fractal clustering of induced seismicity in the geysers geothermal area california summary we analyse earthquakes recorded at the geysers geothermal field in california an area where industrial activity induces seismicity the seismicity is characterized by the seismic b value and d the fractal dimension of earthquake hypocentres measured from sliding windows containing 200 events we study a group of events strongly clustered around an injection well over most of the time period examined we find a positive correlation between b and d however during the initiation of injection into a new well we find instead a negative correlation the diverences in correlation are statistically significant at the 1s level but only marginally so at the 2s level these results provide evidence for a transient change in the seismic mechanisms operating and may be explained by a change from conditions of slow stress loading to rapid loading as a result of the build up of the rate of water injection into the reservoir

machine stiffness appropriate for experimental simulation of earthquake processes loading machine stiffness is considered in an attempt to simulate strain accumulation processes prior to failure in earthquakes we impose the similarity condition that the inelastic deformation in a test specimen relative to the loading machine corresponds with that in a spherical seismic region relative to the surrounding earth from this constraint the appropriate ratio between the stiffnesses of machine and specimen is determined to be 1 5 2 3 preliminary experiments were carried out for sandstone samples a qualitative difference between acoustic emission sequences was observed depending on whether the conditions were similar or dissimilar to the earth based on the scaling assumption in the former case a subsidence in acoustic emission activity prior to the main fractures was clearly and systematically observed

compaction permeability evolution and stress path effects in unconsolidated sand and weakly consolidated sandstone abstract the influence of stress paths on the mechanical behavior and coupled permeability evolution of quartz sand packings and soft sandstone have been investigated the proposed approach is mainly based on the performing drained compression tests keeping constant a stress path parameter k defined as the ratio of the horizontal to vertical effective stress increments macroscopic mechanical data and the stress path dependency of permeability have been measured in the elastic brittle and compaction regimes the micro structural characteristics of unconsolidated materials coarse rounded grains glass beads gb and angular grains durance sand ds and a weakly consolidated sandstone otter sherwood sandstone oss have been quantified in intact and deformed states combining petrophysical core analysis and multi scale imaging according to the feasibility of sub sampling of the different materials two different types of hydro mechanical behavior were evidenced for each material the weakly consolidated oss presents a mechanical behavior similar to the consolidated rocks with two distinct regimes of deformation the transition between these two regimes is a function of the stress path on the other hand the sand shows a gradual transition regime requiring the use of a curvature criterion to pick yield stresses this criterion has been validated on the basis of acoustic emissions analysis the modified cam clay model and the elliptical cap model allow capturing efficiently the yield envelopes the permeability measurements were performed with intermediate pressure ports to measure accurately the high permeable sand and to avoid end effects for oss permeability evolutions follow closely the volumetric strain evolutions in both the elastic and plastic regimes and are mainly controlled by the mean pressure before yield at these critical stresses the permeability drops drastically under the influence of deviatoric stress conversely for ds the correlation between strain and permeability is not obvious as permeability reduction is pronounced at an early stage of loading

temporal change in coda q before the tangshan earthquake of 1976 and the haicheng earthquake of 1975 a new earthquake precursor called coda q is demonstrated for the tangshan earthquake of 1976 and for the haicheng earthquake of 1975 the precursor is based on the measurement of coda decay rate for small local earthquakes in the general area that is preparing for a major earthquake during the 3 year period preceding the tangshan earthquake the coda q was about 3 times lower than it was before or after this period a comparable change also occured at the time of the haicheng earthquake the observed change cannot be due to the change in the source parameters such as the focal depth or the source spectrum the coda q precursor appears to detect the same physical process detected by other precursors such as b value quiescence radon water quality vp vs and animal behavior but has advantages over them in its clearer physical mechanism easier sampling and better defined anomalous region

frequency characteristics of acoustic emission in rocks under uniaxial compression and its relation to the fracturing process to failure acoustic emission generated during the uniaxial compression of brittle rocks were counted after band pass filtering throughout the whole process from application of load to failure the count rate monitored through a low pass window was compared statistically with the monitored through a high passs window the emission rate monitored through a low pas window increases more rapidly than that monitored through a high pass window as rock approaches failure two possible explanations for this effect are 1 generation of larger cracks or coalescence of cracks into larger sizes and 2 the relative attenuation of higher frequency wave components this finding is consistent with past observations i e the relative number of large amplitude events increases i e a decrease in b value as rock approaches failure we suggest that larger cracks tend to generate events with larger amplitudes and containing lower frequency components the frequency characteristics of acoustic emissions are used to characterize the microfracturing processes leading to failure the beginning of a sequence of acoustic emission activity is discussed in relation to the stress strain curve and the onset of dilatancy the present observations may be utilized in understanding structural instability of a region where rockbursts or earthquakes can potentially occur if a sizable microseismicmore population is observed through sensors placed near the fault then a potential exists for predicting major earthquakes less

seismic moment distribution summary we review possible errors in determining the size of earthquakes we propose that the size distribution can be effectively defined only for earthquake sequences not for individual earthquakes as a result of the error analysis we question the validity of many reported correlations between the 6 value and spatio temporal characteristics of tectonics and seismicity the study of the distribution of earthquake seismic moment largely avoids the influence of these errors and biases and is therefore superior to the standard analysis of the magnitude frequency law the gamma distribution is applied to describe the distribution of seismic moments of earthquake sequences this distribution which generalizes the well known gutenberg richter g r relation is characterized by two parameters we use the maximum likelihood method to determine the parameters of the gamma distribution for worldwide earthquakes in different depth ranges the empirical distribution is derived from the moment tensor catalogue compiled by the harvard group the p value for scalar seismic moment of earthquake sequences an analogue of the 6 value in the g r magnitude frequency law is close to 1 2 for all earthquakes the value predicted by a critical branching model of seismicity this result which will be improved as more seismic moment data become available provides input for modelling of seismicity based on percolation or on self organized criticality models if 3 equals 1 2 then using the critical branching model we find that most small earthquakes in available catalogues are dependent shocks i e aftershocks even among earthquakes of intermediate magnitudes the number of dependent events may exceed that of independent events

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

statistical physics seismogenesis and seismic hazard the scaling properties of earthquake populations show remarkable similarities to those observed at or near the critical point of other composite systems in statistical physics this has led to the development of a variety of different physical models of seismogenesis as a critical phenomenon involving locally nonlinear dynamics with simplified rheologies exhibiting instability or avalanche type behavior in a material composed of a large number of discrete elements in particular it has been suggested that earthquakes are an example of a self organized critical phenomenon analogous to a sandpile that spontaneously evolves to a critical angle of repose in response to the steady supply of new grains at the summit in this stationary state of marginal stability the distribution of avalanche energies is a power law equivalent to the gutenberg richter frequency magnitude law and the behavior is relatively insensitive to the details of the dynamics here we review the results of some of the composite physical models that have been developed to simulate seismogenesis on different scales during 1 dynamic slip on a preexisting fault 2 fault growth and 3 fault nucleation the individual physical models share some generic features such as a dynamic energy flux applied by tectonic loading at a constant strain rate strong local interactions and fluctuations generated either dynamically or by fixed material heterogeneity but they differ significantly in the details of the assumed dynamics and in the methods of numerical solution however all exhibit critical or near critical behavior with behavior quantitatively consistent with many of the observed fractal or multifractal scaling laws of brittle faulting and earthquakes including the gutenberg richter law some of the results are sensitive to the details of the dynamics and hence are not strict examples of self organized criticality nevertheless the results of these different physical models share some generic statistical properties similar to the universal behavior seen in a wide variety of critical phenomena with significant implications for practical problems in probabilistic seismic hazard evaluation in particular the notion of self organized criticality or near criticality gives a scientific rationale for the a priori assumption of stationarity used as a first step in the prediction of the future level of hazard the gutenberg richter law a power law in energy or seismic moment is found to apply only within a finite scale range both in model and natural seismicity accordingly the frequency magnitude distribution can be generalized to a gamma distribution in energy or seismic moment a power law with an exponential tail this allows extrapolations of the frequency magnitude distribution and the maximum credible magnitude to be constrained by observed seismic or tectonic moment release rates the answers to other questions raised are less clear for example the effect of the a priori assumption of a poisson process in a system with strong local interactions and the impact of zoning a potentially multifractal distribution of epicentres with smooth polygons the results of some models show premonitory patterns of seismicity which could in principle be used as mainshock precursors however there remains no consensus on both theoretical and practical grounds on the possibility or otherwise of reliable intermediate term earthquake prediction

