earthquakes good tidings tidal stresses in the earth s crust don t seem to influence earthquakes water wells on the other hand seem strangely sensitive to seismic activity explanations are now proposed

radionuclide sorption on primary and fracture filling minerals from the east bull lake pluton massey ontario canada abstract the east bull lake gabbroic pluton near massey ontario has been studied as part of the underlying research for the canadian nuclear fuel waste management program the pluton has undergone a complex multiple alteration history the dominant feature of this alteration is replacement of the pyroxenes by calcic amphiboles and by a variety of other alteration minerals particularly in fracture zones these fracture zones would play a dominant role in the eventual transport of radionuclides from a nuclear waste disposal vault through the host rock interactions between radionuclides in solution and minerals lining the fractures may retard migration of these radionuclides a comparative study of the sorptive capacity of fracture zone minerals for radionuclides representing alkaline earths 90 sr alkali metals 137 cs rare earth elements 144 ce actinides 241 am and two anionic forming fission products 75 se and 95m tc was carried out using thin sections from drill core material obtained from fracture zones the distribution of sorbed radionuclides on thin sections was determined by autoradiography in general alteration minerals show enhanced sorptive capacity in comparison to primary rock forming minerals increased k content significantly increases cs sorptin while increased mg and fe content plays a positive role in the removal of sr from solution

shear and normal load perturbations on a two dimensional continuous fault 2 dynamic triggering 1 the influence of normal and shear stress static perturbations on a strike slip fault is addressed on the basis of a two dimensional continuous and quasi dynamic model friction along the fault plane is described using a rate and state friction law with depth variable properties normal and shear stress perturbations result in similar effects in terms of earthquake triggering iftm s is constant t ands being the amplitude of the shear and normal stress fluctuations respectively and m being a constant which can be interpreted as the static friction coefficient on the fault in a coulomb failure model therefore the coulomb stress changecff tm s is a useful tool to account simultaneously for normal and shear stress variations in our model we also show that when estimating the clock advance or clock delay of an earthquake the simple coulomb failure model is at first order in good agreement with our results during the first 90 of the earthquake cycle however it differs significantly during the last 10 due to the sharp velocity increase predicted by the rate and state friction law before rupture this suggests that as long as static variations of stress are concerned realistic fault models using rich laboratory based friction laws like rate and state friction laws may lead topredictions fairly close to the ones made using one of the simplest failure model i e the coulomb failure model this may explain why coulomb stress change computations although often based on drastic approximations have been able in many occasions to explain earthquake triggering sequences index terms 7209 seismology earthquake dynamics and mechanics 7215 seismology earthquake parameters 7260 seismology theory and modeling keywords earthquake triggering static triggering coulomb stress change rate and state friction laws clock advance delay

redistribution of dynamic stress during coseismic ruptures evidence for fault interaction and earthquake triggering we investigate the spatiotemporal evolution of dynamic stress outside a rupturing extended fault the dynamic stress variations caused by a coseismic rupture in a half space are computed by using the discrete wavenumber and reflectivity methods after a transient phase the stress time history evolves to the final static stress value we compare the static stress changes resulting from this model with those computed from a static dislocation model we have applied this method to study the interactions between the first two normal faults which ruptured during the 1980 ms 6 9 irpinia earthquake these two subevents are separated in time by nearly 20 s while the third and last subevent occurred 40 s after the rupture onset we compute the dynamic stress changes caused by the rupture of the first subevent our modeling results show that the dynamic stress peak on the second subevent fault plane is reached between 7 s and 8 s after the rupture initiation on the main fault on the average the static stress level on the second subevent 20 s fault plane is reached nearly after 14 s the dynamic rupture did not jump from a rupturing segment to the adjacent one immediately but the triggering of the 20 s subevent is delayed by roughly 10 s with respect to the instant of occurrence of the dynamic stress peak induced by the 0 s event the dynamic stress pulse propagates along the strike direction of the second subevent fault plane at an average velocity of nearly 3 4 km s the delayed triggering of the second subevent can be interpreted in terms of the frictional properties of the faults in particular rate and state dependent frictional law can explain a delayed instability after a sudden change in stress using the estimated values of the subevent triggering delay and the shear stress change we attempt to constrain the parameter a on the 20 s fault the values here inferred agree well with those resulting from previous studies

laboratory gouge friction seismic like slip weakening and secondary rate and state effects 1 we investigate experimentally the frictional response of a thick sample of simulated fault gouge submitted to very high shear displacements up to 40 m in an annular simple shear apparatus acsa the frictional strength of our granular material exhibits velocity weakening consistent with classical rate and state dependent friction laws the length scale involved in the latter phenomenon is dc 100 m however the evolution of friction is largely dominated by a significant slip weakening active over decimetric distances l 0 5 m interestingly these decimetric frictional length scales are quantitatively compatible with those estimated for natural faults during shearing a thin and highly sheared layer emerges from the thick and slowly deforming bulk of the sample because of the intermittent and non local coupling observed between these two zones we relate the large frictional length scales in our data to the slow structuring of the thick interface

scaling of the critical slip distance for seismic faulting with shear strain in fault zones theoretical and experimentally based laws for seismic faulting contain a critical slip distance1 5 dc which is the slip over which strength breaks down during earthquake nucleation on an earthquake generating fault this distance plays a key role in determining the rupture nucleation dimension6 the amount of premonitory and post seismic slip7 10 and the maximum seismic ground acceleration1 11 in laboratory friction experiments d c has been related to the size of surface contact junctions2 5 12 thus the discrepancy between laboratory measurements of dc 10 5m and values obtained from modelling earthquakes 10 2m has been attributed to differences in roughness between laboratory surfaces and natural faults5 this interpretation predicts a dependence of dc on the particle size of fault gouge2 breccia and wear material but not on shear strain here we present experimental results showing that dc scales with shear strain in simulated fault gouge our data suggest a new physical interpretation for the critical slip distance in which dc is controlled by the thickness of the zone of localized shear strain as gouge zones of mature faults are commonly 102 103 m thick12 17 whereas laboratory gouge layers are 1 10 mm thick our data offer an alternative interpretation of the discrepancy between laboratory and field based estimates of dc

imaging surface contacts power law contact distributions and contact stresses in quartz calcite glass and acrylic plastic a procedure has been developed to obtain microscope images of regions of contact between roughened surfaces of transparent materials while the surfaces are subjected to static loads or undergoing frictional slip static loading experiments with quartz calcite soda lime glass and acrylic plastic at normal stresses to 30 mpa yield power law distributions of contact areas from the smallest contacts that can be resolved 3 5 m2 up to a limiting size that correlates with the grain size of the abrasive grit used to roughen the surfaces in each material increasing normal stress results in a roughly linear increase of the real area of contact mechanisms of contact area increase are by growth of existing contacts coalescence of contacts and appearance of new contacts mean contacts stresses are consistent with the indentation strength of each material contact size distributions are insensitive to normal stress indicating that the increase of contact area is approximately self similar the contact images and contact distributions are modeled using simulations of surfaces with random fractal topographies the contact process for model fractal surfaces is represented by the simple expedient of removing material at regions where surface irregularities overlap synthetic contact images created by this approach reproduce observed characteristics of the contacts and demonstrate that the exponent in the power law distributions depends on the scaling exponent used to generate the surface topography

