efficiently buffer the temperature of fault zones during an earthquake this is a way to explain the low heat flow around active faults and the apparent scarcity of frictional melts in nature

helium and neon isotope systematics in carbon dioxide rich and hydrocarbon rich gas reservoirs the isotopic compositions and elemental abundances of helium and neon were measured in three natural gas reservoirs in the pannonian sedimentary basin of hungary kismarja a co2 rich reservoir and szeghalom south and szeghalom north both ch4 dominated reservoirs are located on topographic basement highs close to the derecske sub basin in eastern hungary mantle derived neon has been identified in mixed ch4 co2 reservoirs in the vienna basin austria this study establishes that mantle derived neon and helium are a characteristic feature of gas reservoirs throughout the neogene extensional basins of hungary and austria regardless of the dominant active gas composition 3he4he ratios within these samples are attributable to a two component mixing between mantlederived and crustal radiogenic helium the percent contribution of mantle derived 4he varies from 2 3 to 17 in contrast neon isotopic ratios indicate that the gases contain a significant component of atmosphere derived neon in addition to the mantle and crustal derived components 20ne 21ne and 22ne abundances can be corrected for this atmospheric contribution calculated contributions of mantle and crustal derived 21ne are between 3 6 21 and 1 37 respectively 20ne22nec and 21ne22nec ratios derived for these atmosphere corrected components correlate with measured r ra values and plot along a single two component mixing line between crustal and mantle isotopic endmembers this is consistent with a model in which simple mixing occurs between crustal and mantle endmembers with fixed he ne ratios the mixing line is defined by a hyperbolic constant k where k 4he22ne rad 4 he22ne mntl with a mean value of 67 3 11 8 based on estimated values of 0 47 for 21ne22nerad and 1 62 0 03 107 for 4he21ne rad kennedy et al 1990 values of 7 61 106 for he22ne rad and 11 3 104 for 4he22ne mntl can be calculated for the pannonian basin gases this 4he22ne mntl value is indistinguishable within error from the value of 8 04 104 calculated for rare gases in natural gases from the vienna basin these results clearly establish that the continental expression of mantle derived rare gases in continental extensional systems in austria and hungary is distinct and consistently different from that of gases discharging at the spreading ridges where best estimates of 4he22ne mntl are 8 1 11 3 times higher 9 10 105 staudacher et al 1989 given the remarkable agreement in the continental expression of mantle derived gases throughout the pannonian and vienna basins it is difficult to attribute the observed neon enrichment helium depletion with respect to morb gases to fractionation related to lithospheric transport processes kinetic fraction ation processes involved in transport through the crust might be expected to produce a much wider variation in the observed he ne elemental ratios the consistent order of magnitude neon enrichment observed throughout these gas fields instead implies that mantle derived fluids in these continental extensional systems may be sourced in a region of the mantle distinct from that supplying the mid ocean spreading ridges

4he diffusion in 40ar retentive minerals abstract 4he vacuum diffusion experiments were carried out by stepwise degassing minerals normally used in k ar chronometry such as hornblende nepheline and others the study aims at 4he diffusion parameters for uppermost crustal temperatures and thus at he retentivities of the minerals the results indicate long time near quantitative 4he retention in hornblende pyroxene langbeinite and perhaps nepheline for 0 5 mm grains at least at temperatures below about 70 c sanidine and muscovite lose 4he over geologic periods at lower temperatures only the minerals of the first group are possible u th he chronometers or geothermometers provided that other dating problems such as alpha loss and u migration can be settled closure temperatures for rapidly cooling 2 mm hornblende crystals are found to be in the range of 190 to 170 c tentative u he indices indicate that diffusion parameters of the same order of magnitude as determined for the he retentive minerals may be used to select minerals to be further examined for their u th he datability e g ore minerals

isotopic variations of the fixed nitrogen in metamorphic rocks the content and isotopic composition of fixed nitrogen nh 4 n in metasedimentary rocks phyllites mica schists and gneisses from prograde metamorphic zones in the erzgebirge saxon ore mountains g d r were measured and show that the nitrogen content decreases and 15n increases with increasing metamorphic grade the geochemical and isotopic behaviour of nitrogen during contact metamorphism was investigated in a mica schist profile through the contact aureole of a granitic pluton the nitrogen concentrations of the wall rocks decrease towards the contact whereas the 15n values become higher various model experiments were carried out in the laboratory to explain the changes observed in the rock samples the nitrogen isotope fractionation factor was determined for the isotopic exchange in the system ammonium sulphate ammonia at 250 c 1 0143 0 0012 and 350 c 1 0128 0 0006 in accordance with the model experiments the conclusion seems warranted that the change in nitrogen content and its isotopic composition in rocks during metamorphic processes depend mainly on the metamorphic temperature

is the initial thermal state of a fault relevant to its dynamic behavior the prediction ofimpending earthquakes undoubtedly remains one of the most pursued goals of modern seismology within the framework of a deterministic description of earthquake faulting the initial state of the fault system and the choice of the governing model describing its rheological behavior play a fundamental role in the description of the earthquake recurrence in classical models of faulting this initial stateisbasicallydescribedbytheinitialshear stressdistribution priortothenextearth quakeevent andbytheinitialslidingvelocity inthispaper byassumingarate state andtemperature dependent rheology weinvestigatewhether theinitialthermalstate of the fault can also have a significant role in earthquake dynamics our numerical results clearly demonstrate that the initial temperature greatly influences the cosesimic slip and thus the earthquake magnitude the released stress and thus the radiated energy andtheintereventtime i e theearthquakerecurrence despitetheremainingissueson theconceptofearthquakecyclicity ourresultscancontributetothelivelydebateonthe deterministic hazard assessment illuminating that the temperature field also plays a fundamental role in earthquake dynamics not only because it controls possible phase changes and the chemical environment of the fault zone but also because it affects the response of a brittle fault and earthquake cycles