a comparison of seismic and structural measurements of scaling exponents during tensile subcritical crack growth abstract the observed fractal nature of both fault length distributions and earthquake magnitude frequency distributions suggests that there may be a relationship between the structure of active fault systems and the resulting seismicity in previous theoretical work a positive correlation between the exponent d from the fracture length distribution and the seismic or acoustic emission ae b value has been inferred from a simple dislocation model of the seismic source here we present the first experimental evidence for a correlation between d and b from a series of tensile fracture mechanics tests on crystalline rock carried out in different environmental conditions both air dry and water saturated and at ambient temperature and pressure the microseismic acoustic emissions were monitored during subcritical crack growth under controlled conditions of constant stress intensity k i and quantitative analyses of the resulting fracture patterns were carried out on the same specimens it is found that ae b values ranging from 1 0 to 2 3 correlate negatively with the normalized stress intensity k i k ic where k ic is the fracture toughness of the specimen the microcrack length distribution exponent d ranges from 1 0 to 1 7 fluid presence has a first order influence on both the ae and structure produced in these experiments for experiments at low stress intensity or high fluid content the activation of the stress corrosion mechanism for k i k ic leads to a greater relative proportion both of small cracks and of low amplitude acoustic emissions reflected in higher values of d and b the exponent d is found to correlate positively with the ae b value

stress triggering and stress memory observed from acoustic emission records in a salt mine summary some of the most interesting questions in geosciences are whether results from laboratory experiments can be applied to processes in the earth crust and whether in situ studies with high spatio temporal resolution can bridge the gap between laboratory work and seismology in this study acoustic emission ae activity caused by stress changes due to the backfilling of a cavity in an abandoned salt mine is studied to answer questions regarding 1 the dependence of ae event rates event distribution and b value on the stress state 2 the stress memory effect of rock kaiser effect 3 the possibility to detect significant changes in the system like the initiation of macrocracks and 4 the possibility to estimate future activity from previous ae records the large number of events studied 3 10 5 allows a spatial resolution of the order of 1 m and a temporal one on the order of 1 hr stress changes are created due to the thermal expansion and contraction of the rock mass in response to the temperature changes caused by the backfilling a roughly 20 50 50 m section of the mining complex just above the backfilled cavity is well covered by a network of 24 piezo electric receivers and poses an optimal volume for the study results of a 2 d finite element thermoelastic stress model are in agreement with the spatio temporal ae event distribution in addition to the initial upward migration of the ae event front which correlates with the calculated stress field the rock salt exhibits a pronounced kaiser effect for the first few thermal loading cycles throughout the whole study region the deviation from the kaiser effect during later loading cycles seems to be caused by the initiation of a planar macroscopic crack which is subsequently reactivated ae activity tends to concentrate along this macrocrack calculated b values decrease before and increase after the supposed initiation of the macrocrack supporting this explanation in intact rock volumes not subjected to macrocracking a linear relation between the maximum event rate and the calculated absolute coulomb stress increase is observed this indicates that future maximum ae event rates can be estimated from expected loading ae activity during stress loading cycles is most prominent in regions with coulomb stress maxima indicating possible shear cracking that has the potential to create macrocracks strong bursts of ae activity observed during thermal unloading phases are concentrated in regions for which the minimum principal stress becomes tensile these regions exhibit significantly higher b values than those active during thermal loading we interpret these weak events with tensile microcracking

fractals in geology and geophysics the definition of a fractal distribution is that the number of objectsn with a characteristic size greater thanr scales with the relationn r d the frequency size distributions for islands earthquakes fragments ore deposits and oil fields often satisfy this relation fractals were originally introduced by mandelbrot to relate the length of a coastline to the length of the measuring stick this application illustrates a fundamental aspect of fractal distributions scale invariance the requirement of an object to define a scale in photographs of many geological features is one indication of the wide applicability of scale invariance to geological problems scale invariance can lead to fractal clustering geophysical spectra can also be related to fractals these are self affine fractals rather than self similar fractals examples include the earth s topography and geoid

identifying fault heterogeneity through mapping spatial anomalies in acoustic emission statistics 1 seismicity clusters within fault zones can be connected to the structure geometric complexity and size of asperities which perturb and intensify the stress field in their periphery to gain further insight into fault mechanical processes we study stick slip sequences in an analog laboratory setting analysis of small scale fracture processes expressed by acoustic emissions aes provide the possibility to investigate how microseismicity is linked to fault heterogeneities and the occurrence of dynamic slip events the present work connects x ray computer tomography ct scans of faulted rock samples with spatial maps of b values slope of the frequency magnitude distribution seismic moments and event densities our current experimental setup facilitates the creation of a series of stick slips on one fault plane thus allowing us to document how individual stick slips can change the characteristics of ae event populations in connection to the evolution of the fault structure we found that geometric asperities identified in ct scan images were connected to regions of low b values increased event densities and moment release over multiple stick slip cycles our experiments underline several parallels between laboratory findings and studies of crustal seismicity for example that asperity regions in lab and field are connected to spatial b value anomalies these regions appear to play an important role in controlling the nucleation spots of dynamic slip events and crustal earthquakes

precursors to the shear failure of rock discontinuities active geophysical monitoring of potential failure along mechanical discontinuities in rock requires identification of precursory signatures to failure in geophysical signals active ultrasonic monitoring of shear failure along frictional discontinuities was performed to determine the signatures of potential failure an instrumented direct shear apparatus was used to apply a constant shearing rate to a discontinuity that was held under a constant normal stress transmitted and reflected compressional and shear waves were recorded during the shearing process ultrasonic precursors were identified as distinct maxima in the amplitude of transmitted shear waves as well as minima in the amplitude of reflected shear waves that occurred well before the peak shear strength of a frictional discontinuity the precursors are linked to changes in the local shear specific stiffness along the discontinuity while the discontinuity s macroscopic shear strength continues to increase prior to failure

deformation regime and long term precursors to eruption at large calderas rabaul papua new guinea eruptions at large calderas are normally preceded by variable rates of unrest that continue for decades or more a classic example is the 1994 eruption of rabaul caldera in papua new guinea which began after 23 years of surface uplift and volcano tectonic vt seismicity at rates that changed unevenly with time by an order of magnitude although the vt event rate and uplift rate peaked in 1983 1985 eruptions only began a decade later and followed just 27 hours of anomalous changes in precursory signal here we argue that the entire 23 years of unrest belongs to a single sequence of damage accumulation in the crust and that in 1991 1992 the crust s response to applied stress changed from quasi elastic elastic deformation with minor fault movement to inelastic deformation predominantly by fault movement alone the change in behaviour yields limiting trends in the variation of vt event rate with deformation and can be quantified with a mean field model for an elastic crust that contains a dispersed population of small faults the results show that identifying the deformation regime for elastic brittle crust provides new criteria for using precursory time series to evaluate the potential for eruption they suggest that in the quasi elastic regime short term increases in rates of deformation and vt events are unreliable indicators of an imminent eruption but that in the inelastic regime the precursory rates may follow hyperbolic increases with time and offer the promise of developing forecasts of eruption as much as months beforehand

failure zone development in andesite as observed from acoustic emission locations and velocity changes three dimensional 3 d locations of acoustic emissions ae and p wave velocity anisotropy were investigated to analyse the failure zone development in two yugawara andesites the results were obtained during the deformation of rock under incremental loading and short duration creep regimes at a confining pressure of 40 mpa creep tests were carried out at a constant differential stress of about 90 95 of short term fracture strength of andesites for the single and two stage creep experiments in both the experiments the ae activity during loading was mostly confined to two region where ae hypocentres concentrated near the cylindrical surface of the rock prior to fault nucleation the localized zone of microcracks attained a spheroidal shape in one sample and planar shape in the other p wave velocity anisotropy increased to 44 as final failure approached in the rock the loading rate and the magnitude of creep stress were found to have a profound influence on the details of failure zone development and times of failure of the rocks studied

diagnosis of time of increased probability tip for volcanic earthquakes at mt vesuvius the possibility of intermediate term earthquake prediction at mt vesuvius by means of the cn algorithm is explored cn was originally designed to identify the times of increased probability tips for the occurrence of strong tectonic earthquakes with magnitude m m0 within a region a priori delimited here the cn algorithm is applied for the first time to the analysis of volcanic seismicity the earthquakes recorded at mt vesuvius during the period from february 1972 to june 2004 are considered and the magnitude threshold m0 selecting the events to be predicted is varied within the range 3 0 3 3 satisfactory prediction results are obtained by retrospective analysis when a time scaling is introduced in particular when the length of the time windows is reduced by a factor 2 5 3 with respect to the standard version of cn algorithm more than 90 of the events with m m0 occur within the tip intervals with tips occupying about 30 of the total time considered the control experiment seismic history demonstrates the stability of the obtained results and indicates that the cn algorithm can be applied to monitor the preparation of impending earthquakes with m 3 0 at mt vesuvius

analysis of temporal and spatial heterogeneity of magnitude frequency distribution inferred from earthquake catalogues summary statistical models are introduced for the simultaneous estimation of the changes of the b value and detection rate of earthquakes in a catalogue which develops in time or varies in space the three characteristic parameters including b for the magnitude frequency distribution of detected earthquakes are represented by respective three b spline functions of time or location in a space an objective bayesian method is adopted for the optimal estimate of such functions with many unknown coefficients the present procedure is applied to earthquake catalogues for japanese seismic activity