triggering of the ms 5 4 little skull mountain nevada earthquake with dynamic strains abstract we have developed an approach to test the viability of dynamic strains as a triggering mechanism by quantifying the dynamic strain tensor at seismogenic depths we focus on the dynamic strains at the hypocenter of the m s 5 4 little skull mountain lsm nevada earthquake this event is noteworthy because it is the largest earthquake demonstrably triggered at remote distances 280 km by the m s 7 4 landers california earthquake and because of its ambiguous association with magmatic activity our analysis shows that if dynamic strains initiate remote triggering the orientation and modes of faulting most favorable for being triggered by a given strain transient change with depth the geometry of the most probable lsm fault plane was favorably oriented with respect to the geometry of the dynamic strain tensor we estimate that the magnitude of the peak dynamic strains at the hypocentral depth of the lsm earthquake were 4 strain 2 mpa which are 50 smaller than those estimated from velocity seismograms recorded at the surface we suggest that these strains are too small to cause mohr coulomb style failure unless the fault was prestrained to near failure levels the fault was exceptionally weak and or the dynamic strains trigger other processes that lead to failure

earthquake nucleation and triggering on an optimally oriented fault seismic surface waves from large distant earthquakes commonly trigger smaller earthquakes however delay times of hours to days between the surface waves and the triggered earthquakes weaken the causal connection furthermore when there is no delay the triggered earthquakes are typically too small or too obscured to obtain reliable source mechanisms we present observations of instantaneous triggering of a strike slip earthquake in central alaska shear strain from the optimally aligned teleseismic love wave induced a 24 s exponential foreshock signal leading to the triggered earthquake this nucleation phase and the alignment of the triggered earthquake source mechanism with the teleseismic stress field reveal the behavior of an existing fault under well calibrated strain conditions the alaska earthquake provides the first observation of combined nucleation and triggering and it suggests that transient stresses during nucleation may influence the subsequent earthquake rupture laboratory and theoretical studies of nucleation and triggering may help discriminate between different interpretations for the alaska earthquake

a new observation of dynamically triggered regional seismicity earthquakes in greece following the august 1999 izmit turkey earthquake the m w 7 4 izmit turkey earthquake triggered widespread regional seismicity in greece over a study region extending from 400 km to nearly 1000 km away from the epicenter small events began immediately after the passage of the mainshock surface waves suggesting that the transient stresses of the seismic waves were the trigger the increase in cataloged earthquakes in ordinary continental crust is a new observation and is statistically significant at the 95 level unlike the previous example of distant triggering during the landers earthquake the activated seismicity occurred entirely in non volcanic areas the greek sites were triggered by waves with amplitudes at least a factor of 3 lower than the observed triggering threshold for imperial valley we speculate that dynamic triggering on a regional scale results in countrywide episodes of increased seismicity or superswarms in regions with low triggering thresholds such as greece

static stress transfer and earthquake triggering no lower threshold in sight the main objective of this study is to see if a lower threshold for earthquake triggering exists resolving this issue is important for the understanding of earthquake mechanics and for the purpose of hazard analysis we compute the cumulative static stress changes imposed on 63 m 4 5 earthquakes in central california between 1969 and 1998 by adding the stress changes imposed by all previous m 4 5 earthquakes as a function of time prior to the events we find that 85 of the cumulative stress changes at the time of rupture are positive for stress change magnitudes of 10 kpa 0 1 bar and 70 are positive for stress changes of 10 kpa 0 1 bar as well as 1 kpa 0 01 bar a comparison between these results and those obtained for synthetic catalogs in which the timing or focal mechanisms of the earthquakes were randomized shows that this degree of triggering is very unlikely to be found in a random catalog thus we conclude that no lower threshold for earthquake triggering in central california has been found we show that the temporal distribution of stress changes that discourage failure is consistent with the theoretical prediction that the time delay increases with the magnitude of the stress change

absence of earthquake correlation with earth tides an indication of high preseismic fault stress rate because the rate of stress change from the earth tides exceeds that from tectonic stress accumulation tidal triggering of earthquakes would be expected if the final hours of loading of the fault were at the tectonic rate and if rupture began soon after the achievement of a critical stress level we analyze the tidal stresses and stress rates on the fault planes and at the times of 13 042 earthquakes which are so close to the san andreas and calaveras faults in california that we may take the fault plane to be known we find that the stresses and stress rates from earth tides at the times of earthquakes are distributed in the same way as tidal stresses and stress rates at random times while the rate of earthquakes when the tidal stress promotes failure is 2 higher than when the stress does not this difference in rate is not statistically significant this lack of tidal triggering implies that preseismic stress rates in the nucleation zones of earthquakes are at least 0 15 bar h just preceding seismic failure much above the long term tectonic stress rate of 10 4 bar h

pressure temperature and time indicators in mafic schist their application to reconstructing the polymetamorphic history of vermont mineral chemistry and overgrowth relationships in mafic schist from vermont are shown to be sensitive indicators of pressure temperature and relative time and to be good chronicles of the paleozoic history of this polymetamorphic terrane within the common assemblage amphibole chlorite epidote plagioclase quartz ti phase carbonate k mica fe 3 oxide electron microprobe analyses show that increasing metamorphic grade as defined by intercalated pelitic schist is recorded by an increase in the edenite glaucophane and tschermakite contents of amphibole in the anorthite content of plagioclase and in the substitution of al vi f 2 ti al iv for fe 2 mg mn si in biotite chlorite and muscovite with increasing pressure the glaucophane component of amphibole increases for medium pressure metamorphism the albite oligoclase gap is in the garnet zone where amphibole has between 1 2 and 1 8 formula proportion al iv and al vi fe 3 ti cr this gap is below the garnet isograd in low pressure mafic schist where al iv and al vi fe 3 ti cr in the amphibole are both less than 0 6 mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two ordovician taconic and two devonian acadian events silurian devonian mafic schist in northeastern vermont records two periods of low pressure devonian metamorphism these events are expressed by medium pressure and low pressure metamorphism in the cambrian and ordovician rocks to the west and south in the pre silurian rocks high pressure and medium pressure metamorphism is assigned to the ordovician the observed high pressure metamorphism is confined to a 110 by 40 km area along the green mountain anticlinorium axis in north central vermont

some characteristics of the stress field of the 1995 hyogo ken nanbu kobe earthquake we investigate the characteristics of the stress field associated with the 1995 hyogo ken nanbu kobe earthquake we use for the study a tomographic model of the fault slip inferred from the numerous near field recordings and we calculate the space and time evolution of shear stress on the fault during the earthquake we show that the spatial distribution of stress drop is very heterogeneous the apparent strength of the fault at the onset of the earthquake is low of the order of 1 mpa or less which indicates that the pre earthquake tectonic shear stress was close to the static friction over most of the fault at many locations the stress drop rotates significantly during sliding as was originally proposed by spudich 1992 we use the requirement of colinearity between the directions of maximum shear stress and instantaneous slip to determine the initial stress on the fault at the onset of the earthquake the pre earthquake tectonic stress varies greatly over the fault and ranges from about 1 to 10 mpa average values of the initial and final shear stresses over the fault are 3 3 and 1 6 mpa respectively indicating that about half of the pre earthquake tectonic stress was released during the earthquake there is a relatively strong correlation between the initial and final stress distributions which suggests that intrinsic fault properties not modified by the earthquake control the spatial distribution of tectonic stress over the fault we show that on average over the fault the dynamic friction coefficient is equal to about 40 of the static friction coefficient diagrams depicting the evolution of shear stress as a function of slip are consistent with slip weakening behavior but their interpretation is questionable because of the poor resolution of the data at high frequency and because of the constraints imposed on the model to perform the inversion