strain localization driven by thermal decomposition during seismic shear field and laboratory observations show that shear deformation is often extremely localized at seismic slip rates with a typical deforming zone width on the order of a few tens of microns this extreme localization can be understood in terms of thermally driven weakening mechanisms a zone of initially high strain rate will experience more shear heating and thus weaken faster making it more likely to accommodate subsequent deformation fault zones often contain thermally unstable minerals such as clays or carbonates which devolatilize at the high temperatures attained during seismic slip in this paper we investigate how these thermal decomposition reactions drive strain localization when coupled to a model for thermal pressurization of in situ groundwater building on rice et al 2014 we use a linear stability analysis to predict a localized zone thickness that depends on a combination of hydraulic frictional and thermochemical properties of the deforming fault rock numerical simulations show that the onset of thermal decomposition drives additional strain localization when compared with thermal pressurization alone and predict localized zone thicknesses of 7 and 13 m for lizardite and calcite respectively finally we show how thermal diffusion and the endothermic reaction combine to limit the peak temperature of the fault and that the pore fluid released by the reaction provides additional weakening of 20 40 of the initial strength

air xe enrichments in elk hills oil field gases role of water in migration and storage hydrocarbons from the elk hills naval petroleum reserve npr 1 bakersfield ca are enriched in heavy noble gases the 132xe 36ar ratios are as high as 576 times the ratio in air and represent the largest relative xe enrichments ever observed in terrestrial fluids the xe isotopic composition is indistinguishable from air we show that these samples cannot be explained by equilibration of oil with air saturated water and secondary enrichment via a rayleigh distillation gas stripping process based on laboratory studies of others with potential petroleum source rocks we believe the source of this enriched heavy noble gas component was adsorbed air initially trapped in on the source rocks that was expelled and mixed with the hydrocarbons during expulsion and primary migration kr and xe enrichments decrease with increasing 36ar concentration we propose a model in which an initial kr xe enriched hydrocarbon becomes diluted with noble gases extracted from air saturated groundwater during expulsion migration and storage the model generates an integrated water hydrocarbon ratio for the production fluid which indicates a minimal role for water in hydrocarbon expulsion and migration the results are interpreted to provide time geometrical constraints on the mechanisms by which hydrocarbons can migrate as a separate phase

serpentinization and the origin of hydrogen gas in kansas hydrogen gas occurs in ten kansas wells near the mid continent rift system since 1982 two wells have yielded small amounts of gases containing an average of 29 37 mole h2 the remainder being chiefly n2 with only traces of hydrocarbons isotopic compositions for hydrogen dgrd 740 to 836 pmil imply near ambient about 10 c equilibration temperatures for the gases which are among the most deuterium depleted in nature and resemble the h2 rich gases described from ophiolites in oman isotopic values for the kansas n2 differ slightly from those of the atmosphere but not enough to rule out an atmospheric origin because they are low in ch4 and co2 expected byproducts of biogenic activity the gases are probably abiogenic in origin the existence of such gases near a major rift system containing mafic rocks and not far from known kimberlites is consistent with an origin from reactions involving fe 2 oxidation for example during serpentinization because the gases may be associated with kimberlites and deep seated rifting mantle outgassing is possible but such an origin would be difficult to reconcile with the low isotopic temperatures the h2 gases from kansas and elsewhere seem to be too low in pressure to have commercial value however neither the kansas gases nor those from other h2 occurrences have been adequately examined to assess their importance as potential resources

importance of thermochemical pressurization in the dynamic weakening of the longmenshan fault during the 2008 wenchuan earthquake inferences from experiments and modeling 1 we determined the internal structure and mineral composition of the yingxiu beichuan fault zone at the zhaojiagou exposure and measured frictional and transport properties of the fault rocks collected to gain a better understanding of dynamic weakening mechanisms during seismic fault motion this fault is a major fault in the longmenshan fault system that caused the 2008 wenchuan earthquake the exposure studied is located midway along the surface rupture close to where the largest coseismic displacement occurred high velocity friction experiments reveal exponential slip weakening from a peak friction toward a steady state value slip weakening is more pronounced for water dampened gouge than dry gouge suggesting thermal pressurization the fault gouge has a very low permeability 10 21 m2 at 165 mpa effective pressure and is surrounded by fault breccia with a permeability of 10 19 to 10 17 m2 grading into less permeable fractured country rocks the fault zone thus exhibits a conduit barrier structure allowing fluid flow only in the breccia zone we numerically modeled coseismic slip weakening including thermal pressurization and mineral dehydration decarbonation basing our calculation on measured frictional and transport properties and on the slip history inferred for the wenchuan earthquake the results indicate that 1 thermochemical pressurization played an important role in causing dynamic slip weakening 2 the slip weakening distance is similar to the seismologically determined values and 3 pore pressures might have exceeded the normal stress thus maintaining temperatures below 600 c interestingly enough heat was generated to fully remove and thermally pressurize the interlayer water from smectite contributing an excess pore pressure of 6 mpa in addition we found that the incorporation of state dependent fluid properties predicts much more efficient fluid pressurization than using constant properties the dramatic weakening predicted probably offers a compelling explanation for the large coseismic displacement and slip acceleration observed near beichuan city

the fluid dynamics of solid mechanical shear zones shear zones in outcrops and core drillings on active faults commonly reveal two scales of localization with centimeter to tens of meters thick deformation zones embedding much narrower zones of mm scale to cm scale the narrow zones are often attributed to some form of fast instability such as earthquakes or slow slip events surprisingly the double localisation phenomenon seem to be independent of the mode of failure as it is observed in brittle cataclastic fault zones as well as ductile mylonitic shear zones in both a very thin layer of chemically altered ultra fine grained ultracataclasite or ultramylonite is noted we present an extension to the classical solid mechanical theory where both length scales emerge as part of the same evolutionary process of shearing the host rock we highlight the important role of any type of solid fluid phase transitions that govern the second degree localisation process in the core of the shear zone in both brittle and ductile shear zones chemistry stops the localisation process caused by a multiphysics feedback loop leading to an unstable slip the microstructural evolutionary processes govern the time scale of the transition between slow background shear and fast intermittent instabilities in the fault zone core the fast cataclastic fragmentation processes are limiting the rates of forming the ultracataclasites in the brittle domain while the slow dynamic recrystallisation prolongs the transition to ultramylonites into a slow slip instability in the ductile realm