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

theory of time dependent rupture in the earth at a velocity x which is a well defined function of k this propagation is stable and quasi static and is referred to as subcritical crack growth we shall use the terms stable quasistatic and subcritical to mean propagation at velocities much less than the sonic velocities of the medium this behavior results from stress induced corrosion at the crack tip the principal corrodent for the present application being h20 this behavior has been firmly established in the laboratory for mode i tensile cracks in a wide variety of materials ineluding silicates and silicate glasses see cholz 1968a 1972a martin 1972 wiederhorn and bolz 1970 atkinson 1979 lawn and wilshaw 1975 knott 1973 the form of the relationship between k and x does not vary significantly with the material only the parameters in the law vary as an example we show in figure 1 data on subcritical crack growth in quartz after atkinson 1979 the empirical relationship is found to take the form

quasi static modelling of stress histories during the earthquake cycle precursory seismic and aseismic stress release summary a functional form describing non linearity in the stress history during the quasistatic phase of anelasticity in the final stages of semi brittle failure is presented semi brittle failure is known to occur in laboratory rock samples and the existence of earthquake precursors indicates that the earth s upper crust also exhibits an anelastic response in the final stages of the buildup to dynamic failure the functional form of the stress history depends on the amount of localized aseism stress relaxation on a fault nucleation patch and the amount of relatively more distributed seismic damage around the fault measured by a catalogue of earthquakes or acoustic emissions aseismic stress relaxation on the nucleation patch is modelled assuming slip occurs behind a crack front which is growing due to subcritical crack growth by the mechanism of stress corrosion under these conditions it is impossible to model correctly the observed duration of intermediate term earthquake precursors given realistic estimates of the size of the nucleation patch 0 1 1 km and the stress corrosion index n where 10 n 50 therefore intermediate term earthquake precursors must result from more distributed seismic or aseismic damage coupled with fluid transport in a volume much larger than that of the nucleation patch in terms of seismic moment or seismic energy release the precursory seismic damage in the quasi static anelastic phase is dominated by the largest magnitudes for the usual case of b 2c 3 the smaller events dominate the remote stress drop in terms of the inferred power law exponent d 3b c of the fault length distribution d 2 corresponds to distributed damage and d 2 and strain softening and shear localization occurs when d 2

classification of earthquake precursors from a fracture mechanics model abstract a new classification of earthquake precursors is proposed based on a fracture mechanics approach which includes the effects of stress and the acceleration of a fracture through an asperity two phases of earthquake warning associated with intermediate term and short term anomalies can be related in a simple manner to two peaks in the stress intensity the first broad peak is further split into two halves associated with precursory strain release strain hardening followed by precursory stress drop strain softening and the second peak responds to the transition from quasi static fracture and slip to dynamic rupture the model predicts a seismic b value anomaly which is in good agreement with laboratory experiment and some aspects of empirical observation of earthquakes in particular evidence is presented of the distinctive short term b value anomaly predicted by the second stress intensity peak and a critical foreshock related b value of 0 5 other earthquake precursors can also be classified into this unifying model and it is suggested that those anomalies which respond to the cumulative production of microcracks e g coda q 1 could produce a more distinctive two stage anomaly after differentiation with respect to time such a two stage anomaly especially if it were observed in several independent earthquake precursors could be a very useful method of deciding when a low level earthquake warning should be changed to a higher level warning

detailed analysis of acoustic emission activity during catastrophic fracture of faults in rock abstract the detailed time space distribution of acoustic emission ae events during the catastrophic fracture of rock samples containing a pre existing joint or potential fracture plane is obtained under triaxial compression using a high speed 32 channel waveform recording system and the results are discussed with respect to the prediction and characterization of catastrophic fault failure ae activity is modeled quantitatively in terms of the seismic b value of the magnitude frequency relation the self excitation strength of the ae time series and the fractal dimension of ae hypocenters consistent with previous studies on rock samples containing a fracture plane with several asperities the present analyses reveal three long term phases of ae activity associated with damage creation on heterogeneous faults each clearly identifiable based on the above parameters a long term decreasing trend and short term fluctuation of the b value in the phase immediately preceding dynamic fracture are identified as characteristic features of the failure of heterogeneous faults based on the experimental results it is suggested that precursory anomalies related to earthquakes and other events associated with rock failure are strongly dependent on the heterogeneity of the fault or rock mass a homogeneous fault or rock mass appears to fracture unpredictably without a consistent trend in precursory statistics while inhomogeneous faults fracture with clear precursors related to the nature of the heterogeneity

coupling among the terrestrial sulfur carbon and oxygen cycles numerical modeling based on revised phanerozoic carbon isotope record abstract numerical modeling of the terrestrial oxygen budget based on the revised 13ccarb record by veizer et al 1980 has shown that total photosynthetic oxygen has varied between 7 and 10 of its average reservoir size 3 2 1022 g during the last 800 myr as a result of oscillations of the sedimentary reservoir of organic carbon calculated curves of oxygen evolution display a distinct minimum in the early paleozoic framed by two maxima in the latest proterozoic and the mesozoic the sympathetic relationship observed between the curves of total oxygen evolution and respective functions for the partial reservoir of sulfate bound oxygen suggests that the o2 required for an additional conversion of sulfide to sulfate was most probably provided by excess burial of organic carbon the results of the modeling thus adding credence to current interpretations proposed for the negative correlation between the secular 13 c 12 c and 34 s 32 s trends

experimental and modeling study of fluid pressure driven fractures in darley dale sandstone 1 we report measurements of acoustic emissions ae generated during formation and growth of pressure driven fractures in cylindrical samples of darley dale sandstone that were co axially pre drilled in order to allow an internal pressure to be applied a set of 3 to 6 fractures initiate at the wall of the internal bore at a fluid pressure around three times that of the confining pressure but only 3 propagate to the outer wall of the sample time and spatial distributions of acoustic emissions show two distinct bursts of activity associated with initiation and propagation respectively a particle flow code pfc model has been used to reproduce the mechanics of fracture initiation and the time and spatial distributions of ae in both the experiments and the model we observe a distinct phase of accelerating ae activity preceding fracture formation

microcracking during triaxial deformation of porous rocks monitored by changes in rock physical properties ii pore volumometry and acoustic emission measurements on water saturated rocks abstract we report results from a series of laboratory triaxial deformation experiments performed on samples of darley dale sandstone at servo controlled constant strain rate and constant pore fluid pressure during deformation the volume of water either expelled from the samples during compaction or injected into the samples during dilatancy in order to maintain a constant pore fluid pressure was continuously monitored throughout each experiment in addition the parameters of differential axial stress axial strain and acoustic emission ae characteristics were also recorded complete ae waveforms were also captured and stored by means of a transient recorder and fourier analysis of these waveforms was subsequently carried out the reported experiments were all performed in a conventional high pressure gas medium triaxial cell but with the pore fluid pressure maintained constant using a new servo controlled fluid pressure intensifier and pore volumometer the ae and pore volumometry measurements on water saturated rocks reported in this paper are complementary to the measurements of elastic wave propagation parameters made on dry rocks and reported in a companion paper in this issue the two suites of data have been integrated to infer relations between crack density parameters calculated from elastic wave velocity measurements and the directly measured pore volume during deformation our results show a distinct positive correlation between changes in the axial crack density parameter x and changes in the pore volume during deformation once fluid expulsion due to elastic pore collapse has been corrected for these two parameters have then been combined to obtain an estimate of changes in the mean aspect ratio of propagating dilatant cracks the results suggest that under moderate confining pressures axially aligned dilatant cracks are likely to bow open elastically prior to significant crack extension and growth

