containment of stick slip failures on a simulated fault by pore fluid injection unstable stick slip events have been confined to a portion of a finely ground 40 by 200 cm simulated fault in a granite sample by selectively injecting water onto the ends of the stressed fault source parameters of eight confined events generated at normal stresses between 0 7 and 4 1 mpa have been measured dynamic stress drops increased from 0 06 to 0 38 mpa with increasing normal stress such that the ratio of stress drop to normal stress is about 0 09 fractional shear stress drops of these events are less than 0 2 total fault slip measured at the center of the slipped portion ranged from 8 5 to 38 9 mu m maximum relative slip velocities at the same location ranged from 2 5 to 34 7 cm s slip velocity increased with increasing dynamic stress drop

anisotropic changes in p wave velocity and attenuation during deformation and fluid infiltration of granite fluid infiltration and pore fluid pressure changes are known to have a significant effect on the occurrence of earthquakes yet for most damaging earthquakes with nucleation zones below a few kilometers depth direct measurements of fluid pressure variations are not available instead pore fluid pressures are inferred primarily from seismic wave propagation characteristics such as v p v s ratio attenuation and reflectivity contacts we present laboratory measurements of changes in p wave velocity and attenuation during the injection of water into a granite sample as it was loaded to failure a cylindrical sample of westerly granite was deformed at constant confining and pore pressures of 50 and 1 mpa respectively axial load was increased in discrete steps by controlling axial displacement anisotropic p wave velocity and attenuation fields were determined during the experiment using an array of 13 piezoelectric transducers at the final loading steps 86 and 95 of peak stress both spatial and temporal changes in p wave velocity and peak to peak amplitudes of p and s waves were observed p wave velocity anisotropy reached a maximum of 26 transient increases in attenuation of up to 483 db m were also observed and were associated with diffusion of water into the sample we show that velocity and attenuation of p waves are sensitive to the process of opening of microcracks and the subsequent resaturation of these cracks as water diffuses in from the surrounding region symmetry of the orientation of newly formed microcracks results in anisotropic velocity and attenuation fields that systematically evolve in response to changes in stress and influx of water with proper scaling these measurements provide constraints on the magnitude and duration of velocity and attenuation transients that can be expected to accompany the nucleation of earthquakes in the earth9s crust

damped regional scale stress inversions methodology and examples for southern california and the coalinga aftershock sequence 1 we present a new focal mechanism stress inversion technique to produce regional scale models of stress orientation containing the minimum complexity necessary to fit the data current practice is to divide a region into small subareas and to independently fit a stress tensor to the focal mechanisms of each subarea this procedure may lead to apparent spatial variability that is actually an artifact of overfitting noisy data or nonuniquely fitting data that does not completely constrain the stress tensor to remove these artifacts while retaining any stress variations that are strongly required by the data we devise a damped inversion method to simultaneously invert for stress in all subareas while minimizing the difference in stress between adjacent subareas this method is conceptually similar to other geophysical inverse techniques that incorporate damping such as seismic tomography in checkerboard tests the damped inversion removes the stress rotation artifacts exhibited by an undamped inversion while resolving sharper true stress rotations than a simple smoothed model or a moving window inversion we show an example of a spatially damped stress field for southern california the methodology can also be used to study temporal stress changes and an example for the coalinga california aftershock sequence is shown we recommend use of the damped inversion technique for any study examining spatial or temporal variations in the stress field

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

magnitude 7 level earthquakes a new lower limit of self similarity in seismic scaling relationships microfractures occurring in a rock sample that are called acoustic emission ae events show some similar features to earthquakes however it remains to be shown whether or not ae equate to ultramicroearthquakes in this study we show the existence of magnitude 7 level earthquakes based on seismological analyses of ae source parameters advances in multichannel broadband high speed continuous recording of ae under seismogenic pressure conditions has facilitated increasingly robust measurement source parameters of ae show that ae events satisfy the same scaling relationship as natural earthquakes in which seismic moment is inversely proportional to the cube of corner frequency this result suggests that both millimeter scale fractures and natural earthquakes of kilometer scale ruptures are highly similar as physical processes hence ae events can be interpreted as ultramicroearthquakes having a magnitude of about 7 these results demonstrate that laboratory observation is an effective approach in studying natural earthquake generation process