earthquake slip weakening and asperities explained by thermal pressurization for the earth s crust to release accumulated stress in the form of an earthquake a fault must weaken and give way otherwise the movement would quickly grind to a halt due to the surrounding pressure this weakening is not really understood laboratory experiments simply don t match seismologists estimates of the distance a fault must slip before it becomes very weak a new study concludes that the missing ingredient is water geological studies and measurements show the slip zones to be narrow impermeable structures water already in the fault zone can quickly become pressurized by frictional heating as the rocks start to move catalysing large earthquakes from minor movements an earthquake occurs when a fault weakens during the early portion of its slip at a faster rate than the release of tectonic stress driving the fault motion1 2 this slip weakening occurs over a critical distance dc understanding the controls on dc in nature is severely limited however because the physical mechanism of weakening is unconstrained conventional friction experiments typically conducted at slow slip rates and small displacements have obtained dc values that are orders of magnitude lower than values estimated from modelling seismological data for natural earthquakes3 4 5 6 here we present data on fluid transport properties of slip zone rocks and on the slip zone width in the centre of the median tectonic line fault zone japan we show that the discrepancy between laboratory and seismological results can be resolved if thermal pressurization of the pore fluid7 8 9 10 is the slip weakening mechanism our analysis indicates that a planar fault segment with an impermeable and narrow slip zone will become very unstable during slip and is likely to be the site of a seismic asperity

character origin and occurrence of natural gases in the anadarko basin southwestern kansas western oklahoma and texas panhandle u s a abstract natural gas production in the anadarko basin comes from three geographically separated areas that can be differentiated by age of reservoir and by inferred nature of organic thermal origin of the gases in the central basin non associated gases are produced mainly from upper mississippian and pennsylvanian sandstones gas samples are from reservoirs as much as 6588 m deep gases become isotopically heavier 13 c 1 values range from 49 8 to 33 2 and chemically drier c 2 values range from 1 33 with increasing level of thermal maturity gases were generated mainly from interbedded shales with type iii kerogen during the mature and post mature stages of hydrocarbon generation deviations from the trend are due to vertical migration and mixing of gases generated at different levels of thermal maturity over the past 250 myr in the giant panhandle hugoton field non associated gases are generally produced from permian carbonates at depths of 13 c 1 value is 43 2 mean c 2 value is 14 because organic rich mature source rocks are not present in the area gases probably were generated in the central basin from pennsylvanian or older source rocks during the mature stage of hydrocarbon generation this interpretation implies migration over distances as much as several hundred kilometers in the sooner trend associated gases are produced from silurian devonian and mississippian carbonates at depths as great as 2950 m and were generated from type ii kerogen during the mature stage of hydrocarbon generation associated oil usually correlates with extracts of the upper devonian and lower mississippian woodford shale gases are isotopically lighter mean 1 3 c 1 value is 43 9 and chemically wetter mean c 2 value is 14 than those derived from type iii kerogen at an equivalent level of thermal maturity

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

a magnus opus helium neon and argon isotopes in a north sea oilfield this study of the magnus oilfield located in the east shetland basin northern north sea represents the most detailed investigation of noble gas isotope systematics in a liquid hydrocarbon reservoir yet undertaken samples from nine producing wells across this middle jurassic field were taken and the helium neon and argon isotopic ratios and abundances in the oil were determined both the helium and the neon isotope systematics require a contribution from a mantle source if the mantle endmember is modeled using mid ocean ridge mor values 2 3 4 5 of the 4he and 4 3 6 2 of the 21ne is mantle derived the remainder of the 4he and 9 0 12 0 of the 21ne is crustal radiogenic and the remaining 21ne is atmosphere derived the resolved mantle derived he ne ratio is quite distinct from upper mantle values estimated from mor samples but indistinguishable from values resolved in other regions of continental extension this result provides strong evidence that the subcontinental lithospheric mantle not only has a different noble gas inventory to the convecting mantle under mors but also has a near uniform composition the quantity of radiogenic noble gas associated with the magnus oil groundwater system can only be accounted for by production predominantly from outside the volume of the magnus sandstone aquifer reservoir drainage area and the associated kimmeridge clay source rock formation and together with the mantle derived noble gases provides strong evidence for cross formational communication with deeper regions of the crust the 20ne and 36ar must have been input into the oil phase by interaction with an air equilibrated groundwater noble gas partitioning between a seawater derived groundwater and the oil phase at the average magnus sandstone aquifer temperature requires a subsurface seawater oil volume ratio of 110 40 to account for both the 20ne and 36ar concentrations in the central and southern magnus samples the volume of groundwater which has equilibrated with the magnus oil is indistinguishable from the static volume of water estimated to be in the down dip magnus aquifer reservoir drainage volume this suggests that the magnus oil has obtained complete equilibrium with the groundwater in the reservoir drainage volume probably during secondary migration and further suggests that concurrent cementation of the magnus sandstone aquifer has occurred with little or no large scale movement of air equilibrated groundwater through the aquifer system higher concentrations of 20ne and 36ar in the n magnus oil cannot be accounted for by equilibration with a seawater derived groundwater this is qualitatively consistent with the earlier and more mature oil in this section equilibrating with freshwater which is known to have been trapped in the crest of the reservoir structure

energy taken up by co seismic chemical reactions during a large earthquake an example from the 1999 taiwan chi chi earthquake 1 frictional heat in a fault zone during earthquake slip transiently induces chemical reactions that may use energy released during the earthquake we estimated the energy used by such reactions ec by a numerical analysis incorporating frictional heat thermal diffusion chemical kinetics and energy conservation and found that ec has an auto feedback effect that inhibits temperature rise in fault zone during the 1999 taiwan chi chi earthquake estimated ec was 0 43 mj m2 corresponding to 0 79 of the frictional heat generated this low percentage probably reflects the low initial concentrations of reactive materials however in the case of a fault with abundant reactive materials ec could reach 50 of the frictional heat and the auto feedback effect could be large at this case ec is a nonnegligible component on earthquake energy budget and can affect fault mechanics