fault model for preseismic deformation at parkfield california we construct an earthquake instability model to estimate precursory faulting and ground deformation before the next moderate m 5 5 6 earthquake on the san andreas fault near parkfield california the quasi static model simulates fault slip fault shear stress and ground deformation for all stages of repeated earthquake cycles unstable slip which is the analog of a moderate parkfield mainshock is caused by overall failure of a postulated strong area or patch of the fault zone the brittle patch and surrounding weaker viscous fault are defined by the sign of a spatially varying coefficient of a slip rate dependent fault law stable failure of the patch leading up to mainshock failure causes slip rate to increase at depth amplitudes and timescales of the resulting preseismic deformation anomalies are found for the current earthquake cycle at parkfield starting just after the 1966 mainshock in two ways in the first way we vary parameter values in a sequence of simulations so as to find the simulation giving best agreement with locations moments and recurrence times of past moderate earthquakes and also with fault creep and trilateration line length changes since the 1966 mainshock in the second way we assign values of most parameters from laboratory friction experiments and a temperature depth profile inferred from heat flow data overall agreement between model results and observations is comparable for the two methods in all simulations preseismic deformation anomalies arise from slip rate increase near the model earthquake focus and start several years or less before instability preseismic anomalies in surface fault creep and line length are always too small to detect in current measurements however for some simulations constrained mainly by field data the predicted anomalies in borehole dilatation are large enough to detect

on the mechanics of earthquake afterslip we propose a model for earthquake afterslip based on rate and state variable friction laws in the model afterslip is attributed to the interaction of a velocity weakening region at depth within which earthquakes nucleate with an upper region of velocity strengthening frictional behavior the existence of this upper region is supported by independent seismologic observations and the results of laboratory friction experiments in our model afterslip is the result of relaxation of a stress perturbation within the velocity strengthening region which arises when an earthquake propagates into that region from below we derive the stress perturbation and its decay from the friction constitutive law using a simple 1 degree of freedom approximation for the elastic interaction between the fault and its surroundings this approximation is based on thickness averaged displacements and slip velocities within the velocity strengthening region which is assumed to slip as a rigid block coseismic and postseismic slip are coupled through the thickness averaged stiffness k of the velocity strengthening region we assume k to be inversely proportional to the thickness of this region which means that thicker velocity strengthening regions have a greater tendency to arrest coseismic slip we model the afterslip time histories of the 1966 parkfield and 1987 superstition hills earthquakes and relate the model parameters to physical parameters which may govern the rheologic behavior of the faults in accord with field observations our model predicts 1 that afterslip on some faults scales with the thickness of the unconsolidated sedimentary cover and 2 that proportionally more afterslip occurs for earthquakes in which coseismic surface slip is small compared with coseismic slip at depth velocity strengthening frictional behavior is to be expected for faults within poorly consolidated sediments and for those that contain significant gouge zones about 500 m within their shallow regions 3 5 km combining our results with those of recent laboratory friction studies indicates that relatively young faults with little accumulated fault gouge should exhibit little afterslip

the nature of groundwater flow in fractured rock evidence from the isotopic and chemical evolution of recrystallized fracture calcites from the canadian precambrian shield abstract the isotope geochemistry of fracture calcites in three precambrian plutons on the canadian shield has been investigated in order to understand the paleohydrogeological conditions in fractured crystalline rock fracture calcites of ancient hydrothermal origins in the chalk river and east bull lake plutons exhibit 18o enrichment and 13c depletion trends resulting from recent low temperature calcite recrystallization under open system conditions for oxygen but semiclosed for carbon and under extremely variable time integrated water rock calcite ratios this process causes recycling of elements with calcite distribution coefficients 1 rare earths manganese and possibly iron from the precursor calcite to younger calcites as well as calcite control over the 87 sr 86 sr ratio of the groundwater within the chalk river pluton the large but variable water rock calcite mole ratios calculated from the shifts in the stable isotopic compositions of fracture calcites are compatible with fracture flow models that invoke flow channeling within single fractures that also contain regions of immobile porosity quantification of isotopic shifts resulting from recrystallization requires that the initial isotopic composition of the precursor hydrothermal calcite be well constrained unlike the chalk river and east bull lake plutons hydrothermal fracture calcites in the white lake pluton which intrudes marble rich country rocks of the grenville supergroup have highly variable 13c and 18o values this is attributed to mixing of carbon from magmatic and sedimentary reservoirs and to oxygen isotopic exchange between hydrothermal fluids and carbonate country rocks at the time of intrusion convective circulation of meteoric groundwater from the surrounding 13c rich carbonate rocks up to 6 6 resulted in 13c values as heavy as 3 0 for the fracture calcites in the pluton

implications of halide leaching on 36cl studies at yucca mountain nevada 1 chlorine 36 was generated from nuclear tests in the 1950s and 1960s and has been used to identify fast flow paths at yucca mountain the proposed repository for high level nuclear waste fabryka martin et al 1997 1998 bomb pulse 36cl brought into the subsurface by infiltrating rainwater presumably resides along fracture surfaces because of the extremely low rock matrix permeability however leaching a rock sample to extract this salt inevitably extracts pore water chloride cl and rock matrix chloride thereby making it difficult to obtain reproducible measurements or detect the specific bomb pulse signatures complexities introduced by these sources of older chloride include dilution of bomb pulse 36cl cl ratios for samples from strata with a high cl concentration variations in measured ratios as a function of leaching time rock chip size and the differing effects of active leaching from those of passive leaching this work provides both a conceptual model and a mathematical solution for the leaching processes and examines the role of sample leaching in the 36cl studies of yucca mountain rocks an analytical solution is derived for the diffusion of cl and 36cl in composite media rock matrix and water to accommodate variable diffusivity this solution is subsequently used to develop a leaching model that takes into account bomb pulse signal matrix pore water and relatively hard to leach components isolated fluid inclusion and mineral boundary salts the model is then applied to samples from stratigraphic units at yucca mountain to obtain leachate concentrations from different setup methods protocols including duration chip size and gravitational settling of the water rock mixture the model results show that the probability of detecting a 36cl cl bomb pulse signal is severely diminished under longer leaching times and smaller rock fragment sizes and that leaching times of 1 to 10 hours are most likely to be successful in detecting a bomb pulse signal bomb pulse 36cl cl ratios are more likely to be observed if pore water cl concentrations were initially low prior to the introduction of bomb pulse carrying water

triggering of earthquake aftershocks by dynamic stresses it is thought that small static stress changes due to permanent fault displacement can alter the likelihood of or trigger earthquakes on nearby faults many studies of triggering in the near field particularly of aftershocks rely on these static changes as the triggering agent and consider them only in terms of equivalent changes in the applied load on the fault here we report a comparison of the aftershock pattern of the moment magnitude mw 7 3 landers earthquake not only with static stress changes but also with transient oscillatory stress changes transmitted as seismic waves that is dynamic stresses dynamic stresses do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone these dynamically weakened faults may fail after the seismic waves have passed by and might even cause earthquakes that would not otherwise have occurred we find similar asymmetries in the aftershock and dynamic stress patterns the latter being due to rupture propagation whereas the static stress changes lack this asymmetry previous studies have shown that dynamic stresses can promote failure at remote distances but here we show that they can also do so nearby