source mechanism of mining induced seismic events resolution of double couple and non double couple models abstract traditional assumption about a seismic source as a double couple dc can represent a serious limitation for describing mechanisms of mining induced seismic events in mining conditions more general process than a shear slip can occur due to stress concentration on mined out areas we assume both non dc components within the dipole description of the source the moment tensor and a single force we processed five seismic events recorded at driefontein gold mine in south africa to demonstrate that waveform modeling is not successful enough due to poor information about the medium inversion of amplitudes of direct p and s waves is more prospective due to their smaller sensitivity to detail of the medium and possibility to correct them for ray bending and shear wave splitting prior to the inversion moreover we are able to estimate the error of the resulting mechanism due to neglect of ray bending all five events processed display non dc mechanisms which are significant at 95 considering the mismodelling of the rock mass by a homogeneous isotropic medium the source mechanisms are in agreement with the underground mining and geological structures one event has a sub vertical pressure single dipole complemented by an implosion which is the body force equivalent of a collapsing horizontal crack or a burst of a pillar supporting the hangingwall of a mined out area two events have mechanisms close to a pressure single couple all three events are located on pillars two out of these three events can be assigned with a downward single force as an alternative mechanism which matches the amplitude data successfully it is the force equivalent of a mined out area collapse when a rock mass falls from the hangingwall to the footwall the events are however rather strong which would require too large rock mass fallen one of the remaining events is a nearly horizontal tensile single couple corresponding to a vertical tensional fault it could be originated by a break of the hangingwall of a mined out area however the hypocenter of this event is deep into the footwall were there is no mining taking place the last event is mostly dc accompanied by a compensated compressive dipole which may be associated to nearly vertical dip slip along a fault in the hangingwall this event is located on the margin of a wide pillar which supports the interpretation

a robust calibration technique for acoustic emission systems based on momentum transfer from a ball drop we describe a technique to estimate the seismic moment of acoustic emissions and other extremely small seismic events unlike previous calibration tech niques it does not require modeling of the wave propagation sensor response or signal conditioning rather this technique calibrates the recording system as a whole and uses a ball impact as a reference source or empirical green s function to correctly apply this technique we develop mathematical expressions that link the seismic mo ment m0 of internal seismic sources i e earthquakes and acoustic emissions to the impulse or change in momentum p of externally applied seismic sources i e me teor impacts or in this case ball impact we find that at low frequencies moment and impulse are linked by a constant which we call the force moment rate scale factor c f mm0 p this constant is equal to twice the speed of sound in the material from which the seismic sources were generated next we demonstrate the calibration technique on two different experimental rock mechanics facilities the first example is a saw cut cylindrical granite sample that is loaded in a triaxial apparatus at 40 mpa confining pressure the second example is a 2 m long fault cut in a granite sample and deformed in a large biaxial apparatus at lower stress levels using the empirical cali bration technique we are able to determine absolute source parameters including the seismic moment corner frequency stress drop and radiated energy of these magnitude 2 5 to 7 seismic events

geological modeling of submeter scale heterogeneity and its influence on tracer transport in a fluvial aquifer 1 we developed a new model of aquifer heterogeneity to analyze data from a single well injection withdrawal tracer test conducted at the macrodispersion experiment made site on the columbus air force base in mississippi usa the physical heterogeneity model is a hybrid that combines 3 d lithofacies to represent submeter scale highly connected channels within a background matrix based on a correlated multivariate gaussian hydraulic conductivity field the modeled aquifer architecture is informed by a variety of field data including geologic core sampling geostatistical properties of this hybrid heterogeneity model are consistent with the statistics of the hydraulic conductivity data set based on extensive borehole flowmeter testing at the made site the representation of detailed small scale geologic heterogeneity allows for explicit simulation of local preferential flow and slow advection processes that explain the complex tracer response from the injection withdrawal test based on the new heterogeneity model advective dispersive transport reproduces key characteristics of the observed tracer recovery curve including a delayed concentration peak and a low concentration tail importantly our results suggest that intrafacies heterogeneity is responsible for local scale mass transfer