temporal variations in seismic event rate and b values from stress corrosion constitutive laws abstract main i g meredith p g and sammonds p r 1992 temporal variations in seismic event rate and b values from stress corrosion constitutive laws in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 233 246 a model is developed to characterise the state of damage for a fractal population of cracks in terms of a mean energy release rate g which depends on the stress the event rate n and the seismic b value n is assumed to be proportional to the total number of potentially active cracks and b to be proportional to the exponent d of the crack length distribution g is then positively correlated with n and negatively correlated with b consistent with experimental observation based on stress intensity as a constitutive variable the model predicts that higher b values are associated either with lower stress intensity or greater material heterogeneity both experiment and theory predict intermediate term seismic quiescence of similar magnitude to that observed in the field 45 90 for only a small simultaneous reduction in stress and stress intensity 2 7 or the equivalent mean energy release rate the theoretical model predicts three different types of quiescence associated with increasing constant or decreasing b value the first two are associated with stable intermediate term quiescence due to a reduction in g and the third is associated with unstable short term quiescence associated with constant or increasing g the theory can be used to infer relative changes in g from acoustic emissions in the laboratory experiments to date show that quiescence of the order 10 can be seen in the laboratory at strain rates of 10 5 s 1 but that this has the characteristics of short term rather than intermediate term quiescence

microcracking during triaxial deformation of porous rocks monitored by changes in rock physical properties i elastic wave propagation measurements on dry rocks we present results from two series of triaxial deformation experiments performed on dry samples of two sandstones darley dale and gosford carried out at confining pressures from 25 mpa to 200 mpa over this pressure range the mode of failure in both these sandstones passes from localized brittle failure with a clear through going fault to distributed cataclastic flow during these experiments stress strain compressional wave velocity vp and shear wave velocity vs measurements were made simultaneously in the direction of the maximum principal compressive loading axis initial application of the hydrostatic confining pressure causes both vp and vs to increase and upon raising the axial stress above the confining pressure both velocities increase further at first generally by only a few percent but then decrease as dilatant crack growth commences during dilatancy vs decreases proportionately more than vp and this decrease is generally of the order of 10 15 these velocity measurements allow changes in rock physical properties to be calculated along with the axial and transverse crack volume density parameters x and z the results from two selected tests are analysed in detail these tests were chosen because they exhibit a typical brittle shear failure and b typical ductile cataclastic flow respectively the full interrogating elastic waveforms were also recorded during testing and these have been used to calculate the seismic quality factors qp and qs to our knowledge this is the first time this has been reported for rock samples undergoing triaxial deformation the changes in q values generally exhibit similar trends to those observed in the velocity measurements but the percentage changes in q are an order of magnitude greater suggesting that this parameter is a more sensitive measure of dilatant crack damage the measurements on dry rock samples reported here provide the basis for comparison with measurments of changes in complementary physical properties made on water saturated rock samples under the same experimental conditions and reported in a companion paper in this issue read et al 1995

earthquake precursors in japan precursor time and detectability abstract earthquake precursor data amounting to 418 in number accumulated in japan are analyzed most data are obtained under the nation wide earthquake prediction programme the precursor time t versus main shock magnitude m relationship is different for different groups of precursors a linear relation such as log t a bm where a and b are constants seems to hold for precursor disciplines such as land deformation revealed by geodetic survey and tide gauge observation ground tilt observed by a water tube tiltmeter anomalous seismic activity and geomagnetic field change although a similar relationship seems to hold for the disciplines of earth currents radon concentration underground water etc the precursor time is substantially smaller for these disciplines than for the former disciplines the above characteristics of precursor time suggests that precursors closely connected to the dilatancy generation seem to be observed at an earlier stage of seismogenetic activity precursors probably associated with diffusion of underground water into the dilatant region seem to follow for a number of precursor disciplines such as foreshock ground tilt observed by a pendulum tiltmeter resistivity by a variometer and so on no dependence of t on m is found the relationship between m and the logarithmic epicentral distance d measured in kilometers is different from discipline to discipline relying on empirical and theoretical studies of coseismic strain step it turns out that precursors found by geodetic and geomagnetic observations seem to reflect a crastal strain of the order of 10 7 10 6 precursors obtained by a resistivity variometer seem to represent a strain of the order of 10 9 10 8 precursors of other disciplines seem to correspond to a strain of 10 8 10 7 it is clear that precursors can be detected further from the epicenter as the main shock magnitude becomes larger in some favourable cases a precursor can be observed even where the epicentral distance is several times larger than that of the epicentral area

role of pore fluids in the generation of seismic precursors to shear fracture a systematic study of temporal changes in seismic b values defined as the log linear slope of the earthquake frequency magnitude distribution has shown that large earthquakes are often preceded by an intermediate term increase in b followed by a decrease in the months to weeks before the earthquake1 the onset of the b value increase can precede earthquake occurrence by as much as 7 years a recently proposed fracture mechanics model of the earthquake source2 explains these temporal fluctuations in b in terms of the underlying physical processes of time varying applied stress and crack growth the model predicts two minima in b separated by a short lived maximum here we report the results of controlled laboratory deformation experiments done in simulated upper crustal conditions on both air dried and water saturated rock specimens as found in previous experiments3 5 shear fracture in dry specimens is characterized by a decline in b during anelastic deformation to a single minimum reached just before failure but in water saturated specimens when pore fluid volume is kept constant by servo control we also observe a second intermediate term b value minimum so reproducing the double b value anomaly predicted by the model2

the 1980 pressure response and flank failure of mount st helens usa inferred from seismic scaling exponents the cataclysmic 18 may 1980 eruption at mount st helens was preceded by intense seismic activity marking the mechanical response of the volcanic edifice to interior pressurisation this seismicity is analysed to yield the temporal change in the seismic scaling exponent d inferred from the seismic b value that in turn is related to the seismic moment of an earthquake time evolution of d preceding the eruption onset reveals 1 a major decrease in d occurring over only a few days at the end of march 2 a steady but stepped decrease in d steps 5 10 days occurring from the end of march to early may 3 a sharp decrease in d in early may and 4 steady low values of d occurring 2 3 days before the eruption onset this response is interpreted as major ruptures formed at the end of march arresting and participating in but not triggering the ultimate failure of the flank rather the rate of interior fracturing slowed in the 2 months preceding the 18 may 1980 major blast and the triggering of failure is consistent with interior gas overpressurisation the occurrence of two swarms of low frequency seismic events and the high values of the harmonic tremor indicate the action of interior pressurisation on a cycle of 20 25 days solutions are applied to represent the harmonic interior pressurisation of the edifice by gas exsolving from the volcano core the transient radial migration of overpressured gas may reduce the apparent strength of the edifice and ultimately trigger failure of the flank importantly this mechanism is capable of triggering flank failure both after multiple core pressurisation cycles have been sustained and as core pressures are low and diminishing and may be a minimum these twin attributes are both apparent in the seismic record for mount st helens used as a proxy for the unrecorded timing and magnitude of gas pressurisation at the volcano core d 2004 elsevier b v all rights reserved

a reinterpretation of the precursory seismic b value anomaly from fracture mechanics summary a new model is presented which can explain the major temporal fluctuations in seismic b value in terms of the underlying physical processes of time varying applied stress and crack growth under conditions of constant strain rate the model predicts two minima in the b value separated by a temporary maximum of inflexion point a corollary being that a single broad maximum would be expected in scattering attenuation these fluctuations in b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence we present preliminary results from controlled laboratory experiments and report recent field observations both of which are consistent with the predicted form and in particular exhibit the distinctive second short term anomaly which reaches a value bc 0 5 during the critical phase

spatio temporal evolution of volcano seismicity a laboratory study we report a laboratory and microstructural study of a suite of deformation experiments in which basalt from mount etna volcano is deformed and fractured at an effective confining pressure representative of conditions under a volcanic edifice 40 mpa particular attention was paid to the formation of a fracture and damage zone with which to stimulate coupled hydro mechanical interactions that create the various types of seismicity recorded on volcanic edifices and which usually precede eruption location of ae events through time shows the formation of a fault plane during which waveforms exhibit the typical high frequency characteristics of volcano tectonic vt earthquakes we found that these vt earthquakes were particularly pronounced when generated using dry samples compared to samples saturated with a pore fluid water vt events generated during deformation of water saturated sample are characterised by a distinctive high frequency onset and a longer low frequency coda exhibiting properties often seen in the field as hybrid events we present evidence that hybrid events are in fact the common type of volcanic seismic event with either vt or low frequency lf events representing end members and whose proportion depend on pore fluid being present in the rock type being deformed as well as how close the rock is to failure we find a notable trend of reducing instances of hybrid events leading up to the failure stage in our experiments suggesting that during this stage the pore fluid present in the rock moves sufficiently quickly to provide a resonance seen as a lf coda our data supports recent modeling and field studies that postulate that hybrid events generated in volcanic areas are likely to be generated through the interaction of hydrothermal fluids moving through a combination of pre existing microcrack networks and larger faults such as those we observe in forensic post test examination