evidence of instantaneous dynamic triggering during the seismic sequence of year 2000 in south iceland 1 we analyze the coseismic stress perturbation during the 17 june 2000 south iceland seismic sequence the main shock ms 6 6 was followed by three large events within a few tens of seconds 8 26 and 30 s located within 80 km the aim of this paper is to investigate short term fault interaction and instantaneous triggering this happens when a fault perturbed by a stress change fails before the end of the transient stress perturbation we compute the shear normal and coulomb stress changes as functions of time in a stratified elastic half space by using discrete wave number and reflectivity methods we calculate dynamic stresses caused by the main shock at the hypocenters of these three subsequent events our numerical results show that the onset of the last two events is slightly delayed with respect to the arrival time of the second positive peak of coulomb stress variation while the first event occurred after the first positive stress peak we have also analyzed the response of a spring slider system representing a fault governed by a rate and state dependent friction law perturbed by shear and normal stress variations caused by the main shock the fault response to the computed stress perturbations is always clock advanced we have found suitable constitutive parameters of the modeled fault that allow the instantaneous dynamic triggering of these three earthquakes if the initial sliding velocity is comparable with the tectonic loading velocity we obtained failure times close to the observed origin times for low values of the initial effective normal stress

hydrothermal synthesis of amphiboles along the tremolite pargasite join and in the ternary system tremolite pargasite cummingtonite we report here an investigation of amphibole synthesis within the ternary system ca 2mg5si8o22 oh 2naca2mg4al3si6o22 oh 2 mg7si8o22 oh 2 tr pg mc to define better the limits of solid solution for amphiboles formed along and near the join tremolite pargasite hydrothermal syntheses were conducted in the range of 750 1000 c and 1 6 kbar syntheses at 10 mol compositional intervals along the tr pg join produced incomplete yields of amphibole and the resultant amphiboles were found to be associated with varying amounts of accessory clinopyroxene plagioclase and gehlenite syntheses of end member pargasite with progressively greater amounts of the mc component produced a very good amphibole yield for a pargasite composition containing 2 5 mol mc the persistence of clinopyroxene or gehlenite and the improved yield of pargasitic amphibole with minor addition of the mc component indicate a shift toward mc enrichment for amphiboles made from bulk compositions directly on the tr pg join syntheses within the tr pg mc ternary system suggest that the ca rich limit of solid solution is a slightly curved field in the tr pg mc field amphiboles made along this join showed a very systematic change in the 151 and 3 31 peak spacing with mol pg providing a simple technique for deducing the composition of amphiboles made near the tremolite pargasite join electron microprobe analysis confirms that the amphiboles for the most part conform to the pargasite substitution the one exception being amphiboles formed near 80 90 mol pg compositions which proved both difficult to synthesize 75 wt amphibole yield and displayed a small but significant excess of na in their structure high yields of amphibole could be formed from this same bulk composition with the use of a dilute naoh solution instead of pure water suggesting that there was insufficient na in the starting mixture to allow complete amphibole formation structural refinements using x ray diffraction xrd rietveld refinement confirmed the high na content for this sample and also revealed a non linear trend in the unit cell volume with composition with a maximum at the 80 mol pg bulk composition the difficulty in synthesizing amphiboles near this bulk composition and its unusually high volume suggest that immiscibility in natural hornblendes may initiate with increasing pressure for pargasite rich bulk compositions

a petrochemical study of calcic amphiboles from the east bull lake anorthosite gabbro layered complex district of algoma ontario calcic amphiboles are ubiquitous in the east bull lake anorthosite gabbro complex northeastern ontario the mode of occurrence suggests the amphiboles replaced clinopyroxene throughout the stratigraphic levels of the pluton but they are also prolific in the top younger units three types of amphiboles namely 1 tremolite actinolite 2 actinolitic hornblende and 3 hornblende were identified the composition of the amphiboles shows that they define a continuous series without any obvious compositional miscibility gap the analyses further indicated that the amphiboles evolved from actinolite to hornblende through coupled substitution involving edenite and tschermakite end members among the three coexisting amphiboles chlorine is concentrated in hornblende this is due to preferential location of hornblende along grain boundaries and or its favourable structure

multiple alteration events in the east bull lake anorthosite gabbro layered complex ne ontario canada evidence from fracture mineralogy and40ar 39ar dating abstract the east bull lake anorthosite gabbro layered complex contains a variety of alteration minerals some of the more common ones are calcic amphiboles biotite epidote adularia quartz chlorite calcite prehnite pumpellyite laumontite gypsum iron hydroxides and clays the mode of occurrence and the data related to the stability of the alteration minerals suggest that they were formed under pressure temperature conditions of 1 epidote amphibolite greenschist facies 2 prehnite pumpellyite facies 3 zeolite facies and 4 low temperature mineral facies 40 ar 39 ar data of hornblende and adularia indicate that the pluton is affected by distinct alteration events two mafic dyke intrusions that overlap the alteration events are recognised in the pluton synthesis of available radiometric ages suggests that the pluton intruded at 2472 70ma and was subjected to alteration as late as the paleozoic and cenozoic eras

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

interplay of seismic and aseismic deformations during earthquake swarms an experimental approach observations of earthquake swarms and slow propagating ruptures on related faults suggest a close relation between the two phenomena earthquakes are the signature of fast unstable ruptures initiated on localized asperities while slow aseismic deformations are experienced on large stable segments of the fault plane the spatial proximity and the temporal coincidence of both fault mechanical responses highlight the variability of fault rheology however the mechanism relating earthquakes and aseismic processes is still elusive due to the difficulty of imaging these phenomena of large spatiotemporal variability at depth here we present laboratory experiments that explore in great detail the deformation processes of heterogeneous interfaces in the brittle creep regime we track the evolution of an interfacial crack over 7 orders of magnitude in time and 5 orders of magnitude in space using optical and acoustic sensors we explore the response of the system to slow transient loads and show that slow deformation episodes are systematically accompanied by acoustic emissions due to local fracture energy disorder features of acoustic emission activities and deformation rate distributions of our experimental system are similar to those in natural faults on the basis of an activation energy model we link our results to the rate and state friction model and suggest an active role of local creep deformation in driving the seismic activity of earthquake swarms

accuracy of bathymetric assessment by locally analyzing radar ocean wave imagery february 2008 in this paper the error source in assessing the bathymetry by a recently presented method the dispersive surface classificator is discussed this method is based on the analysis of x band radar image sequences of sea surface waves to determine spatial maps of hydrographic parameters to implement this objective the radar deduced bathymetry is validating by multibeam echosounder data the accuracy of the method is high in homogeneous areas and reduced at the areas of bathymetric gradient and lower but comparable with multibeam echosounder data under the assumption of the spatial resolution the identification of systematic correlation of the absolute value of the error with the slope is significant and insignificant with the actual depth itself the spatial correlation of the error illustrates that the direction of the wave field influences the neighboring grid cell in the same direction the application of the method during crucial weather conditions is the main advantage and permits the accurate operational nearshore monitoring for several applications