shear failure of a granite pin traversing a sawcut fault abstract fault heterogeneities such as bumps bends and stepovers are commonly observed on natural faults but are challenging to recreate under controlled laboratory conditions we study deformation and microseismicity of a 76 mm diameter westerly granite cylinder with a sawcut fault with known frictional properties an idealized asperity is added by emplacing a precision ground 21 mm diameter solid granite dowel that crosses the center of the fault at right angles this intact granite pin provides a strength contrast that resists fault slip upon loading to 80 mpa in a triaxial machine we first observed a m 4 slip event that ruptured the sawcut fault slipped 40 m but was halted by the granite pin with continued loading the pin failed in a swarm of thousands of m 6 to m 8 events known as acoustic emissions aes once the pin was fractured to a critical point it permitted complete rupture events m 3 on the sawcut fault stick slip instabilities subsequent slip events were preceded by clusters of foreshock like aes all located on the fault plane and the spatial extent of the foreshock clusters is consistent with our estimate of a critical nucleation dimension h we also identified an aseismic zone on the fault plane surrounding the fractured rock pin a post mortem analysis of the sample showed a thick gouge layer where the pin intersected the fault suggesting that dilatancy of this gouge propped open the fault and prevented microseismic events in its vicinity recorded microseismicity separates into three categories slip on the sawcut fault fracture of the intact rock pin and off fault seismicity associated with pin related rock joints we found that pin fracture events were exclusively implosive anticrack even though the shear process zone was overall dilatant this shows how aseismic effects can lead to unexpected seismic manifestations of certain faulting processes

experimental investigation of acoustic emissions and their moment tensors in rock during failure abstract we study acoustic emissions aes associated with shear and tensile failures around a horizontal borehole in a sandstone sample subjected to triaxial stress the aim is to relate the ae event rate to macroscopic observations of sample deformation and the percentage of isotropic and deviatoric components of the seismic moment tensors to the expected failure mechanisms the horizontal hole interferes with the applied load and forms a strongly spatially dependent anisotropic stress field focusing the crack initiation into both shear and tensile failures the recorded aes follows reasonably well existing damage models but the elastic solution of hoop stress does not represent the onset of failure around the borehole the focal mechanisms correlate with the orientation of macroscopic fractures in the sample events close to the borehole show a higher fraction of isotropic percentage in moment tensors compared to events occurring in the macroscopic fracture featuring higher double couple percentages the inhomogeneous stress field due to the borehole and the stress induced damage is strongly affecting the axial and radial velocities which in turn affect the waveforms of the recorded aes and the resulting moment tensors the vp vs ratio obtained from the ratio of isotropic to compensated linear vector dipole components of the moment tensors is close to that obtained from ultrasonic velocity measurements

earthquakes as multiscale dynamic ruptures with heterogeneous fracture surface energy 1 we propose a model of the wide scale growth of dynamic rupture during an earthquake based on our multiscale simulation of a planar crack in a three dimensional homogeneous elastic space a simple slip weakening law governs the fracture friction processes and its characteristic parameters slip weakening distance and fracture surface energy have multiscale heterogeneous distributions we consider a set of randomly distributed circular patches whose diameter is proportional to the fracture surface energy each patch represents an asperity between irregular fault surfaces and the size number relation of the patches obeys power law statistics we assess rupture propagation from a small instability using a boundary integral equation method with a renormalization technique although most events stop shortly after their initiation some grow triggering neighboring patches of similar size small and large events show statistically self similar properties of rupture growth and stop spontaneously without requiring a special stopping mechanism the rupture velocity locally exceeds the shear wave speed but globally remains subshear speed due to the increase of the average fracture energy as the rupture grows the relation between size and frequency of events is a power law which is explained by the triggering probability between patches as a consequence of statistically self similar random triggering growth we observe a distinct main phase in seismic waves similar to those of natural earthquakes but we cannot estimate the final size of the event from the initial part of the seismic waves if this is true for the real earthquakes predicting the size of a future earthquake would be quite difficult

premonitory acoustic emissions and stick slip in natural and smooth faulted westerly granite 1 a stick slip event was induced in a cylindrical sample of westerly granite containing a preexisting natural fault by loading at constant confining pressure of 150 mpa continuously recorded acoustic emission ae data and computer tomography ct generated images of the fault plane were combined to provide a detailed examination of microscale processes operating on the fault the dynamic stick slip event considered to be a laboratory analog of an earthquake generated an ultrasonic signal that was recorded as a large amplitude ae event first arrivals of this event were inverted to determine the nucleation site of slip which is associated with a geometric asperity on the fault surface ct images and ae locations suggest that a variety of asperities existed in the sample because of the intersection of branch or splay faults with the main fault this experiment is compared with a stick slip experiment on a sample prepared with a smooth artificial saw cut fault surface nearly a thousand times more ae were observed for the natural fault which has a higher friction coefficient 0 78 compared to 0 53 and larger shear stress drop 140 compared to 68 mpa however at the measured resolution the ultrasonic signal emitted during slip initiation does not vary significantly between the two experiments suggesting a similar dynamic rupture process we propose that the natural faulted sample under triaxial compression provides a good laboratory analogue for a field scale fault system in terms of the presence of asperities fault surface heterogeneity and interaction of branching faults