the isotopic compositions of molecular nitrogen implications on their origins in natural gas accumulations abstract the isotopic compositions of n2 15 n atm imply the geochemical origins of molecular nitrogen in natural gas accumulations but the irregular variation of 15 n n 2 for most petroliferous basins puzzles scientists we believe that this is due to multiple origins of n2 which are often mixed together and leads to the irregularities in gas pools the four petroliferous basins the west siberian basins in russia the yinggehai basin of china the californian great valley basin of usa and the mid european basin have significantly different isotope characteristics of n2 which may provide insight into the isotope geochemical implications of n2 in natural gas accumulations n2 with 19 15 n n 2 10 may be from sedimentary organic matter at the immature and the early mature stage ro 0 6 values of 10 15 n n 2 2 may indicate that n2 originates from mature including high mature sedimentary organic matter ro 0 6 2 0 values of 2 15 n n 2 15 n n 2 0 and n2 ar 38 84 suggest that the atmosphere is the origin values of 1 15 n n 2 15 n n 2 4 is the feature of n2 from saltpeter in evaporites values of 4 15 n n 2 18 indicate that n2 maybe derived from post mature sedimentary organic matter ro 2 0

dynamic weakening and amorphization in serpentinite during laboratory earthquakes the mechanical properties of serpentinites are key factors in our understanding of the dynamics of earthquake ruptures in subduction zones especially intermediate depth earthquakes here we performed shear rupture experiments on natural antigorite serpentinite which showed that friction reaches near zero values during spontaneous dynamic rupture propagation rapid coseismic slip 1 m s although it occurs over short distances 1 mm induces significant overheating of microscale asperities along the sliding surface sufficient to produce surface amorphization and likely some melting antigorite dehydration occurs in the fault walls which leaves a partially amorphized material the water generated potentially contributes to the production of a low viscosity pressurized melt explaining the near zero dynamic friction levels observed in some events the rapid and dramatic dynamic weakening in serpentinite might be a key process facilitating the propagation of earthquakes at intermediate depths in subduction zones

pore fluid pressure development in compacting fault gouge in theory experiments and nature the strength of fault zones is strongly dependent on pore fluid pressures within them moreover transient changes in pore fluid pressure can lead to a variety of slip behavior from creep to unstable slip manifested as earthquakes or slow slip events the frictional properties of low permeability fault gouge in nature and experiment can be affected by pore fluid pressure development through compaction within the gouge layer even when the boundaries are drained here the conditions under which significant pore fluid pressures develop are analyzed analytically numerically and experimentally friction experiments on low permeability fault gouge at different sliding velocities show progressive weakening as slip rate is increased indicating that faster experiments are incapable of draining the pore fluid pressure produced by compaction experiments are used to constrain the evolution of the permeability and pore volume needed for numerical modeling of pore fluid pressure build up the numerical results are in good agreement with the experiments indicating that the principal physical processes have been considered the model is used to analyze the effect of pore fluid pressure transients on the determination of the frictional properties illustrating that intrinsic velocity strengthening behavior can appear velocity weakening if pore fluid pressure is not given sufficient time to equilibrate the results illustrate that care must be taken when measuring experimentally the frictional characteristics of low permeability fault gouge the contribution of compaction induced pore fluid pressurization leading to weakening of natural faults is considered cyclic pressurization of pore fluid within fault gouge during successive earthquakes on larger faults may reset porosity and hence the capacity for compaction weakening

ammonium concentration and nitrogen isotope composition in metasedimentary rocks from different tectonometamorphic units of the european variscan belt the content and isotopic composition of ammonium nh4 were measured by using kjeldahl distillation for 63 samples of predominantly metasedimentary rocks from two different geological settings of the european variscides i e the erzgebirge and the zone of erbendorf vohenstrauss in germany the studied erzgebirge schists represent an early paleozoic passive margin sequence that reappears in different deep subducted metamorphic units this allows an efficient examination of nitrogen loss and isotope fractionation during prograde metamorphism ammonium is found to be progressively depleted accompanied by a shift in 15n from the low grade units 2 kbar 300 c with 638 124 ppm and 15n 2 2 0 6 to the garnet phyllite unit 9 kbar 470 c with 621 190 ppm and 15n 3 5 0 9 the mica schist eclogite unit 12 kbar 550 c with 394 113 ppm and 15n 3 9 0 8 and to the gneiss eclogite unit 12 kbar 730 c with 99 32 ppm and 15n 7 7 2 0 using equilibrium models for rayleigh distillation and batch volatilization suggest that the nitrogen depletion took place by ammonia release only for the mica schists this isotope fractionation can be explained by the loss of molecular nitrogen in comparison with the early paleozoic schists the proterozoic ortho and paragneisses of the erzgebirge gneiss unit 6 8 kbar 650 c contain significantly lower amounts of ammonium 70 ppm and reach relatively low 15n values 2 5 to 3 6 the paragneisses from the zone of erbendorf vohenstrauss zev sampled from the ktb pilot hole represents a separate tectonometamorphic unit that has undergone polyphase metamorphic conditions with the major imprint being amphibolite facies 7 kbar 650 700 c the metagraywackes have nh4 contents of 80 ppm and 15n of approximately 6 gneisses in the vicinity of a late cataclastic shear zone at 2000 m depth are characterized by a shift in nh4 250 ppm and 15n up to 12 which suggest late fluid rock interactions with a nitrogen and 15n enriched fluid

thermal constraints on crustal rare gas release and migration evidence from alpine fluid inclusions the palfris marl of the helvetic alpine nappes contains four distinct vein fill generations ch4 rich gas is found in abundant fluid inclusions within these carbonate veins while free ch4 gas has also been produced from exploratory boreholes through this formation the stable isotope and helium neon and argon isotopic composition of these fluids has been determined a constant radiogenic 40ar concentration of 1 25 0 13 1 ppm in these differently sited fluids requires an intimate association between the 40arrad source and the hydrocarbon phase this can only be reasonably explained if the 40arrad was input into the hydrocarbon phase during hydrocarbon generation migration or storage prior to entrapment in the fluid inclusions stable isotope results constrain the maturity of hydrocarbon production while fluid inclusion formation pressures and temperatures record values of up to 2 5 kbars and 250 c these values place limits on the range of thermal conditions in which the hydrocarbon 40arrad relationship was established all fluids within inclusions also contain radiogenic 4he40ar values at predicted crustal production ratios these observations provide the first evidence that both 4herad and 40arrad can be quantitatively released on a regional scale bounded by the thermal conditions required to produce the hydrocarbon phase and the conditions under which the fluid inclusions were formed t 190 250 c these results require that negligible quantities of excess 40arrad decoupled from 4herad have been released into this system given the wide array of mechanisms which can potentially cause decoupling of these two species this result provides an important constraint on the role of these processes within the sedimentary fluid regime in contrast the free borehole gas contains excess radiogenic 4he and 21ne relative to 40arrad in proportions which can be accounted for by local production and subsequent diffusion from the surrounding marl the latter pattern is consistent with rare gas migration in lower temperature environments a conceptual model which considers both diffusional and metamorphic release of helium and argon and the ability of the surrounding fluid regime to transport the rare gases from their respective mineral production sites is consistent with both these results and data from regional rare gas studies