frequency magnitude statistics and spatial correlation dimensions of earthquakes at long valley caldera california summary intense earthquake swarms at long valley caldera in late 1997 and early 1998 occurred on two contrasting structures the first is defined by the intersection of a northnorthwesterly array of faults with the southern margin of the resurgent dome and is a zone of hydrothermal upwelling seismic activity there was characterized by high b values and relatively low values of d the spatial fractal dimension of hypocentres the second structure is the pre existing south moat fault which has generated largemagnitude seismic activity in the past seismicity on this structure was characterized by low b values and relatively high d these observations are consistent with lowmagnitude clustered earthquakes on the first structure and higher magnitude divuse earthquakes on the second structure the first structure is probably an immature fault zone fractured on a small scale and lacking a well developed fault plane the second zone represents a mature fault with an extensive coherent fault plane

stress corrosion cracking of quartz a note on the influence of chemical environment abstract tensile mode i stress corrosion growth of cracks on the a plane of quartz in a direction normal to z has been studied in de ionized water in 2n hcl and in 2n naoh solutions at 20 c the double torsion testing method was used to obtain crack velocity v stress intensity factor ki curves the stress corrosion index n where v kin was 19 3 for 2n hcl 12 8 for deionized water and 9 5 for 2n naoh the degree of influence of reagent chemistry on crack velocity was most marked at low ki values at 0 5 kic 3 orders of magnitude increase in crack velocity results from substituting 2n naoh for 2n hcl as the corrosive medium but decreased monotonically on raising ki so that close to kic there was no clearly discernible influence on crack growth rates ca 10 3 m s 1 these observations are explained in terms of a model of crack propagation in which at low ki values the chemistry of the bulk fluid environment controls the corrosion reaction the rate of crack growth is enhanced by increasing environmental oh concentration at high ki or velocity however the chemical composition of the quartz surface may control crack propagation rates through its influence on the chemistry of the crack tip fluid as the ph of natural pore fluids is likely to vary widely in different geological environments these results should be noted when invoking stress corrosion to explain geophysical phenomena

seismic quiescence and asperities in the tonga kermadec arc highly significant temporal and spatial seismicity rate changes were found along the tongaskermadec plate boundary the standard deviate z test was used to compare systematically the rates in different periods and different volumes and to test for uniqueness of anomalies found the noaa hypocenter data file showed a pronounced decrease in the number of small magnitude earthquakes reported after 1969 at the same time as such a reporting decline exists in the worldwide data therefore only earthquakes with mb 4 9 were studied a search for quiescence before recent mainshocks yielded two uniquely significant precursory anomalies one missed mainshock and one false alarm quiescence started 63 months before the january 1976 m 8 0 kermadec and 25 months before the june 1977 tonga m 7 2 earthquakes the december 1975 m 7 5 shock was not preceded by a seismicity rate change the seismic gaps in the tonga kermadec arc with sufficient seismicity for analysis 17 22 and 31 34 show constant rates up to the present therefore on the basis of the quiescence hypothesis earthquakes are not forecast for these segments the stress drops of 380 interface events were estimated using the mb ms method the average was uniform along the island arc with the exception of two segments with average of less than half the normal values these segments coincided with the two largest recent interface ruptures however in the northern tonga the was low before the 1975 rupture whereas in the kermadec the 1976 aftershocks showed low asperities could not be identified on the basis of high stress drops a systematic study of the seismicity rate as a function of space defined large variations along the arc the areas of highest and lowest seismicity rate correlate with the northern end of the trench and with the louisville ridge intersection respectively however other large rate contrasts exists between arc segments without obvious tectonic differences this implies that seismicity doughnut patterns do not reliably define precursors one segment of outstandingly high seismicity rate 30 31 s is identified as a major asperity because it contains a large number of m 6 events and a large number of recent mainshock aftershock sequences and because it stopped the great 1976 rupture we propose that the only other segment of high seismicity rate near 20 5 s also represents a major asperity

fracture toughness and subcritical crack growth during high temperature tensile deformation of westerly granite and black gabbro abstract the double torsion testing method has been used to determine catastrophic and subcritical crack propagation parameters for pre cracked specimens of westerly granite and black gabbro under a number of environmental conditions the critical stress intensity factor for catastrophic crack propagation fracture toughness of granite and gabbro has been measured at temperatures from 20 to 400 c in a vacuum at 20 c the fracture toughness of westerly granite was 1 79 0 02 mpa m 1 2 and for two blocks of black gabbro it was 3 03 0 08 mpa m 1 2 and 2 71 0 15 mpa m 1 2 respectively these values are very close to those reported by other investigators for tests conducted in air of ambient humidity at room temperature for both rocks fracture toughness at first increased slightly and then decreased steadily on raising the temperature above ambient conditions this behaviour is explained in terms of the density and distribution of thermally induced microcracks as determined by quantitative optical microscopy subcritical crack growth behaviour has been studied at temperatures up to 300 c and under water vapour at pressures of 0 6 to 15 kpa both the load relaxation and incremental constant displacement rate forms of the double torsion testing method were utilised to generate stress intensity factor crack velocity diagrams crack growth was measured over the velocity range 5 10 3 to 10 7 m s 1 increasing both temperature and water vapour pressure resulted in substantially higher crack growth rates the overall effect of raising the temperature over the range studied here 20 300 c was to increase the crack growth rate in granite and gabbro by 5 and 7 orders of magnitude respectively at constant stress intensity factor and vapour pressure of water for both rocks the slopes of stress intensity factor crack velocity curves were sensitive to changes in both temperature and water vapour pressure at low values of the latter parameter slopes fell substantially on raising the water vapour pressure but were relatively insensitive to changes in temperature at these higher pressures no subcritical crack growth limit was encountered estimates of the uncertainty in our experimental data are given from the results of multiple load relaxation experiments on westerly granite specimens we estimate the uncertainty in position of stress intensity factor crack velocity curves along the stress intensity axis to be c 10 of the fracture toughness and the uncertainty in slope of such curves to be c 12 problems associated with the extrapolation of our experimental data to regions of higher effective confining pressure in the earth s crust are discussed

controls on strain localization in a two dimensional elastoplastic layer insights into size frequency scaling of extensional fault populations 1 we use a two dimensional 2 d finite element code to investigate how mechanical properties and boundary conditions influence progressive strain localization during extension of a 2 d elastoplastic layer the deforming medium is modeled using a strain softening von mises rheology with a gaussian heterogeneity in yield strength distributed randomly in space specifically we examine the effects of changing 1 the thickness of the deforming layer 2 the basal boundary condition 3 the range of strengths in the deforming layer 4 the strength loss on failure and 5 the total accommodated strain discrete zones of plastic shear strain are observed to nucleate and grow progressively during each experiment in order to quantify and thus discriminate between the deformation patterns produced under differing conditions we calculate the size frequency distributions of the shear band populations size is defined as the total plastic strain represented by each discrete structure a wide range of size frequency distributions is observed including both power law and exponential as well as distributions that show breaks in scaling with a transition from power law at small sizes to exponential at large sizes we show that these variations in population statistics are directly reflecting a change in strain localization in space and time caused by changing model parameters furthermore by making an analogy between the model shear bands and tectonic faults we provide insights into key features of fault size frequency distributions that have been measured in continental and oceanic extensional settings and also observed in analogue experiments