ultrasonic velocities acoustic emission characteristics and crack damage of basalt and granite acoustic emissions ae compressional p shear s wave velocities and volumetric strain of etna basalt and aue granite were measured simultaneously during triaxial compression tests deformation induced ae activity and velocity changes were monitored using twelve p wave sensors and eight orthogonally polarized s wave piezoelectric sensors volumetric strain was measured using two pairs of orthogonal strain gages glued directly to the rock surface p wave velocity in basalt is about 3 km s at atmospheric pressure but increases by 50 when the hydrostatic pressure is increased to 120 mpa in granite samples initial p wave velocity is 5 km s and increases with pressure by 20 the pressure induced changes of elastic wave speed indicate dominantly compliant low aspect ratio pores in both materials in addition etna basalt also contains high aspect ratio voids in triaxial loading stress induced anisotropy of p wave velocities was significantly higher for basalt than for granite with vertical velocity components being faster than horizontal velocities however with increasing axial load horizontal velocities show a small increase for basalt but a significant decrease for granite using first motion polarity we determined ae source types generated during triaxial loading of the samples with increasing differential stress ae activity in granite and basalt increased with a significant contribution of tensile events close to failure the relative contribution of tensile events and horizontal wave velocities decreased significantly a concomitant increase of double couple events indicating shear suggests shear cracks linking previously formed tensile cracks

slip rate dependent friction as a universal mechanism for slow slip events a growing body of observations worldwide has documented fault slip transients that radiate little or no seismic energy the mechanisms that govern these slow slip events sses and their wide range of depths slip rates durations stress drops and recurrence intervals remain poorly known here we show that slow slip can be explained by a transition from rate weakening frictional sliding at low slip rates towards rate neutral or rate strengthening behaviour at higher slip rates as has been observed experimentally we use numerical simulations to illustrate that this rate dependent transition quantitatively explains the experimental data for natural fault rocks representative of materials in the source regions of sses with a standard constant parameter rate and state friction law sses arise only near the threshold for slip instability the inclusion of velocity dependent friction parameters substantially broadens the range of conditions for slow slip occurrence and produces a wide range of event characteristics which include stress drop duration and recurrence as observed in nature upscaled numerical simulations that incorporate parameters consistent with laboratory measurements can reproduce geodetic observations of repeating sses on tectonic faults we conclude that slip rate dependent friction explains the ubiquitous occurrence of sses in a broad range of geological environments a transition from rate weakening to rate strengthening frictional behaviour with increasing slip rate could explain the observed diversity of slow slip events on faults according to numerical simulations

preslip and cascade processes initiating laboratory stick slip recent modeling studies have explored whether earthquakes begin with a large aseismic nucleation process or initiate dynamically from the rapid growth of a smaller instability in a cascade up process to explore such a case in the laboratory we study the initiation of dynamic rupture stick slip of a smooth saw cut fault in a 76 mm diameter cylindrical granite laboratory sample at 40 120 mpa confining pressure we use a high dynamic range recording system to directly compare the seismic waves radiated during the stick slip event to those radiated from tiny m 6 discrete seismic events commonly known as acoustic emissions aes that occur in the seconds prior to each large stick slip the seismic moments focal mechanisms locations and timing of the aes all contribute to our understanding of their mechanics and provide us with information about the stick slip nucleation process in a sequence of 10 stick slips the first few microseconds of the signals recorded from stick slip instabilities are nearly indistinguishable from those of premonitory aes in this sense it appears that each stick slip begins as an ae event that rapidly 20 s grows about 2 orders of magnitude in linear dimension and ruptures the entire 150 mm length of the simulated fault we also measure accelerating fault slip in the final seconds before stick slip we estimate that this slip is at least 98 aseismic and that it both weakens the fault and produces aes that will eventually cascade up to initiate the larger dynamic rupture