origin of nitrogen rich natural gases in the california great valley evidence from helium carbon and nitrogen isotope ratios abstract dry natural gases produced from the sacramento and northern san joaquin basins contain up to 87 nitrogen 15n measurements for gases with n2 8 vary from 0 9 to 3 5 methane 13c values vary from 61 to 15 3 he 4 he ratios indicate the presence of mantle derived helium in addition to radiogenic crustal helium and suggest that the gases may originate from mixing between three principal components a hydrocarbon rich gas with 3 he 4 he 0 1r a derived from the microbial and thermogenic alteration of sedimentary organic matter a hydrocarbonrich gas with 3 he 4 he 2 75r a derived from the localized pyrolysis of sediments by magmatic volatiles and a nitrogenrich gas with 3 he 4 he 1r a the nitrogen rich component is characterized by methane 13c less negative than 25 and n 2 ar ratios as high as 22 000 nitrogen concentrations tend to increase towards the basement along the shallow eastern margin of the valley suggesting that the nitrogen rich component is derived from metasedimentary rocks deep within the crust the coast ranges fraciscan complex which was thrust well beneath the great valley during late mesozoic and early cenozoic subduction appears to be the most likely source

low to high velocity frictional properties of the clay rich gouges from the slipping zone of the 1963 vaiont slide northern italy the final slip of about 450 m at about 30 m s of the 1963 vaiont landslide italy was preceded by 3 year long creeping phase which was localized in centimeter thick clay rich layers 60 70 smectites 20 30 calcite and quartz here we investigate the frictional properties of the clay rich layers under similar deformation conditions as during the landslide 1 5 mpa normal stress 2 10 7 to 1 31 m s slip rate and displacements up to 34 m experiments were performed at room humidity and wet conditions with biaxial torsion and rotary shear apparatus the clay rich gouge was velocity independent to velocity weakening in both room humidity and wet conditions in room humidity experiments the coefficient of friction decreased from 0 47 at v 0 70 m s full lubrication results from the formation of a continuous water film in the gouge the vaiont landslide occurred under wet to saturated conditions the unstable behavior of the landslide is explained by the velocity weakening behavior of the vaiont clay rich gouges the formation of a continuous film of liquid water in the slipping zone reduced the coefficient of friction to almost zero even without invoking the activation of thermal pressurization this explains the extraordinary high velocity achieved by the slide during the final collapse

genetic characterization of natural gases natural gases can be characterized genetically using four properties c2 concentration carbon and hydrogen isotope variations in methane and carbon isotope variation in ethane three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters in these diagrams compositional fields have been defined for primary gases biogenic thermogenic associated and thermogenic nonassociated and for gases which result from mixing of these gases these fields are strictly empirical and comprise compositional variations found in about 500 natural gases the isotopic and compositional variations in natural gases can be described in terms of 1 processes during formation of the gases such as bacterial fermentation or maturation of organic matter and 2 processes during secondary migration mixing of primary gases is an important and common process migration of gases predominantly affects the c2 concentration whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged allowing an assessment of the origin of migrated gases and properties of their source rocks the formation of gas from humic organic matter and coals is not yet clear from published data the diagrams use data from various basins and areas interstitial gases from the gulf of california are entirely of bacterial origin traces of thermogenic gases are formed only in the vicinity of dolerite sills gases in the south german molasse basin and in the vienna basin are of bacterial mixed and thermogenic origins data from the north italian po basin provide examples for genetic characterization of migrated gases

shear zone broadening controlled by thermal pressurization and poroelastic effects during model earthquakes as a result of the comminution that takes place over numerous earthquake cycles mature faults are characterized by thick layers of pulverized gouge with finite porosity that is saturated with water at seismogenic depths the heat generated during earthquakes raises the gouge temperature and thermal expansion of the pore fluid and surrounding solids produce elevated pore pressures that cause fault strength to decrease in the process known as thermal pressurization building upon this framework we describe a model that imposes a plane strain configuration and shows that the stress variations caused by porothermoelasticity promote the mohr coulomb failure of previously undeformed regions except in special cases where the friction is rate strengthening we find that the frictional strength must vary throughout the post failure region which we identify in our model with the shear zone we introduce a strain rate function that describes the overall influence of distributed slip on energy dissipation and fault strength as the shear zone thickness expands using typical fault parameters at 7 km depth the shear zone reaches several millimeters of thickness after 1 s sliding at an overall rate of 1 m s the expansion of the shear zone limits the temperature rise to several hundred degrees celsius and the average fault strength falls to about a tenth of the static frictional strength

a fault constitutive relation accounting for thermal pressurization of pore fluid 1 the heat generated in a slip zone during an earthquake can raise fluid pressure and thereby reduce frictional resistance to slip the amount of fluid pressure rise depends on the associated fluid flow the heat generated at a given time produces fluid pressure that decreases inversely with the square root of hydraulic diffusivity times the elapsed time if the slip velocity function is crack like there is a prompt fluid pressure rise at the onset of slip followed by a slower increase the stress drop associated with the prompt fluid pressure rise increases with rupture propagation distance the threshold propagation distance at which thermally induced stress drop starts to dominate over frictionally induced stress drop is proportional to hydraulic diffusivity if hydraulic diffusivity is 0 02 m2 s estimated from borehole samples of fault zone material the threshold propagation distance is 300 m the stress wave in an earthquake will induce an unknown amount of dilatancy and will increase hydraulic diffusivity both of which will lessen the fluid pressure effect nevertheless if hydraulic diffusivity is no more than two orders of magnitude larger than the laboratory value then stress drop is complete in large earthquakes