geodetic observations of postseismic creep in the decade after the 1999 izmit earthquake turkey implications for a shallow slip deficit the relationship between aseismic slip and tectonic loading is important for understanding both the pattern of strain accumulation along a fault and its ability to generate large earthquakes we investigate the spatial distribution and temporal evolution of aseismic creep on the western north anatolian fault naf using time series analysis of envisat interferometric synthetic aperture radar insar data covering the full extent of the 1999 izmit and duzce earthquake ruptures and spanning 2002 2010 discontinuities in the line of sight velocity across the fault imply that fault creep reaches the earth s surface at an average fault parallel rate of 5 mm yr along an 80 km section of the naf by combining insar and published gps velocities we are able to extract the vertical and east west components of motion and show that the adapazari basin is subsiding at a rate of 6 mm yr vertical motions have biased previous estimates of creep in this region the displacement time series close to the fault is consistent with an afterslip model based on rate and state friction which predicts a rapid deceleration in fault creep rate after the izmit earthquake to a near steady state 5 mm yr after 5 years projecting our model 200 years into the future we find that the cumulative displacement of 1 1 3 m is insufficient to account for the shallow coseismic slip deficit observed in previous studies distributed off fault deformation in the shallow crust or transient episodes of faster slip are likely required to release some of the long term strain during the earthquake cycle

on the lognormal distribution of oxides in igneous rocks using magnetic susceptibility as a proxy for oxide mineral concentration summary trace element concentrations in igneous rocks are frequently lognormally distributed and shaw 1961 suggested that minor minerals in igneous rocks might also be lognormally distributed there is accumulating evidence to show that bulk magnetic susceptibility bms often closely follows a lognormal distribution in fresh igneous rocks but because bms is dependent upon oxide grain size and mineralogy it is not obvious why this should be so we have adopted shaw s simple theoretical model as an argument to account for the lognormal distribution of bms the obvious prevailing conditions must be that the original melt was uniform that pressure temperature and oxygen fugacity were constant and that the minerals were formed at equilibrium in addition because bms is also a function of grain size and mineralogy the distribution of these latter parameters must be insensitive to changes in p t etc at equilibrium it appears that there must be a limit to the lognormal law in trying to apply it to minerals of greater concentration however bms proxies for oxide content and because it does closely follow the lognormal distribution it thus appears that in crystalline igneous rocks the lognormal law is obeyed by minerals of concentrations at least up to a few percent knowledge of the mean and variance of such distributions can be useful in a variety of petrological applications especially in drillcore logging a simple example is presented

compositional variation of calcic amphiboles in mineoka metabasites central japan and its bearing on the actinolite hornblende miscibility relationship abstract calcic amphiboles from the metabasites of the medium pressure type from mineoka central japan are chemically continuous from al poor actinolite to al rich hornblende even in a single grain calcic amphibole with the intermediate composition between actinolite and hornblende was confirmed to be a single phase by analytical electron microscope and x ray diffraction studies calcic amphiboles from mineoka metabasites transitional between greenschist and epidote amphibolite facies are characterized by the continuous deficiency of ca which is due to the increase of crossite and cummingtonite components from actinolite to hornblende in metabasites of medium pressure type however calcic amphiboles with low ca deficiency have the compositional gap between actinolite and hornblende it is most probable that the miscibility gap in calcic amphiboles of metabasites of medium pressure type tends to be narrowed with increase of crossite and cummingtonite components

earthquake nucleation on model faults with rate and state dependent friction effects of inertia laboratory studies suggest that earthquake nucleation involves a transition from quasi static slip when inertial effects are negligible to inertia driven dynamic motion this transition occurs via quasi dynamic motion during which the effects of inertia become increasingly important the characteristics of this transition which depend on frictional properties of the fault determine the observability of earthquake nucleation by seismic geodetic or other means by investigating the role of inertia during nucleation we obtain a quantitative definition of the limiting velocity vin which marks the end of quasi dynamic motion and the onset of instability for reasonable friction parameters and fault widths we obtain estimates of vin for crustal faults to study the roles of inertia stiffness and friction parameters on preseismic motion we simulate triggered instabilities in a one dimensional model of a homogeneous fault with rate and state variable friction in most of our simulations triggering is achieved by applying a stress perturbation to an initially creeping fault under steady state friction we also investigate triggering on faults which are initially locked and overstressed compared to their nominal frictional strength due to time dependent healing we study the amount up and duration tp of preseismic slip as a function of system mass m and other model parameters for crustal faults we interpret the relevant mass as a product of density and fault width and find that wider fault zones result in smaller up and larger tp both up and tp are proportional to the system stiffness k the characteristic slip distance dc and the friction constitutive parameter a and inversely proportional to the size of the triggering event we find greater up and tp for constitutive laws which allow strengthening at zero velocity compared to laws that require slip or a combination of slip and aging for state evolution in contrast to quasi static modeling our simulations suggest a minimum stress perturbation criterion for instability which may be interpreted in terms of a strain threshold for triggered seismicity

fault stability inferred from granite sliding experiments at hydrothermal conditions seismicity on crustal faults is concentrated in the depth interval 1 3 to 12 15km tse and rice 1986 suggested that the lower bound on seismicity is due to a switch with increasing temperature from velocity weakening destabilizing to velocity strengthening stabilizing friction they inferred this transition from friction data for dry granite however pore fluids exist at elevated temperatures throughout the crust and may strongly influence strength and sliding behavior we present new data from sliding experiments on granite at elevated t 23 to 600 plus elevated ph2o 100 mpa our results show velocity strengthening at room temperature but velocity weakening from 100 to 350 c except at 250 from 350 to 600 there are systematic trends from velocity weakening to strong velocity strengthening and from high to low friction neither trend was seen in tests on dry granite the velocity dependence data imply the potential for unstable slip in the interval 100 to 350 using a geotherm to map temperature to depth this interval closely matches the observed earthquake distribution

suppression of large earthquakes by stress shadows a comparison of coulomb and rate and state failure stress shadows generated by california s two most recent great earthquakes 1857 fort tejon and 1906 san francisco substantially modified 19th and 20th century earthquake history in the los angeles basin and in the san francisco bay area simple coulomb failure calculations which assume that earthquakes can be modeled as static dislocations in an elastic half space have done quite well at approximating how long the stress shadows or relaxing effects should last and at predicting where subsequent large earthquakes will not occur there has however been at least one apparent exception to the predictions of such simple models the 1911 m 6 0 earthquake near morgan hill california occurred at a relaxed site on the calaveras fault we examine how the more complex rate and state friction formalism based on laboratory experiments might have allowed the 1911 earthquake rate and state time to failure calculations are consistent with the occurrence of the 1911 event just 5 years after 1906 if the calaveras fault was already close to failure before the effects of 1906 we also examine the likelihood that the entire 78 years of relative quiet only four m 6 earthquakes in the bay area after 1906 is consistent with rate and state assumptions given that the previous 7 decades produced 18 m 6 earthquakes combinations of rate and state variables can be found that are consistent with this pattern of large bay area earthquakes assuming that the rate of earthquakes in the 7 decades before 1906 would have continued had 1906 not occurred these results demonstrate that rate and state offers a consistent explanation for the 78 year quiescence and the 1911 anomaly although they do not rule out several alternate explanations