a comparison of seismicity characteristics and fault structure between stick slip experiments and nature fault zones contain structural complexity on all scales this complexity influences fault mechanics including the dynamics of large earthquakes as well as the spatial and temporal distribution of small seismic events incomplete earthquake records unknown stresses and unresolved fault structures within the crust complicate a quantitative assessment of the parameters that control factors affecting seismicity to better understand the relationship between fault structure and seismicity we examined dynamic faulting under controlled conditions in the laboratory by creating saw cut guided natural fractures in cylindrical granite samples the resulting rough surfaces were triaxially loaded to produce a sequence of stick slip events during these experiments we monitored stress strain and seismic activity after the experiments fault structures were imaged in thin sections and using computer tomography the laboratory fault zones showed many structural characteristics observed in upper crustal faults including zones of localized slip embedded in a layer of fault gouge laboratory faults also exhibited a several millimeter wide damage zone with decreasing micro crack density at larger distances from the fault axis in addition to the structural similarities we also observed many similarities between our observed distribution of acoustic emissions aes and natural seismicity the aes followed the gutenberg richter and omori utsu relationships commonly used to describe natural seismicity moreover we observed a connection between along strike fault heterogeneity and variations of the gutenberg richter b value as suggested by natural seismicity studies areas of low b value marked the nucleation points of large slip events and were located at large asperities within the fault zone that were revealed by post experimental tomography scans our results emphasize the importance of stick slip experiments for the study of fault mechanics the direct correlation of acoustic activity with fault zone structure is a unique characteristic of our laboratory studies that has been impossible to observe in nature

evidence for precursory changes in the frequency magnitude b value summary a clear case has been established for a significant increase in the slope of the frequency magnitude relation for periods of up to six years before a number of earthquakes in new zealand and california in a region of about 200 km diameter in central new zealand the b value increased from near unity to more than 2 0 at the end of 1968 and remained abnormally high until the beginning of 1975 an earthquake of magnitude 6 0 followed in 1977 january within 50 km of the location which had registered the highest b value the anomaly was detected in contour maps of b values windowed in space and time then resolved more precisely in time using the cusum technique it was statistically significant at a level higher than 99 99 per cent compared with the average rate of moment release in this region there was a deficit of 1 3 x 10 nm during the anomalous period which is equivalent to an earthquake of magnitude 6 08 this phenomenon of a high b value has been found before all shallow new zealand earthquakes of magnitude 6 0 or more since 1965 wherever the seismicity rate and detection threshold were adequate to resolve it further the only statistically significant occurrences of high b value were those which were followed by large earthquakes two of magnitude 6 8 and 6 0 and a swarm of equivalent magnitude 5 3 the phenomenon has also been found before the san fernando and coalinga earthquakes in california both of which were thrust events but it is not apparent before two large californian earthquakes with strike slip mechanisms observations of high b value are consistent with reported instances of quiescence in background seismicity in fact what has been interpreted as quiescence may probably be more accurately described as a localized high b value the duration of the anomaly is not a simple function of magnitude different strain rates in the regions examined may account for this the available data suggest that careful monitoring of the b value may provide a tool for medium term earthquake prediction although perhaps not in all tectonic environments

imaging slow failure in triaxially deformed etna basalt using 3d acoustic emission location and x ray computed tomography we have deformed basalt from mount etna italy in triaxial compression tests under an effective confining pressure representative of conditions under a volcanic edifice 40 mpa and at a constant strain rate of 5 similar to 10 6 s 1 despite containing a high level of pre existing microcrack damage etna basalt retains a high strength of 475 mpa we have monitored the complete deformation cycle through contemporaneous measurements of axial strain pore volume change compressional wave velocity change and acoustic emission ae output we have been able to follow the complete evolution of the throughgoing shear fault without recourse to any artificial means of slowing the deformation locations of ae events over time yields an estimate of the fault propagation velocity of between 2 and 4 mm s 1 we also find excellent agreement between ae locations and post test images from x ray microtomography scanning that delineates deformation zone architecture

correlation of microseismic and chemical properties of brittle deformation in locharbriggs sandstone constant stress rate loading the exit pore fluid was analyzed after the tests for the concentration of dissolved ions and acoustic emission was monitored in real time throughout the tests the exit pore fluid silica concentration and microcrack damage derived from the acoustic emission ae data both exhibited an exponential increase during the strain hardening phase of deformation damage parameters inferred from the ae data predict the stress strain curves adequately or at least up to the point of strong microcrack coalescence the damage parameters and silica signal were strongly correlated by a power law relationship the observed environment and strain rate dependence of mechanical properties can hence be attributed uniquely to time dependent crack growth by the stress corrosion mechanism index terms 1045 geochemistry low temperature geochemistry 5102 physical properties of rocks acoustic properties 5104 physical properties of rocks fracture and flow keywords stress corrosion acoustic emission silica dissolution subcritical crack growth deformation rate brittle fracture citation ojala i o b t ngwenya i g main and s c elphick correlation of microseismic and chemical properties of brittle deformation in locharbriggs sandstone j geophys res 108 b5 2268 doi 10 1029 2002jb002277 2003

a study on the time dependent microfracturing and strength of oshima granite abstract uniaxial compressive loading experiments where axial strain rate or dilatant volumetric strain rate was kept constant were carried out using a servo controlled hydraulic testing machine the rock studied was oshima granite in order to investigate the effect of the strain rate on the strength a theory based on the assumption of redistribution of the pre existing microcracks due to subcritical crack growth was examined results expected from the theory were well borne out by the observations the strength was expressed by power functions of both axial strain rate and dilatant strain rate the stress corrosion index of oshima granite appeared to be 31 3 which agreed well with the values reported previously when the axial strain rate was held constant the acoustic emission rate increased exponentially and the b value the exponent in the frequency magnitude relation of the acoustic emissions decreased with increasing applied stress when the dilatant volumetric strain rate was kept constant the acoustic emission rate and the b value remained constant and independent of the stress increase furthermore when the dilatant strain rate was held constant at a higher value the b value was constant at a lower level as the decrease in the b value indicates the higher dilatant strain rate the b value is one of the most useful key parameters in the fracture predictions

earthquake magnitude recent research and current trends abstract the magnitude recommendations adopted by the iaspei assembly at zurich in 1967 have had both a stabilizing and a stimulating effect on magnitude determinations and related research from 1967 onwards one magnitude research paper has appeared on the average almost every week thus making this parameter the most studied one in seismology new facilities and more accurate methods e g concerning instrumental equipment and interpretation techniques have made it possible to improve earlier achievements the application of magnitude scales has been extended in all respects e g with regard to epicentral distances focal depths wave types and wave periods magnitude frequency relations have become the most investigated equations within seismology observationally as well as theoretically they have wide applications e g for estimating the maximum magnitudes of future earthquakes an important item in earthquake prediction the magnitudes provide significant information on other source parameters such as wave energy fault length seismic moment relations between different types of magnitude yield valuable information on source properties for instance relations between magnitudes based on body and surface waves are used for efficient discrimination between earthquakes and underground explosions it is our purpose to review the magnitude development in the post zurich period 1967 1980 partly for geologists tectonophysicists and engineers who need an overview partly for seismologists who need an introduction to an overwhelmingly comprehensive literature

the b value as an earthquake precursor a systematic study of b values in new zealand has shown that within the vicinity of forthcoming large earthquakes there is initially an increase in b and then a return to normal the caracas and san fernando earthquakes show the same phenomenon which is related to that of seismic gaps

relative stress variations as determined by b values from earthquakes in circum pacific subduction zones abstract assuming a relation of b to stress state the possibility of globe wide stress variation and transmission was investigated the noaa earthquake data file served to determine the temporal change in b of log n a bm from 1963 to 1975 periods of six to eight years are observed in the b values stress pattern for most circum pacific areas south america tonga kermadec new hebrides kamchatka and eastern aleutians in the kurils fore and aftershock sequences of large earthquakes seem to mask any characteristic global pattern that might exist these sequences exhibit low b values high stress through the time of foreshocks and early stages of aftershocks followed by rapid increase in b values decrease in stress use of a worldwide earthquake data file clearly yields less resolution of the temporal b variation than the use of local network studies published by other authors incidental to the study 1124 earthquakes of the noaa data file yield the ms mb relations ms 1 16mb 0 835 for 4 5 mb 6 and log10ms 0 1432mb 0 0629 formb 6 with correlative coefficients of 0 994 and 0 992 respectively