fault rock injections record paleo earthquakes fault rocks such as pseudotachylyte melt or granular gouge are sometimes injected into cracks off a fault surface the width to height ratio of the injection veins is a direct measure of the shear strain in the wall rock required to accommodate the injection so the aspect ratios preserve a record of the overpressure opening the cracks and injecting melt or fluidized granular material from the fault zone we measured the aspect ratios for 201 pseudotachylyte injections and 29 granular injections they have aspect ratios of 0 2 0 177 0 025 and 0 227 0 077 respectively within 99 confidence these are significantly proportionally wider than the aspect ratios of ordinary dikes from 10 4 to 10 3 the injection aspect ratios exceed the usual limit of elastic strain in the wallrock 1 so we infer that the injection veins were accommodated by permanent deformation by microcracking and slip on foliation surfaces in the host rock which is difficult to detect observationally the fully elastic model therefore offers an upper bound on the stress required to open these injection veins 10 8 210 10 pa with higher pressures in stiffer host rock lithologies these overpressures cannot be achieved by expansion of the fault rock fluids during melting or gouge fluidization mechanisms as the pressure along the fault is limited to 10 7

inferring earthquake physics and chemistry using an integrated field and laboratory approach abstract earthquakes are the result of a combination of 1 physico chemical processes operating in fault zones which allow ruptures to nucleate and rock friction to decrease with increasing slip or slip rate and 2 of the geometrical complexity of fault zones in this review paper we summarize recent experimental findings from high velocity conducted at about 1 m s slip rate or typical seismic slip rates rock friction experiments with an emphasis on potential dynamic weakening mechanisms melt lubrication nanopowder lubrication etc and how these mechanisms might be recognized by means of microstructural and mineralogical studies in exhumed fault zones we discuss how earthquake source parameters coseismic fault strength weakening distances energy budgets etc might be derived from the field and laboratory experiments additionally we discuss what needs to be considered in terms of fault zone geometry and morphology focusing on fault surface roughness in order to develop models of realistic fault surfaces and present theoretical considerations for microphysical modeling of laboratory data at seismic slip rates with an emphasis on the case of melt lubrication all experimental data and in the case of melt lubrication microphysical models indicate that faults must be very weak

high velocity frictional properties of a clay bearing fault gouge and implications for earthquake mechanics 1 frictional properties of natural kaolinite bearing gouge samples from the median tectonic line sw japan have been studied using a high velocity rotary shear apparatus and deformed samples have been observed with optical and electron scanning and transmission microscopy for a slip velocity of 1 m s 1 and normal stresses from 0 3 to 1 3 mpa a dramatic slip weakening behavior was observed x ray diffraction analysis of deformed samples and additional high velocity friction experiments on pure kaolinite indicate kaolinite dehydration during slip the critical slip weakening distance dc is of the order of 1 to 10 m these values are extrapolated to higher normal stresses assuming that dc is rather a thermal parameter than a parameter related to a true characteristic length the calculation shows that dimensionally dc 1 n2 where n is the normal stress applied on the fault the inferred dc values range from a few centimeters at 10 mpa normal stress to a few hundreds of microns at 100 mpa normal stress microscopic observations show partial amorphization and dramatic grain size reduction down to the nanometer scale localized in a narrow zone of about 1 to 10 m thickness fracture energy gc is calculated from the mechanical curves and compared to surface energy due to grain size reduction and energies of mineralogic transformations we show that most of the fracture energy is either converted into heat or radiated energy the geophysical consequences of thermal dehydration of bonded water during seismic slip are then commented in the light of mineralogical and poromechanical data of several fault zones which tend to show that this phenomenon has to be taken into account in most of subsurface faults and in hydrous rocks of subducted oceanic crust

mantle helium in sacramento basin natural gas wells abstract helium isotope ratios in sacramento basin natural gas wells show a strong mantle signal the3he 4he ratios range from 0 11 times the atmospheric ratio 0 11 ra in the rio vista field to 2 75 ra in the moon bend field indicating that 1 to 34 of the helium is mantle derived 3he 4he versus ch4 4he ratios provide evidence of two component mixing between crustal and magmatic end members extrapolation of the linear regression line to ch4 4he 0 gives a hypothetical magmatic end member 3he 4he ratio of 3 84 ra half the typical mantle ratio this indicates that the magmatic end member may actually represent a mixture of mantle and crustal helium gases which deviate from the simple two component mixture can be explained by addition of pure methane radiogenic 4he or a high n2 he component with 3he 4he 0 6 ra to 1 0 ra the ch4 3he ratio of the magmatic end member remains poorly constrained 0 to 3 109 and one cannot rule out the possibility that a significant proportion of the methane in some fields may be of deep earth origin however fields with the highest 3he 4he ratios are associated with buried plio pleistocene intrusives which have up arched sediments to form hydrocarbon traps the methane in these fields may have been produced by rapid thermal alteration of the intruded sediment elsewhere the methane appears either to have migrated from deeply buried sediments in the western basin or to have been produced by local microbial activity

flash heating and local fluid pressurization lead to rapid weakening in water saturated fault gouges coseismic fault displacement is quite large at shallow depths in some earthquakes and it implies that fault gouges and sediments have extremely low dynamic friction during seismic slip however the dynamic weakening mechanisms of gouges under wet conditions are still not well constrained here we present direct evidence for the occurrence of flash heating and local fluid pressurization in water saturated gouges by performing low to high velocity v 10 m s to 1 m s friction experiments in a pressure vessel under conditions specially designed to suppress weakening effects of bulk thermal and compaction induced pressurization the tested gouges exhibit transition from velocity strengthening to drastic velocity weakening as slip rates increase strong dynamic weakening starts to occur at v 0 04 m s at the initiation of sliding 0 1 m which is much more efficient than previously reported in terms of the weakening velocity and distance furthermore the onset of weakening is always accompanied by an instantaneous dilatancy 10 25 m which is much larger than that observed in dry tests and in contrast with gradual changes displayed in the wet tests without dynamic weakening numerical modeling integrated with microstructural observation reveals that bulk thermal pressurization cannot explain the experimental results while flash weakening triggered by vaporization of water layers on around asperity contacts and the resultant local fluid pressurization may be responsible for the observed rapid weakening concomitant with instantaneous dilatancy given high efficiency of such weakening process fluid infiltrated faults could be weakened more rapidly than previously recognized during seismic slip