transient triggering of near and distant earthquakes we demonstrate qualitatively that frictional instability theory provides a context for understanding how earthquakes may be triggered by transient loads associated with seismic waves from near and distance earthquakes we assume that earthquake triggering is a stick slip process and test two hypotheses about the effect of transients on the timing of instabilities using a simple spring slider model and a rate and state dependent friction constitutive law a critical triggering threshold is implicit in such a model formulation our first hypothesis is that transient loads lead to clock advances i e transients hasten the time of earthquakes that would have happened eventually due to constant background loading alone modeling results demonstrate that transient loads do lead to clock advances and that the triggered instabilities may occur after the transient has ceased i e triggering may be delayed these simple clock advance models predict complex relationships between the triggering delay the clock advance and the transient characteristics the triggering delay and the degree of clock advance both depend nonlinearly on when in the earthquake cycle the transient load is applied this implies that the stress required to bring about failure does not depend linearly on loading time even when the fault is loaded at a constant rate the timing of instability also depends nonlinearly on the transient loading rate faster rates more rapidly hastening instability this implies that higher frequency and or longer duration seismic waves should increase the amount of clock advance these modeling results and simple calculations suggest that near tens of kilometers small moderate earthquakes and remote thousands of kilometers earthquakes with magnitudes 2 to 3 units larger may be equally effective at triggering seismicity our second hypothesis is that some triggered seismicity represents earthquakes that would not have happened without the transient load i e accumulated strain energy would have been relieved via other mechanisms we test this using two new seismicity models that 1 are inherently unstable but slide at steady state conditions under the background load and 2 are conditionally stable such that instability occurs only for sufficiently large perturbations for the new seismicity models very small amplitude transients trigger instability relative to the clock advance models the unstable steady state models predict that the triggering delay depends inversely and nonlinearly on the transient amplitude as in the clock advance models we were unable to generate delayed triggering with conditionally stable models for both new seismicity models the potential for triggering is independent of when the transient load is applied or equivalently of the prestress unlike in the clock advance models in these models a critical triggering threshold appears to be inversely proportional to frequency further advancement of our understanding will require more sophisticated quantitative models and observations that distinguish between our qualitative yet distinctly different model predictions

dynamic triggering of creep events in the salton trough southern california by regional m 5 4 earthquakes constrained by geodetic observations and numerical simulations abstract since a regional earthquake in 1951 shallow creep events on strike slip faults within the salton trough southern california have been triggered at least 10 times by m 5 4 earthquakes within 200 km the high earthquake and creep activity and the long history of digital recording within the salton trough region provide a unique opportunity to study the mechanism of creep event triggering by nearby earthquakes here we document the history of fault creep events on the superstition hills fault based on data from creepmeters insar and field surveys since 1988 we focus on a subset of these creep events that were triggered by significant nearby earthquakes we model these events by adding realistic static and dynamic perturbations to a theoretical fault model based on rate and state dependent friction we find that the static stress changes from the causal earthquakes are less than 0 1 mpa and too small to instantaneously trigger creep events in contrast we can reproduce the characteristics of triggered slip with dynamic perturbations alone the instantaneous triggering of creep events depends on the peak and the time integrated amplitudes of the dynamic coulomb stress change based on observations and simulations the stress change amplitude required to trigger a creep event of a 0 01 mm surface slip is about 0 6 mpa this threshold is at least an order of magnitude larger than the reported triggering threshold of non volcanic tremors 2 60 kpa and earthquakes in geothermal fields 5 kpa and near shale gas production sites 0 2 0 4 kpa which may result from differences in effective normal stress fault friction the density of nucleation sites in these systems or triggering mechanisms we conclude that shallow frictional heterogeneity can explain both the spontaneous and dynamically triggered creep events on the superstition hills fault

stress strain changes and triggered seismicity at the geysers california the principal results of this study of remotely triggered seismicity in the geysers geothermal field are the demonstration that triggering initiation of earthquake failure depends on a critical strain threshold and that the threshold level increases with decreasing frequency or equivalently depends on strain rate this threshold function derives from 1 analyses of dynamic strains associated with surface waves of the triggering earthquakes 2 statistically measured aftershock zone dimensions and 3 analytic functional representations of strains associated with power production and tides the threshold is also consistent with triggering by static strain changes and implies that both static and dynamic strains may cause aftershocks the observation that triggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process assuming the physical processes underlying earthquake nucleation to be the same gomberg this issue discusses seismicity triggered by the mw 7 3 landers earthquake its constraints on the variability of triggering thresholds with site and the implications of time delays between triggering and triggered earthquakes our results enable us to reject the hypothesis that dynamic strains simply nudge prestressed faults over a coulomb failure threshold sooner than they would have otherwise we interpret the rate dependent triggering threshold as evidence of several competing processes with different time constants the faster one s facilitating failure and the other s inhibiting it such competition is a common feature of theories of slip instability all these results not surprisingly imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceedence of some constant threshold but also the requirements for generating instabilities

cyclic deformation and chemical transport in the folson lake fault zone east bull lake anorthosite gabbro complex evidence for seismic pumping abstract the east bull lake anorthosite gabbro a late archean 2472 24ma magmatically layered lopolith at the southern margin of the superior province has experienced regional retrograde metamorphism and is transected by the folson lake fault zone a structure of regional extent the fault zone displays repeated episodes of ductile brittle deformation and vein development it has been extensively metasomatized in domains marked by the sequential development of quartz adularia epidote chlorite laumontite calcite and clay fe oxyhydroxide downhole geochemical profiles through the fault zone show coherent patterns for zr hf zr th ta nb and cr co ni which are interpreted to reflect variations in primary igneous compositions by alteration insensitive elements metasomatism is marked by peaks of sio2 k2o or na2o with al2o3 fe mn and mg and ca plus volatiles corresponding to domains of quartz adularia or albite chlorite and laumontite plus calcite respectively the analogue elements th and u are present at uniformly low levels averaging 1 1 and 0 4 ppm respectively thorium covaries with zr such that variations of abundance reflect primary differences whereas th u ratios scatter between 1 5 to 10 indicating minor remobilization of u strontium has been extensively redistributed within the fault zone and cs covaries with k retrograded areas of the pluton have whole rock 18 values ranging between 4 5 and 7 2 per mil both below and above primary values of 5 7 per mil indicating open system behaviour during regional retrograde metamorphism within the fault zone hydrothermal reservoirs involved in later k metasomatism were at temperatures of 150 250 c where 18 o h2o 2 6 to 1 7 per mil likely representing isotopically and chemically evolved deep pore fluids expelled during crustal activity related to tectonic processes in the nearby grenville province calcite and laumontite developed by the advection through the fault zone of ca rich thermal brines at temperatures around 50 200 c where 18oh2o 15 to 0 per mil such fluids were evolved formation brines with a major component of meteoric h2o expelled in the lower paleozoic the multiple deformation metasomatism events are interpreted in terms of episodic seismic pumping on a fault that was intermittently active over a time scale in excess of 1 ga