fracture in westerly granite under ae feedback and constant strain rate loading nucleation quasi static propagation and the transition to unstable fracture propagation new observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of westerly granite using acoustic emission ae monitoring by conducting the tests under different loading conditions the fracture process is demonstrated for quasi static fracture under ae feedback load a slowly developing unstable fracture loaded at a slow constant strain rate of 2 5 10 6 s and an unstable fracture that develops near instantaneously loaded at a fast constant strain rate of 5 10 5 s by recording a continuous ultrasonic waveform during the critical period of fracture the entire ae catalogue can be captured and the exact time of fracture defined under constant strain loading three stages are observed 1 an initial nucleation or stable growth phase at a rate of 1 3 mm s 2 a sudden increase to a constant or slowly accelerating propagation speed of 18 mm s and 3 unstable accelerating propagation in the 100 ms before rupture the high level of ae activity as seen on the continuous record prevented the location of discrete ae events a lower bound estimate of the average propagation velocity using the time to rupture and the existing fracture length suggests values of a few m s however from a low gain acoustic record we infer that in the final few ms the fracture propagation speed increased to 175 m s these results demonstrate similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments it is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding the controls on fracture nucleation

earthquake nucleation on rate and state faults aging and slip laws we compare 2 d quasi static earthquake nucleation on rate and state faults under both aging and slip versions of the state evolution law for both versions mature nucleation zones exhibit 2 primary regimes of growth well above and slightly above steady state corresponding respectively to larger and smaller fault weakening rates well above steady state aging law nucleation takes the form of accelerating slip on a patch of fixed length this length is proportional to b 1 and independent of a where a and b are the constitutive parameters relating changes in slip speed and state to frictional strength under the slip law the nucleation zone is smaller and continually shrinks as slip accelerates the nucleation zone is guaranteed to remain well above steady state only for values of a b that are low by laboratory standards near steady state for both laws the nucleation zone expands the propagating front remains well above steady state giving rise to a simple expression for its effective fracture energy g c this fracture energy controls the propagation style for the aging law g c increases approximately as the square of the logarithm of the velocity jump this causes the nucleation zone to undergo quasi static crack like expansion to a size asymptotically proportional to b b a 2 for the slip law g c increases only as the logarithm of the velocity jump and crack like expansion is not an option instead the nucleation zone grows as an accelerating unidirectional slip pulse under both laws the nucleation front propagates at a velocity larger than the slip speed by roughly b divided by the logarithm of the velocity jump where is the effective elastic shear modulus for this prediction to be consistent with observed propagation speeds of slow slip events in subduction zones appears to require effective normal stresses as low as 1 mpa

laboratory generated m 6 earthquakes we consider whether mm scale earthquake like seismic events generated in laboratory experiments are consistent with our understanding of the physics of larger earthquakes this work focuses on a population of 48 very small shocks that are foreshocks and aftershocks of stick slip events occurring on a 2 0 m by 0 4 m simulated strike slip fault cut through a large granite sample unlike the larger stick slip events that rupture the entirety of the simulated fault the small foreshocks and aftershocks are contained events whose properties are controlled by the rigidity of the surrounding granite blocks rather than characteristics of the experimental apparatus the large size of the experimental apparatus high fidelity sensors rigorous treatment of wave propagation effects and in situ system calibration separates this study from traditional acoustic emission analyses and allows these sources to be studied with as much rigor as larger natural earthquakes the tiny events have short 3 6 s rise times and are well modeled by simple double couple focal mechanisms that are consistent with left lateral slip occurring on a mm scale patch of the precut fault surface the repeatability of the experiments indicates that they are the result of frictional processes on the simulated fault surface rather than grain crushing or fracture of fresh rock our waveform analysis shows no significant differences other than size between the m 7 to m 5 5 earthquakes reported here and larger natural earthquakes their source characteristics such as stress drop 1 10 mpa appear to be entirely consistent with earthquake scaling laws derived for larger earthquakes

high resolution mechanics of a microearthquake sequence unique data from microseismic ms and acoustic emission ae systems monitoring a common rock volume have been analyzed from the underground research laboratory in canada the two 16 sensor systems were installed to investigate stress induced microcracking around clay and concrete bulkheads as part of the tunnel sealing experiment single and double ms events are found to be spatially associated with clusters of between 19 86 higher frequency ae events each ae cluster is elongated with the longest axis between 15 and 50 cm the ae events occurring before the associated ms event are termed foreshocks and for two of the three analyzed ae clusters an acceleration in the ae frequency and cumulative magnitude occurs upto the time of the ms event however with one ae cluster very few foreshocks are recorded possibly indicating a more homogeneous failure plane time independent moment tensors were determined with the ms events showing significant deviatoric sources the majority of the ae events have deviatoric mechanisms with a few crack opening and crack closure type events also determined this study highlights the benefits of using small scale seismic systems to investigate the temporal fracture mechanics of microcrack formation and allows comparisons with larger more damaging seismicity