ammonium as a biomarker in precambrian metasediments abstract the following article assesses the potential of ammonium as a biomarker in precambrian metasediments a modern marine environment is considered to highlight the processes that contribute to the isotopic composition of organic nitrogen notably the roles played by nitrogen fixing organisms cyanobacteria and denitrifying bacteria the carbon and nitrogen biogeochemical cycles are compared and contrasted unlike carbon the major nitrogen isotope variations may be controlled by local environmental conditions such as season of the year and water depth however such local environmental factors may have been superimposed on more gradual changes related to the long term chemical evolution of the exterior environments of the earth the great potential of nitrogen as a biomarker is that the breakdown product of organic nitrogen ammonium behaves like a trace element and is stable in high temperature crustal processes thus ammonium may survive as a biomarker when all other evidence for former life has been destroyed and ammonium studies may be able to identify sedimentary pelitic components in high grade precambrian terrains previous results from precambrian rocks are reviewed to highlight that very little work has been completed to date and that only very general conclusions can be drawn much work needs to done in order to understand the basic biogeochemistry and geochemistry of crustal nitrogen and it is suggested that the best way to proceed is by numerous detailed case studies the interpretation of isotope ratios in ancient sediments usually assumes uniformitarianism and the justification of this approach is considered briefly overall this article highlights the under developed potential of precambrian ammonium biogeochemistry and suggests area for future research not only does the development of the subject have implications for the search for former life on mars but it may also open up new areas of precambrian research

seismic slip record in carbonate bearing fault zones an insight from high velocity friction experiments on siderite gouge pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip we report from high velocity friction experiments on siderite bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite calcite and quartz initially at room temperature under normal stresses of 0 6 1 3 mpa and at seismic slip rates of 1 3 2 0 m s in all cases gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and co 2 gas as confirmed by co 2 measurement x ray diffraction analyses and transmission electron microscopy the weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude those changes can be recognized in natural fault zones and black gouge in the chelungpu fault zone in taiwan is perhaps such an example thus our results suggest that thermal decomposition in shallow crustal faults can be an important co seismic process not only for dynamic fault weakening but also for leaving seismic slip records

nitrogen isotope compositions of metasedimentary rocks in the catalina schist california implications for metamorphic devolatilization history abstract in the catalina schist subduction zone metamorphic complex california metasedimentary rocks show a decrease in n concentration and an increase in 15nair with increasing metamorphic grade lowest grade lawsonite albite rocks contain 632 185 ppm n with 15 n 1 9 0 6 whereas highgrade amphibolite equivalents contain 138 76 ppm n with 15 n 4 3 0 8 loss of n accompanied devolatilization reactions that evolved h2o rich c o h s n fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite biotite garnet and kyanite whole rock n concentrations of up to 200 ppm in veins and pegmatites produced during high p t metamorphism reflect the redistribution of n during devolatilization and partial melting of the metasedimentary rocks bulk fluid rock n isotope fractionations 15n 15nfluid 15nrock of 1 5 1 were calculated with the rayleigh distillation equation taking into account variability in rock composition by comparison of samples with similar k2o concentrations these fractionations are similar to but slightly lower than published calculated fractionations for n2 nh4 exchange at the temperature range of 350 600 c over which most of the devolatilization occurred in the catalina schist approximately 3 4 to 2 25 the n systematics appear to be explained by n2 nh4 exchange and a devolatilization process intermediate in behavior to batch volatilization and rayleigh distillation the observed shifts in n concentration and 15n cannot be explained by nh3 nh4 exchange at these temperatures using the equilibrium models the distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization e g b cs u similarity of the calculated fluid 15n 1 5 to 5 5 with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using n as a tracer of large scale volatile transport

nitrogen isotope record of fluid rock interactions in the skiddaw aureole and granite english lake district the skiddaw granite and its contact metamorphic aureole in the english lake district provide an excellent opportunity to test the capability of the n isotope system to trace devolatilization and large scale transfer of crustal fluids in the aureole skiddaw group metasedimentary rocks with relatively uniform lithology and major element compositions show a dramatic decrease in n content toward the granite contact from or 800 ppm at distances 2 5 km from the contact to 1 5 km these rocks have extremely uniform delta 15 n air near 3 7 per mil whereas closer to the contact or 1 km delta 15 n is shifted to higher values up to 8 7 per mil the coupled decreases in n content and increases in delta 15 n are compatible with the removal of n having low delta 15 n in fluids during continuous prograde devolatilization reactions involving the break down of white mica and the stabilization of biotite cordierite and andalusite bearing assemblages in the same metasedimentary rocks the lack of obvious trends in major element concentrations including sio 2 tio 2 sio 2 al 2 o 3 and the ratios of other major oxides to tio 2 and al 2 o 3 with distance from the granitic contact is consistent with minimal change in major element composition during the contact metamorphism ratios of whole rock n b rb and ba concentrations to whole rock k 2 o content are believed to reflect the differing fluid mica partitioning and involving varying relative proportions of white mica and biotite of these trace elements during devolatilization reactions greisenized skiddaw granite from a borehole is enriched in n range of 17 225 ppm for whole rocks and white mica separates relative to the unaltered granite whole rock 30 ppm and has delta 15 n of 1 0 to 4 8 per mil the n concentrations and delta 15 n of the wall rocks and greisenized granites combined with c isotopic data carbonate and carbonaceous matter for the same rocks are consistent with the mobilization of fluids having low delta 15 n and delta 13 c pdb values from the devolatilized aureole into the cooling intrusive body such transport is consistent with the predictions of recent theoretical models of late stage hydrothermal evolution in cooling intrusive systems