the east bull lake intrusive suite remnants of a 2 48 ga large igneous and metallogenic province in the sudbury area of the canadian shield the 2 49 to 2 475 ga intrusions of the east bull lake intrusive suite occur in an east northeast trending discontinuous belt along the presently exposed boundary between the archean superior and the proterozoic southern provinces of the canadian shield near sudbury ontario the east bull lake intrusive suite is part of a regional paleoproterozoic magmatic event extending from 2 49 to 2 44 ga which also includes bimodal volcanic rocks felsic plutons and the regionally extensive hearst and matachewan dike swarms this regional magmatic event is thought to be the result of a mantle plume driven intracontinental rifting event that led to the development of a major basin to the south that was subsequently filled by sedimentary rocks of the huronian supergroup the east bull lake intrusive suite appears to have been emplaced along a major axial rift fault related to this rifting event compelling field and geochemical evidence summarized here indicates that the three largest east bull lake suite intrusions the east bull lake agnew lake and river valley intrusions crystallized from similar low ti high al tholeiitic parent magmas that originated in deeper more basic chambers it is proposed that the primary magmas for the east bull lake suite intrusions were second stage melts derived from a sublithospheric depleted mantle source that had been modified by neoarchean subduction and accretion events most of the petrological characteristics of the east bull lake suite intrusions reflect plagioclase dominated fractional crystallization that generated a pronounced fe enrichment trend in the residual magmas orthopyroxene is the high temperature pyroxene and occurs as a cumulus phase through most of the stratigraphy of each intrusion of the suite local olivine rich cumulates present in all of the major east bull lake suite plutons in some instances appear to be comagmatic with associated plagioclase rich cumulates a distinctive marginal facies comprising brecciated and locally thermally recrystallized and or partially melted footwall rocks is a common feature of the east bull lake suite plutons and indicates that a high energy flow regime existed during the initial stages of emplacement this marginal unit typically grades into a heterolithic inclusion rich gabbronorite that contains erratically distributed mafic autoliths and footwall derived xenoliths chemical contamination of the mineralized matrix of these inclusion bearing rocks can be extreme at a local scale the inclusion rich zone is overlain by a thick interval of undifferentiated plagioclase rich cumulates that locally display spectacular glomerocrystic textures rhythmically and irregularly modally layered leucogabbronorite and gabbronorite make up much of the middle parts of the stratigraphy massive to crudely layered ferrogabbro and ferrosyenite only occur in the uppermost part of the agnew lake intrusion varitextured gabbro and pegmatitic gabbro occur as irregular layers and local veins and pods throughout the stratigraphy the east bull lake suite is currently being explored for contact type platinum group element pge cu ni mineralization that has a clear spatial association with orthopyroxene rich cumulates which are otherwise poorly represented in these intrusions disseminated pge and cu rich sulfide mineralization is concentrated within the inclusion bearing zones but extends upward into overlying plagioclase cumulates and appears to have collected along the lower margins of these intrusions through the combined action of vigorous convection of the resident magma and downward percolation of dense mafic residual liquid the resultant narrow zones of sulfide mineralization were thus derived from a much larger volume of parent magma feeder dikes to the mineralized parts of the intrusions appear to have been saturated in sulfide upon emplacement and had relatively high background pge contents compared to those for other known magmatic pge deposits however the economic potential of the contact type mineralization hinges on the efficacy of the physical concentration process which upgraded the metal content and size of the original sulfide particles inside the magma chambers based on the proposed basal accumulation model the best prospective mineralization is most likely to occur in embayments along the sidewall and floor of the intrusions

earthquake nucleation on faults with rate and state dependent strength abstract dieterich j h 1992 earthquake nucleation on faults with rate and state dependent strength in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 115 134 faults with rate and state dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability calculations with a plane strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip the critical length is a function of normal stress loading conditions and constitutive parameters which include dc the characteristic slip distance if slip starts on a patch that exceeds the critical size the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches solutions have been obtained for a uniform fixed patch model that are in good agreement with results from the plane strain model over a wide range of conditions above the steady state stress the logarithm of the time to instability linearly decreases as the initial stress increases because nucleation patch length and premonitory displacement are proportional to dc the moment of premonitory slip scales by d3c the scaling of dc is currently an open question unless dc for earthquake faults is significantly greater than that observed on laboratory faults premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods excluding the possibility that dc in the nucleation zone controls the magnitude of the subsequent earthquake then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch

elastodynamic analysis for slow tectonic loading with spontaneous rupture episodes on faults with rate and state dependent friction we present an efficient and rigorous numerical procedure for calculating the elastodynamic response of a fault subjected to slow tectonic loading processes of long duration within which there are episodes of rapid earthquake failure this is done for a general class of rate and state dependent friction laws with positive direct velocity effect the algorithm allows us to treat accurately within a single computational procedure loading intervals of thousands of years and to calculate for each earthquake episode initially aseismic accelerating slip prior to dynamic rupture the rupture propagation itself rapid post seismic deformation which follows and also ongoing creep slippage throughout the loading period in velocity strengthening fault regions the methodology is presented using the two dimensional 2 d antiplane spectral formulation and can be readily extended to the 2 d in plane and 3 d spectral formulations and with certain modifications to the space time boundary integral formulations as well as to their discretized development using finite difference or finite element methods the methodology can be used to address a number of important issues such as fault operation under low overall stress interaction of dynamic rupture propagation with pore pressure development patterns of rupture propagation in events nucleated naturally as a part of a sequence the earthquake nucleation process earthquake sequences on faults with heterogeneous frictional properties and or normal stress and others the procedure is illustrated for a 2 d crustal strike slip fault model with depth variable properties for lower values of the state evolution distance of the friction law small events appear the nucleation phases of the small and large events are very similar suggesting that the size of an event is determined by the conditions on the fault segments the event is propagating into rather than by the nucleation process itself we demonstrate the importance of incorporating slow tectonic loading with elastodynamics by evaluating two simplified approaches one with the slow tectonic loading but no wave effects and the other with all dynamic effects included but much higher loading rate

implications of rate and state friction for properties of aftershock sequence quasi static inherently discrete simulations 1 dieterich 1994 modeled the response to a stress step of a population of faults governed by rate and state dependent friction this model assumes that aftershocks nucleate over areas on the fault that at the time of the main shock are already accelerating toward failure and disregards the effect of interactions among aftershocks the main objective of this study is to examine consequences of relaxing these underlying assumptions aftershock activity is simulated using an inherently discrete earthquake fault model with a fault surface governed by an approximate constitutive friction law similar to the one used by dieterich we find that the governing equations in nondimensional form are a function of three main parameters and explore the effect of these parameters on the simulated catalogs we derive a simple expression for the time dependent seismicity response to a stress step that approximates the effect of multiple interactions among aftershocks as a time dependent stressing rate close match is found between the simulated seismicity response to a stress step and that predicted analytically however the numerical simulations show that the effect of the main shock is not only to raise the local seismicity rate but also to systematically modify the earthquake size distribution as a result the actual seismicity rate change early during the aftershock sequence may be higher than that predicted whereas seismicity rate late in the sequence may be lower than that predicted such a modification of the earthquake size distribution can explain observations of lower b values immediately following a stress step

nucleation and early seismic propagation of small and large events in a crustal earthquake model 1 earthquake nucleation and early seismic propagation are studied in a two dimensional strike slip fault model with depth variable properties the fault is governed by the dieterich ruina rate and state friction law we use an efficient and rigorous numerical procedure for elastodynamic analysis of earthquake sequences on slowly loaded faults developed by lapusta et al 2000 we find that for decreasing values of the characteristic slip distance of the friction law small events appear at the transition from the locked to creeping behavior toward the bottom of the seismogenic zone small and large events have very similar nucleation phases in our simulations here by nucleation phase we mean gradually accelerating aseismic slip in a small slowly expanding zone before the breakout of the dynamic seismically detectable event moment acceleration to which velocity seismograms are proportional in early stages of seismic propagation exhibits irregular fluctuations in the form of speedups and slowdowns in the moment release rate consistently with observations as reported by ellsworth and beroza 1995 our simulations show that such irregular moment acceleration can at least in part be due to the heterogeneous stress distribution imprinted on the fault by the arrest of previous small events and by stress concentrations at the borders of creeping regions and to partial arrest of the rupture in velocity strengthening fault regions which inhibit seismic slip index terms 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 3220 mathematical geophysics nonlinear dynamics 7230 seismology seismicity and seismotectonics 3230 mathematical geophysics numerical solutions 8164 tectonophysics stresses crust and lithosphere keywords earthquake nucleation phase event clustering irregular moment release stress concentrations rate and state friction earthquake sequences