source analysis of acoustic emissions in aue granite cores under symmetric and asymmetric compressive loads the effect of stress anisotropy on the brittle failure of granite is investigated under uniaxial compression non standard asymmetric compression tests are performed on cores of aue granite diameter 52 mm length 100 mm in which 20 per cent of the core top surface remains unloaded the edge of the asymmetric steel loading plate acts as a stress concentrator from where a shear rupture is initiated the propagation of the fracture related process zone from top to bottom of the core is mapped by microcrack induced acoustic emissions compared to standard uniaxial tests with symmetric loading in the asymmetric tests both a greater quantity and more localized distributions of emission event hypocentres are observed the maximum event density doubles for asymmetric 20 events per 10 6 m3 compared to symmetric tests the cluster correlation coefficient a measure of strain localization in the faulting process reaches 0 15 for symmetric and 0 30 for asymmetric tests the clustering of events however is found post failure only three different amplitudes are used to determine b values discussed as a possible failure precursor focal amplitudes determined at a 10 mm source distance and maximum amplitudes measured at eight piezoceramic sensors lead to b values that drop before rock failure first pulse amplitudes automatically picked from emission wavelets show no anomaly first motion polarity statistics of amplitudes indicate that a shear crack type radiation pattern is responsible for 70 per cent of the failure of granite irrespective of stress boundary conditions for type s events with an equal percentage of dilatational and compressional first motions focal mechanisms are determined by fitting measured first pulse amplitudes to an assumed double couple radiation pattern while hypocentres of large type s events align parallel to the later fracture plane their fault plane solutions show no coherent pattern spatial views of fracture planes reconstructed from x ray computed tomograms reveal local small scale changes in fracture plane orientation nodal planes from average fault plane solutions of the microscopic acoustic emission events coincide with the overall orientation of the macroscopic fracture plane azimuth strike angle determined from thin sections and tomograms

a laboratory acoustic emission experiment under in situ conditions in this paper we revisit acoustic emission ae data from an in situ rock fracture experiment conducted at the underground research laboratory url in manitoba canada the mine by experiment a large scale excavation response test was undertaken at a depth of 420 m and involved the mechanical excavation of a cylindrical tunnel during the experiment a small array of 16 panametrics v103 ae sensors enclosed a 0 7 m 0 7 m 1 1 m rectangular prism of lac du bonnet granite located in the tunnel wall the v103 sensors were later calibrated in the laboratory and a source parameter analysis was undertaken using a spectral fitting method corner frequency and moment magnitude were found to be inside the ranges 250 khz fc 490 khz and 7 5 mw 6 8 respectively static stress drops ranged from 0 3 to 4 mpa which is consistent with large seismicity recorded at the url

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

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

foreshocks during the nucleation of stick slip instability 1 we report on laboratory experiments which investigate interactions between aseismic slip stress changes and seismicity on a critically stressed fault during the nucleation of stick slip instability we monitor quasi static and dynamic changes in local shear stress and fault slip with arrays of gages deployed along a simulated strike slip fault 2 m long and 0 4 m deep in a saw cut sample of sierra white granite with 14 piezoelectric sensors we simultaneously monitor seismic signals produced during the nucleation phase and subsequent dynamic rupture we observe localized aseismic fault slip in an approximately meter sized zone in the center of the fault while the ends of the fault remain locked clusters of high frequency foreshocks mw 6 5 to 5 0 can occur in this slowly slipping zone 5 50 ms prior to the initiation of dynamic rupture their occurrence appears to be dependent on the rate at which local shear stress is applied to the fault the meter sized nucleation zone is generally consistent with theoretical estimates but source radii of the foreshocks 2 to 70 mm are 1 to 2 orders of magnitude smaller than the theoretical minimum length scale over which earthquake nucleation can occur we propose that frictional stability and the transition between seismic and aseismic slip are modulated by local stressing rate and that fault sections which would typically slip aseismically may radiate seismic waves if they are rapidly stressed fault behavior of this type may provide physical insight into the mechanics of foreshocks tremor repeating earthquake sequences and a minimum earthquake source dimension