a comparison of mechanical behavior and frequency energy relations for two kinds of echelon fault structures through numerical simulation heterogeneities of faults and rock blocks were included in the present numerical simulation to study differences between the compressive and extensional echelon fault structures released elastic strain energy due to shear and tensile failures of elements was calculated an event is defined as a failed element releasing energy for calculation of the frequency energy relation whose negative slope was labled b0 value a cut off factor used to delete a few larger events and a classification factor used to group the remaining events into several grades were introduced the evolution of b0 value at different stages was obtained three kinds of changes in b0 value were found in the compressive echelon fault structure no remarkable change in b0 value is observed during the outward propagation of tensile failure zones also during the jog failure in the extensional fault structure this is due to the fact that tensile failure only increases the number of small events that are originally larger in the compressive echelon fault structure a rapid decrease of b0 value is found during the jog failure since a shear failure zone links two fault tips leading to the occurrence of larger events for both kinds of echelon fault structures b0 value is found to decrease continuously during the fault reactivation this is related to stress or strain enhancement over a larger area in the extensional fault structure a recovery in b0 value in the vicinity of the stress peak is found after a global minimum which is similar to phenomena caused by aftershocks moreover evolution of b0 value at different stages for the same echelon fault structure and that at the same stage for different echelon fault structures are compared and analyzed it is found that the change in fault structure may not lead to the change in b0 value to obtain a more continuous and less fluctuant b0 value curve the suggested values of the cut off and classification factors are 1 8 1 2 and 3 4 respectively the proposed method can also be used to analyze the mechanical behavior and frequency energy relations for other typical or complex fault structures our work can contribute to identify the regions where abnormalities or precursors can be monitored and to assess the positions of larger events

global scale analysis of seismic activity prior to 2004 sumatra andaman mega earthquake abstract following our experience in intermediate term monitoring of seismicity before large earthquakes worldwide and in some regions we apply a similar approach to the analysis of seismicity on global scale several decades before the december 26 2004 sumatra andaman mega earthquake the earth lithosphere is being considered as a single whole representing the ultimate level of the complex earth s hierarchy the study brings up the following questions are there any anomalies of the global seismicity behaviour observed in advance of the mega earthquake do these anomalies correspond to the global scale seismic patterns similar to those detected on the regional scale before great major and strong events in other words does the earth lithosphere considered as a single whole show up an approach of the mega earthquake in the way typical for events on the smaller levels of hierarchy the results of our investigation favour the positive answer to the questions specifically we found that during the decade before the sumatra andaman mega earthquake the earth lithosphere reveals at least in intermediate term scale classical symptoms of instability which can be depicted by known precursory seismic patterns these are i transformation of frequency magnitude distribution ii change in the rate of seismic activity and iii depth redistribution of activity moreover changes of dependencies between magnitudes of different types are detected at the same time the observed global scale patterns of collective behaviour of seismicity may indicate the state of criticality of the earth lithosphere before the sumatra andaman mega earthquake

damage formation associated with bending under a constant moment abstract deformation within the earth s lithosphere is largely controlled by the rheology of the rock fracture and faulting are characterized by elastic rheologies with brittle mechanisms while folding and flow are characterized by plastic and or viscous rheologies due to ductile mechanisms however it has been recognized that deformation that resembles ductile behavior can be produced within the confines of the brittle lithosphere specific examples are folds that form in the shallow crust steep hinges at subduction zones that are accompanied by seismicity and large scale deformation at plate boundaries in these cases the brittle lithosphere behaves elastically with fracture and faulting yet produces ductile behavior in this paper we attempt to simulate such ductile behavior in elastic materials using continuum damage mechanics engineers utilize damage mechanics to model the continuum deformation of brittle materials we utilize a modified form of damage mechanics that represents a reduction in frictional strength of preexisting fractures and faults we use this empirical approach to simulate the bending of the lithosphere under the application of a constant moment we use numerical simulations to obtain elastostatic solutions for plate bending and where the longitudinal stress at a particular node exceeds a yield stress we apply damage to reduce young s modulus at the node damage is calculated at each time step by a power law relationship of the ratio of the yield stress to the longitudinal stress and the yield strain to the longitudinal strain this results in the relaxation of the material due to increasing damage to test our method we apply our damage rheology to an infinite plate deforming under a constant bending moment we simulate a wide range of behaviors from slow relaxation to instantaneous failure over timescales that span six orders of magnitude using this method stress relaxation produces elastic perfectly plastic behavior in cases where failure does not occur for cases of failure we observe a rapid increase in damage leading to failure analogous to the acceleration of microcrack formation and acoustic emissions prior to failure the changes in the rate of damage accumulation in failure cases are similar to the changes in b values of acoustic emissions observed in triaxial compression tests of fractured rock and b value changes prior to some large earthquakes thus continuum damage mechanics can simulate the phenomenon of ductile behavior due to brittle mechanisms as well as observations of laboratory experiments and seismicity

correlating variations of b values and crustal deformations during the 1990s may have pinpointed the rupture initiation of the mw 7 4 izmit earthquake of 1999 august 17 summary the b value increases from 1 1 along straight segments to 1 7 along a releasing bend of the north anatolian fault system between 30 1 e and 30 8 e the tensional nature of the bend is corroborated by extension rates of 0 3 strain yr 1 as obtained from global positioning system gps data fault plane solutions in this area change from abundant right lateral strike slip to oblique slip with normal components in the central part of the bend compatible with the formation of a pull apart basin we suggest that this crustal volume is highly fractured corresponding to a short mean crack length due to the superposition of horizontal and normal faulting as a consequence of low normal stress on the existing cracks frictional sliding is assumed to be the predominant mechanism for the generation of seismic events resulting in high b values the lowest b values b 0 8 in the area investigated are found at the junction between the fault bend and the adjoining segment which runs through the epicentre of the 1999 izmit earthquake at the junction a localized stress concentration is expected from numerical models of seismicity along geometrical barriers thus the site of lowest b has been considered to be the most likely place for a major earthquake a conclusion that is confirmed by the izmit earthquake with epicentre located about 13 km from the anticipated site in early 1992 the spatially averaged b value started to increase along the central and northwestern parts of the fault bend and there was a corresponding period of intensified horizontal extension since no large earthquake was recorded in 1992 it is assumed that the anomalous extension and the associated change in stress have been caused by aseismic fault creep in view of the available seismotectonic information surface displacements due to a slow dislocation at depth were calculated the gps data are reasonably explained by a source of normal fault type at a depth of 10 km and equivalent magnitude mw 5 9 stress changes at seismogenic depths are calculated under the assumption that an increase in b is most likely to be observed along those segments where both deviatoric and mean stress are reduced while the coulomb stress change is positive that is the fault is brought closer to failure for right lateral strike slip the stress condition is fulfilled at the sites of the largest b value increase the surface rupture of the izmit earthquake is associated with known fault traces only in the west and in the east of the study area along the fault bend it detaches itself from mapped fault traces and runs through the regions of positive coulomb stress change caused by the inferred 1992 slow event maximum surface slip 5 m is observed in the immediate vicinity of sites of maximum coulomb stress change 3 bar indicating that the 1992 event could have partially unclamped the rupture plane of the subsequent izmit earthquake we propose that the strong differences in b observed along the complex fault bend can be used to characterize different states of tectonic deformation in space and time

critical point theory of earthquakes observation of correlated and cooperative behavior on earthquake fault systems 1 the critical point theory for earthquakes was originally proposed to explain the scaling relations observed in earthquakes including the gutenberg richter frequency magnitude relation and the omori s law for aftershocks in this model main shocks their foreshocks and aftershocks are all associated with the formation of a correlated cooperative spatial region with high stress until now only indirect evidence of the existence of these correlated regions has been reported here in this paper we present observations and analyses that allow us to directly map the high stress spatially correlated regions preceding four major earthquakes i e the 1992 landers california 1995 kobe japan 1999 chi chi taiwan and 1999 hector mine california earthquakes we therefore conclude that the locations and extent of large main shocks and their immediate aftershocks can be determined from seismicity data taken prior to the main shocks and provide additional evidence in support of the critical point theory for earthquakes

seismic quiescence precursor to the 1983 kaoiki ms 6 6 hawaii earthquake a remarkable seismicity rate decrease of 65 per cent occurred in most of the aftershock volume 2 4 yr before the 1983 kaoiki ms 6 6 main shock if the background seismicity rate 1978 to mid 1981 is extrapolated to the time of the main shock more than 300 earthquakes of ml 1 8 are missing because of the quiescence subvolumes measuring 5 5 km on a side and located within the aftershock zone showed seismicity rate decreases ranging from 0 to 90 per cent the volume of no change had dimensions of approximately 10 3 km it contained the main shock hypocenter and was located near the center of the 10 km radius aftershock area the seismic quiescence in different subvolumes of the aftershock volume surrounding this central nonquiet zone varied somewhat in starting time amount of rate decrease and statistical significance according to the standard deviate z test the reported rate decrease is the most pronounced example of precursory quiescence defined to date the seismicity pattern before the 1983 kaoiki shock conformed to the quiescence hypothesis proposed on the basis of the precursors to the 1975 hawaii main shock no quiescence was observed within the immediate surrounding of the hypocenter while strong rate decreases occurred in most of the rest of the main shock source volume it is thus hypothesized that major asperities which contain high stress levels and from which main ruptures can emanate may be recognizable as volumes of constant seismicity rate within surrounding volumes of quiescence provided that decreases of seismicity rate as a function of time can be defined quantitatively