thermochemical pressurization of faults during coseismic slip 1 during earthquakes frictional heating on the fault plane induces a temperature rise and thus a pore pressure rise which is known as thermal pressurization tp coseismic mineral dehydrations may occur because of this temperature increase and are included within the tp framework dehydrations are modeled as a source term for pore pressure because of the total volume change and as a sink term for temperature because they are endothermic the reaction occurs within the slipping zone when a threshold temperature ts is reached dehydration reaction kinetic is modeled using a first order reaction rate using energy and fluid mass conservation we derive analytically the equations of evolution of pore pressure temperature and reaction extent in the undrained adiabatic case using a constant reaction rate we investigate the values of the kinetic rate constant required to produce a significant effect which are much higher than laboratory data reported in the literature on clay serpentine and phyllosilicate dehydration we show however that such high values can be reached if the temperature dependency of the rate constant is taken into account next we include fluid and heat transport and use an arrhenius law to calculate the rate constant as a function of temperature the subsequent set of differential equations is then solved numerically the main effect of dehydration reactions is an increase of pore pressure and a stabilization of the temperature during slip we explore a wide range of parameters in order to determine in which cases dehydration can be considered as a nonnegligible process for high permeability rocks 10 18 m2 and when the amount of water that can be released is of the order of 10 dehydration is an important mechanism as it delays the onset of melting which would normally occur even within the tp framework if the onset temperature is low compared to the initial temperature t0 ts t0 150 c overpressure can occur if the reactions are highly endothermic and if their kinetic is fast enough frictional melting would not occur unless the dehydration reactions are completed within the slipping zone

molecular nitrogen in natural gas accumulations generation from sedimentary organic matter at high temperatures the occurrence of natural gas accumulations with high percentages up to 100 of molecular nitrogen in various hydrocarbon provinces represents a largely unresolved problem and a serious exploration risk in this context a geochemical and basin modeling study was performed to evaluate the potential of sedimentary organic matter to generate molecular nitrogen the masses of nitrogen present in coals if converted into molecular nitrogen are sufficient to fill commercial gas reservoirs a calculation for gas accumulations in northern germany where percentages of molecular nitrogen range from less than 5 to greater than 90 reveals that the molecular nitrogen generated in underlying coal bearing strata is sufficient to account for the nitrogen gas even in the largest fi lds in addition much of the total nitrogen in clay rich rock types such as shales and mudstones is fixed in sedimentary organic matter and may add to the nitrogen generation capacity of the coals a kinetic interpretation of the nitrogen and methane generation characteristics of humic coals during laboratory pyrolysis indicates that nitrogen is generated from organic matter in sedimentary basins at higher temperatures than methane minimum temperatures for preferential nitrogen generation are in excess of 300 c for extremely low heating rates or isothermal conditions lasting over 300 m y thus nitrogen rich gases are mainly formed in the final stage of gas generation when sedimentary rocks grade into metamorphic rocks applications of the kinetic parameters for the generation of nitrogen and methane from coaly organic matter in carboniferous sedimentary rocks are presented for part of the northwest german basin in this area immense volumes of the methane rich gas gener ted during the late paleozoic and mesozoic were lost to the atmosphere the present gas composition mainly reflects the latest cenozoic gas generation this cenozoic gas is nitrogen rich where the carboniferous source rock sequences are highly mature and deeply buried

natural and experimental evidence of melt lubrication of faults during earthquakes melt produced by friction during earthquakes may act either as a coseismic fault lubricant or as a viscous brake here we estimate the dynamic shear resistance f in the presence of friction induced melts from both exhumed faults and high velocity 1 28 meters per second frictional experiments exhumed faults within granitoids tonalites indicate low f at 10 kilometers in depth friction experiments on tonalite samples show that f depends weakly on normal stress extrapolation of experimental data yields f values consistent with the field estimates and well below the byerlee strength we conclude that friction induced melts can lubricate faults at intermediate crustal depths

coseismic dehydration of serpentinite evidence from high velocity friction experiments abstract high velocity friction hvf experiments on serpentinite under conditions equivalent to large amounts of earthquake slip produced large volumes of water vapor derived from the frictional heating induced dehydration of serpentinite fourier transform infrared ftir and thermogravimetry tg differential scanning calorimetry dsc microspectroscopic analyses show that the water in the slip zone and its bounding zones was released due to dehydration during the hvf experiments our experimental results demonstrate that i the run product consists of 10 wt molten materials with abundant vesicles and 90 wt fine grained clasts ii both serpentine and olivine minerals were melted during high velocity slip 1 1 1 3 m s iii rapid serpentine dehydration occurred in a zone of up to 3 mm wide including the slip zone and its bounding zones accompanying frictional melting during high velocity slip and iv no distinct dehydration or frictional melting occurred during low velocity slip

quasi equilibrium melting of quartzite upon extreme friction quartz minerals in earth s crust are thought to melt at high temperatures laboratory friction experiments however show that metastable melting of quartz on a fault surface can occur at lower temperatures and could lead to large earthquakes

reservoir characterization of the giant hugoton gas field kansas hugoton field in kansas is the largest gas field in north america with cumulative production over 23 tcf infill and deep drilling activity over the last 10 yr have made it possible to build an extensive database of modern wireline log and core data such data formed the basis for a wide ranging reservoir characterization done to obtain critical information for optimum reservoir management of the field reservoir heterogeneity and formation evaluation problems made it difficult to characterize fluid distribution estimate gas in place and determine permeability from wireline log data but few of the problems in this reservoir characterization study are unique to hugoton the techniques described here may be applicable to other reservoirs technologies employed to solve the formation evaluation and lateral variability problems included artificial neural networks resistivity modeling geostatistics and three dimensional grid manipulation copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received april 29 1996 revised manuscript received october 21 1996 final acceptance june 10 1997 2amoco production co 1670 broadway denver colorado 80202 3amoco production co 501 westlake park blvd houston texas 77253 current address altura energy ltd p o box 4294 houston texas 77210 4amoco production co 501 westlake park blvd houston texas 77253 5amoco production co 4502 e 41st st tulsa oklahoma 74102 6amoco production co 501 westlake park blvd houston texas 77253 current address pgs reservoir 10550 richmond ave houston texas 77042 7amoco production co 1670 broadway denver colorado 80202 current address rocky mountain petrophysics p o box 470337 aurora colorado 80047 we thank the hugoton jomt of amoco production company for permission to publish this study we are grateful to mobil exploration and production u s especially w h jamieson jr for permission to use and publish additional core and log data we also thank david reese from amoco denver for his work on equations relating water saturation to permeability porosity and height in the gas column 3 d seismic interpretation by mike bahorich formerly with amoco currently with apache is appreciated thanks go to amoco operations and reservoir engineers too numerous to mention for teamwork and testing of the ideas herein ann pettit and stefan nowina provided invaluable technical support this paper benefited from constructive reviews by robert raynolds joyce budai n c wardlaw salman bloch and virginia riggert tcf trillion cubic feet gcf billion cubic feet