seismic moment tensor and b value variations over successive seismic cycles in laboratory stick slip experiments the formation of fault damage due to slip under high normal stresses can rarely be monitored under in situ conditions to advance our understanding of microfracture processes we investigated stick slip events on westerly granite samples containing the following 1 a planar saw cut fault and 2 a fault developed from a fresh fracture surface we examined temporal changes of seismic moment tensors and b values of acoustic emission ae events during experiment on the saw cut surface small aes exhibiting non double couple components were observed continuously and strong aes displaying double couple components were visible only when approaching the slip onsets sliding on naturally fractured surfaces showed in addition to double couple components significant volumetric contributions especially during the interslip periods and immediately after stick slip events indicating substantial shear enhanced compaction within a relatively broad damage zone the obtained results shed light on how differences in fault structure control the kinematics of microseismicity during different periods of the seismic cycle

spatiotemporal changes faulting regimes and source parameters of induced seismicity a case study from the geysers geothermal field the spatiotemporal kinematic and source characteristics of induced seismicity occurring at different fluid injection rates are investigated to determine the predominant physical mechanisms responsible for induced seismicity at the northwestern part of the geysers geothermal field california we analyze a relocated hypocenter catalog from a seismicity cluster where significant variations of the stress tensor orientation were previously observed to correlate with injection rates we find that these stress tensor orientation changes may be related to increased pore pressure and the corresponding changes in poroelastic stresses at reservoir depth seismic events during peak injections tend to occur at greater distances from the injection well preferentially trending parallel to themaximum horizontal stress direction in contrast at lower injection rates the seismicity tends to align in a different direction which suggests the presence of a local fault during peak injection intervals the relative contribution of strike slip faulting mechanisms increases furthermore increases in fluid injection rates also coincide with a decrease in b values our observations suggest that regardless of the injection stage most of the induced seismicity results from thermal fracturing of the reservoir rock however during peak injection intervals the increase in pore pressure may likewise be responsible for the induced seismicity by estimating the thermal and hydraulic diffusivities of the reservoir we confirm that the characteristic diffusion length for pore pressure is much greater than the corresponding length scale for temperature and also more consistent with the spatial extent of seismicity observed during different injection rates

an improved method for seismic moment tensor inversion of acoustic emissions through assessment of sensor coupling and sensitivity to incidence angle 1 introductionae analysis is frequently used in laboratory experiments asa method to describe the spatio temporal evolution of damage furthermore characterization of the ae sources provides insightinto the micromechanical processes involved in the initiation andcoalescence of cracks in rock samples 1 ae source mechanismanalysis provides additional information on the orientation andmechanisms of individual cracks during nucleation and formationof a fracture 2 simple techniques estimating the faultingmechanisms are based on average polarities 3 using dilatationaland compressional rst p wave motions this allows separatingrecorded aes into tensile shear and compaction sources themoment tensor mt represents a measure of the deformation inthe seismic source 4 it is commonly used to provide informationon fault plane parameters strike dip and rake of the fault thedecomposition of the mt allows estimating the ratio of the volu metric and non volumetric strain in the seismic source e g 5 themt inversion carries information on non double couple ndc anddouble couple dc components in earthquake sources providinginformation onprocesses involved in uid induced seismicity 6 10 and mining induced seismicity 11 13 collins et al 14 investi gated the source mechanisms of seismic events related to fracturingof tunnel walls sixteen ae sensors frequency band 40 400 khz were used to calculate seismic mts of extremely small seismicevents m

imaging hydraulic fractures in a geothermal reservoir 1 an injection experiment at the coso geothermal field in eastern california in march 2005 caused a swarm of microearthquakes that was recorded by a local network of three component digital seismometers high resolution relative hypocenter locations propagated upward and northward on a 700 600 m plane striking n 20 e and dipping 75 to the wnw this plane is a pre existing fault whose surface projection coincides with an active scarp the earthquakes have similar non double couple mechanisms that involve volume increases and the fault plane bisects their dilatational fields implying a process dominated by tensile failure the source types require the additional involvement of subsidiary shear faulting however events before and after the swarm have variable orientations and volume changes of both signs similar tensile shear failure is observed in some natural microearthquake swarms for example at long valley caldera california its occurrence under low fluid pressure may imply a heterogeneous stress field or the induction of thermal stresses by introduction of cold fluid