tectonic evolution of the south altyn nw china constraints by geochemical zircon u pb and lu hf isotopic analysis of the palaeozoic granitic plutons in the mangya area the south altyn orogenic belt saob is one of the most important orogenic belts in nw china consisting of the south altyn continental block and the apa mangya ophiolitic melange belt however its palaeozoic tectonic evolution is still controversial here we present petrological geochemical zircon u pb and lu hf isotopic data for the mangya plutons with the aim of establishing the palaeozoic tectonic evolution we divide the early palaeozoic magmatism in the apa mangya ophiolitic melange belt into four episodes and propose a plate tectonic model for the formation of these rocks during 511 494 ma the south altyn ocean sao was in a spreading stage and some shoshonite series i type granitic rocks were generated from 484 to 458 ma the oceanic crust of the sao subducted northward accompanied by large scale magmatic events resulting in the generation of vast high k calc alkaline series i type granitic rocks during 450 433 ma the sao closed and break off of the subducted oceanic slab occurred with the generation of some high k calc alkaline series i s transitional type granites the saob was in post orogenic extensional environment from 419 to 404 ma and many a type granites were generated

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

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

temporal variations in seismicity during quasi static and dynamic rock failure abstract a comprehensive model is presented which can explain temporal fluctuations in seismic b values in the period leading to mechanical failure in terms of the underlying physical processes of time varying applied stress and stress corrosion enhanced crack growth under conditions of constant strain rate the form of the b value anomaly in the period leading to failure depends on the form of the stress time relationship for the case where dynamic failure occurs at peak stress after a period of strain hardening the model predicts a single cusp like b value anomaly reaching a critically low value of 0 5 at failure for the physically most realistic case where dynamic failure is preceded by a period of precursory strain energy release during strain softening the model predicts two minima in the b value separated by a temporary maximum or inflection point these fluctuations in the b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence for the case of quasi static cataclastic flow the b value mirrors the stress and never falls to the critical value because there is no critical rupture new results from a series of controlled laboratory experiments are presented in which microseismic event rates and b value were monitored contemporaneously with stress time data for all variants of the stress time relationship recent field observations of temporal changes in seismicity rates and b value preceding major earthquakes are also reported both data sets exhibit b value anomalies which are consistent with the model predictions

forecast model for great earthquakes at the nankai trough subduction zone an earthquake instability model is formulated for recurring great earthquakes at the nankai trough subduction zone in southwest japan the model is quasistatic two dimensional and has a displacement and velocity dependent constitutive law applied at the fault plane a constant rate of fault slip at depth represents forcing due to relative motion of the philippine sea and eurasian plates the model simulates fault slip and stress for all parts of repeated earthquake cycles including post inter pre and coseismic stages calculated ground uplift is in agreement with most of the main features of elevation changes observed before and after the m 8 1 1946 nankaido earthquake in model simulations accelerating fault slip has two time scales the first time scale is several years long and is interpreted as an intermediate term precursor the second time scale is a few days long and is interpreted as a short term precursor accelerating fault slip on both time scales causes anomalous elevation changes of the ground surface over the fault plane of 100 mm or less within 50 km of the fault trace

seismicity preceding volcanic eruptions new experimental insights a variety of seismic signals representing different physical mechanisms precedes volcanic eruptions the most meaningful signals are high and low frequency earthquakes and volcanic tremor that have tentatively been related to fracturing and magma transport in the volcanic edifice we provide experimental support for this association by reproducing magma migration while recording seismic signals opening fractures emit high frequency acoustic events while the switch to low frequency and harmonic tremor accompanies the flow of the melt in the fractures discerning between these seismic signals in nature can significantly refine volcanic hazard evaluation

fracture nucleation process in intact rocks abstract yukutake h 1992 fracture nucleation process in intact rocks in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 247 257 we produced tomographic images for cross sections of granite samples by measuring the elastic wave velocity along 20 ray paths at high sampling rates during triaxial compression tests the dynamic fracturing process just before ultimate fracture was investigated from the rapid changes in the associated tomographic images it was found that microcracks concentrated locally in several regions as axial load was increased and the velocity then recovered in some of the intensively microcracked regions 6 60 s before fracture subsequently the amplitude of the elastic waves through the most intensively microcracked region also recovered in a few cases the frequency of the elastic wave through the most intensively microcracked region became extremely high we consider that fracture nucleated in the intensively microcracked regions and infer that several macrocracks occur due to an abrupt coalescence of microcracks in the fracture nucleated region a crack length in the process zone for brittle fracture was evaluated from the stress drop observed just before fracture and the published value of the shear fracture energy for westerly granite we expect from the results of both tomography and stress drops that subsequent instability occurred when one of larger macrocracks formed by the coalescence of the macrocracks reached a critical length in the most intensively nucleated region

crustal heterogeneity highlighted by spatial b value map in the wellington region of new zealand summary we map the b value in the subduction zone of the wellington region new zealand using a high quality earthquake catalogue relocated with a 3 d seismic velocity model consisting of 50 314 events that occurred between 1990 and 2005 in order to investigate heterogeneity in the crust of the overlying plate and in the upper plane of the wadati benioff zone wbz we analyse a series of cross sections perpendicular to the strike of the subduction zone we calculate the b values selecting events with magnitude of completeness 2 4 and depth 65 km and projecting the seismicity within 20 km on each side of the cross sectional planes we observe areas of high b value 1 7 near the plate interface and regions of low b value anomalies are detected both in the wbz in the northwest region below 40 km depth and in the overlying plate in the northern south island at 10 km depth the anomalies are statistically significant based on utsu s p test and the bootstrap method and are not data processing method or parameter dependent we compare the b value distribution with previously determined 3 d distributions of vp vp vs and qp from seismic tomography this comparison suggests that material inhomogeneity caused by fluid filled cracks resulting from dehydration of the subducted slab and subducted sediments is the predominant cause of b value variation in the shallow part of this subduction zone our observations are consistent with a previously proposed conceptual model that fluid distribution in the shallow part of this subduction zone is controlled by the permeability of geological terranes in the overlying plate

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

thermal weakening of the carbonate basement under mt etna volcano italy implications for volcano instability abstract the physical integrity of a sub volcanic basement is crucial in controlling the stability of a volcanic edifice for many volcanoes this basement can comprise thick sequences of carbonates that are prone to significant thermally induced alteration these debilitating thermal reactions facilitated by heat from proximal magma storage volumes promote the weakening of the rock mass and likely therefore encourage edifice instability such instability can result in slow gravitational spreading and episodic to continuous slippage of unstable flanks and may also facilitate catastrophic flank collapse understanding the propensity of a particular sub volcanic basement to such instability requires a detailed understanding of the influence of high temperatures on the chemical physical and mechanical properties of the rocks involved the juxtaposition of a thick carbonate substratum and magmatic heat sources makes mt etna volcano an ideal candidate for our study we investigated experimentally the effect of temperature on two carbonate rocks that have been chosen to represent the deep heterogeneous sedimentary substratum under mt etna volcano this study has demonstrated that thermal stressing resulted in a progressive and significant change in the physical properties of the two rocks porosity wet i e water saturated dynamic poisson s ratio and wet vp vs ratio all increased whilst p and s wave velocities bulk sample density dynamic and static young s modulus dry vp vs ratio and dry dynamic poisson s ratio all decreased at temperatures of 800 c the carbonate in these rocks completely dissociated resulting in a total mass loss of about 45 and the release of about 44 wt of co 2 uniaxial deformation experiments showed that high in situ temperatures 500 c significantly reduced the strength of the carbonates and altered their deformation behaviour above 500 c the rocks deformed in a ductile manner and the output of acoustic emissions was greatly reduced we speculate that thermally induced weakening and the ductile behaviour of the carbonate substratum could be a key factor in explaining the large scale deformation observed at mt etna volcano our findings are consistent with several field observations at mt etna volcano and can quantitatively support the interpretation of 1 the irregularly low seismic velocity zones present within the sub volcanic sedimentary basement 2 the anomalously high co 2 degassing observed 3 the anomalously high vp vs ratios and the rapid migration of fluids and 4 the increasing instability of volcanic edifices in the lifespan of a magmatic system we speculate that carbonate sub volcanic basement may emerge as one of the decisive fundamentals in controlling volcanic stability