determination of constitutive relations of fault slip based on seismic wave analysis constitutive laws define the boundary conditions on fault plane and govern many aspects of earthquake failure although several constitutive laws have been formulated based on laboratory rock experiments and applied to theoretical studies in various fields no actual relation during a natural earthquake has been determined the 1995 kobe earthquake is suitable for detailed kinematic analysis and this enables the first evaluation of constitutive relations for a natural earthquake in this study we determine spatiotemporal slip distribution on an assumed fault plane of the 1995 kobe earthquake by waveform inversion and then solve elastodynamic equations using a finite difference method to determine the stress distribution and constitutive relations on the fault plane an inversion method based on bayes theorem is employed to obtain a spatiotemporal slip distribution and enables us to ensure the objective uniqueness of the solution with numerous parameters and smoothing constraints this slip distribution is then used as part of the boundary condition in the finite difference calculation the time histories of slip and shear stress obtained then provide a constitutive relation at each point on the fault plane they show slip weakening relations almost everywhere on the fault plane while slip rate dependency is not clear the slip weakening behavior has a clear depth dependency indicating that the slip weakening rate d du is smaller in the shallow crust than that in the deep crust this may be associated with the paucity of shallow seismicity observed in the source region of this earthquake as reported for many mature fault systems

the geochemistry age and origin of groundwater in a mafic pluton east bull lake ontario canada abstract the geochemistry of groundwater in the east bull lake gabbro anorthosite pluton near elliot lake ontario canada has been investigated three chemical types of groundwater are present to depths of about 650 m and are controlled by the nature of the groundwater flow systems and the water rock interactions in the fractures ca hco 3 water is present in the recharge area of the local flow system and evolves rapidly along the direction of groundwater flow to a high ph na hco 3 water primarily by cation exchange reactions saline na cl water is present below a depth of about 350 m on the basis of its chemistry and hydraulic head measurements the na cl water is believed to be part of a deep regional groundwater flow system carbon 14 measurements of the na hco 3 water suggest that this water was recharged shortly after the last pleistocene deglaciation this is supported by 18o values that are as much as 6 lighter than shallow modern groundwater despite the high mg content of the enclosing rocks mg2 concentrations in the groundwater of this unit are na hco 3 and na cl waters and hence could still be forming under the present low temperature conditions the stable isotope composition of the na cl water suggests that this water type is not a geochemically evolved product of the overlying waters although present day albitization of the anorthosite could be responsible for the increased ca na ratio in the na cl water compared to the other water types the salinity of this water appears to have been derived outside the pluton possibly as an ancient infiltration into crystalline basement rocks of saline formation waters present in paleozoic sedimentary rocks which formerly overlay the pluton or from nearby proterozoic formations this hypothesis is supported by br cl ratios that are similar to sea water and many present day formation waters and the isotopic composition of the dissolved sulphate the he age of the saline water is of the order of 108 years therefore both the saline water and many of its solutes appear to have ancient origins

on the role of multiple interactions in remote aftershock triggering the landers and the hector mine case studies the observed seismicity rate increase after large earthquakes in sites that are located several source lengths away from the mainshock centroid poses a major problem this is because the static stress change induced by a mainshock in that region seems to be insignificant and the dynamic stress changes can only en hance the seismicity during the passage of the seismic waves but not at later times in quest for a physically viable triggering mechanism for delayed aftershocks in remote sites we examine earthquake activities in remote sites that were triggered by two california earthquakes the magnitude 7 1 hector mine earthquake and the mag nitude 7 3 landers earthquake we introduce a new method for quantifying the de gree to which the triggering effect of each aftershock is locally more important than the triggering effect of the mainshock we apply this method to the landers and the hector mine remote aftershock sequences we show that multiple stress transfers from early aftershocks to later aftershocks played an important role in the enhance ment of both the landers and the hector mine aftershock activities in remote sites we present a time space diagram of the hector mine remote aftershock sequence in the imperial valley which shows that this sequence is made up of several subse quences and that the onset of activity migrated southward

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

modeling afterslip and aftershocks following the 1992 landers earthquake one way to probe the rheology of the lithosphere and fault zones is to analyze the temporal evolution of deformation following a large earthquake in such a case the lithosphere responds to a known stress change that can be assessed from earthquake slip models constrained from seismology and geodesy here we model the postseismic response of a fault zone that is assumed to obey a rate strengthening rheology where the frictional stress varies as a ln e e being the deformation rate and a 0 a rheological parameter the model is simple enough that these parameters can be estimated by inversion of postseismic geodetic data we apply this approach to the analysis of geodetic displacements following the m w 7 3 1992 landers earthquake the model adjusts well the measured displacements and implies a 0 47 0 53 mpa in addition we show that aftershocks and afterslip follow the same temporal evolution and that the spatiotemporal distribution of aftershocks is consistent with the idea that they are driven by reloading of the seismogenic zone resulting from frictional afterslip

seismicity in the western great basin apparently triggered by the landers california earthquake 28 june 1992 within 24 hr after the landers earthquake there were three magnitude 3 4 events in western nevada and an unexpected widespread increase in the rate of small events based on combined catalogs for northern nevada southern nevada and southern california and a model that assumes statistical independence of events in these regions the probability of this happening in a 24 hr period by random chance is less than 10 12 per day therefore there is high statistical confidence that the increased seismicity was triggered by the landers event based on the statistical model we develop a list of 227 earthquakes in the first 83 days following the landers earthquake each of which has no more than 10 probability of occurring by random chance the suspect events are broadly distributed in regions that correlate with historical activity in the great basin the events are not uniquely associated with known volcanic activity or with zones that were previously active with microearthquakes or aftershock sequences the magnitudes of the largest triggered events appear to decrease with distance with time the number of suspect events decreases at a rate comparable to the rate of decrease of aftershocks of the landers and big bear earthquakes three of the most significant triggered events that that occurred were as follows mina 500 km from landers m 4 36 min after landers smith valley 590 km from landers m 3 4 56 min after landers and little skull mountain 280 km from landers m 5 6 22 3 hr after landers the evidence for triggering is particularly strong in the case of the little skull mountain event where an increased rate of microseismicity was evident as soon as small events could be identified in the coda of the landers earthquake we evaluate the seismic history for the past 3 yr to understand what was unique about the landers earthquake this includes identifying the previous earthquakes most likely to have caused the strongest shaking in various frequency bands based on a simple screening model the strains from the landers earthquake were uniquely large at low frequencies but at high frequencies they were exceeded frequently by small more local events thus we hypothesize that the cause of triggering is low frequency dynamic strains at periods of about 10 sec or greater and that there is a regional threshold that must be exceeded since previous events with calculated strains that were almost as strong were not causes of widespread triggering this mechanism for triggering satisfies the criterion of being a relatively rare phenomenon since it is likely to occur only when a large long period wave is radiated into an area where strain has been building slowly toward the point where faults are unstable