source parameters of acoustic emission events and scaling with mining induced seismicity a series of uniaxial compression tests were performed on 96 mm diameter quartzite samples 242 mm in length to understand the scaling of rock fracture processes nine acoustic emission ae sensors glued to each sample monitored the aes resulting from microcracking within the samples in contrast to previous ae studies the sensors were calibrated as velocity transducers so that the output could be compared to mining induced seismicity and natural earthquakes a new hybrid relative moment tensor method was applied to obtain source mechanism solutions for eight clusters of events once the ae rate accelerated prior to failure the event positions are associated with the observed failure planes the moment tensors were found to have double couple components indicating that shearing was occurring the stress drop appears to be constant over the range of moments suggesting the self similar scaling of the fracture response from the laboratory sample to mine seismicity and natural earthquakes over a wide range of length scales similar conclusions can be drawn by considering the apparent stress and source radius the scaling of the peak velocity and peak acceleration parameters is apparently consistent with mining induced seismicity but is considerably affected by high frequency attenuation and the limited bandwidth this is confirmed by the frequency magnitude plot which has a slope of unity the fracture processes in the laboratory are similar to those occurring underground near stope faces and pillars in deep level south african gold mines where there is a high vertical compression and low confinement

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

seismic and aseismic preparatory processes before large stick slip failure natural earthquakes often have very few observable foreshocks which significantly complicates tracking potential preparatory processes to better characterize expected preparatory processes before failures we study stick slip events in a series of triaxial compression tests on faulted westerly granite samples we focus on the influence of fault roughness on the duration and magnitude of recordable precursors before large stick slip failure rupture preparation in the experiments is detectable over long time scales and involves acoustic emission ae and aseismic deformation events preparatory fault slip is found to be accelerating during the entire pre failure loading period and is accompanied by increasing ae rates punctuated by distinct activity spikes associated with large slip events damage evolution across the fault zones and surrounding wall rocks is manifested by precursory decrease of seismic b values and spatial correlation dimensions peaks in spatial event correlation suggest that large slip initiation occurs by failure of multiple asperities shear strain estimated from ae data represents only a small fraction 1 of total shear strain accumulated during the preparation phase implying that most precursory deformation is aseismic the relative contribution of aseismic deformation is amplified by larger fault roughness similarly seismic coupling is larger for smooth saw cut faults compared to rough faults the laboratory observations point towards a long lasting and continuous preparation process leading to failure and large seismic events the strain partitioning between aseismic and observable seismic signatures depends on fault structure and instrument resolution

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

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

spatial and temporal anomalies of soil gas in northern taiwan and its tectonic and seismic implications abstract in this paper we study 1 the spatial anomalies and 2 the temporal anomalies of soil gas in northern taiwan the spatial anomalies of soil gas are related to tectonic faults while the temporal anomalies of soil gas are associated with pre earthquake activities detailed soil gas sampling was systematically performed and the analysis of the collected gas species shows that high helium and nitrogen concentrations appear in samples from specific sites which coincide with the structural setting of the area studied this analysis indicates the possibility of using these soil gases to determine fault zones in the studied area based on the soil gas data a station tapingti for automatic soil gas monitoring was constructed on an appropriate site at the fault zone some anomalous high radon concentrations at certain times can be identified from the dataset which was generated by the continuous monitoring of soil gas for over a year notably many of these anomalies were observed several hours to a few days before the earthquakes ml 3 that occurred in northern taiwan by combining the information of epicenters and fault plane solutions of these earthquakes we find that the shallow earthquakes 20 km were mainly thrust type earthquakes and distributed in greater distances 45 km east of the monitoring site group b such focal mechanisms of earthquakes suggest an extensional and compressional structural domain in the continental crust for group a and group b earthquakes respectively it is suggested that the pre earthquake activities associated with the seismicity of group b may be transmitted along the major decollement in the region below the tapingti station leading to the observed soil gas enhancements

the hierarchical rupture process of a fault an experimental study abstract we describe the detailed faulting process of a naturally healed fault containing geometric and mechanical asperities in a granitic porphyry sample based on data collected with a high speed acoustic emission ae waveform recording system asperity failure is examined using the detailed spatio temporal distribution of ae hypocenters the initial phase of ae activity is also examined using high dynamic range waveforms our experimental results indicate that quasi static nucleation of the heterogeneous fault is associated with the failure of asperities on the fault plane the fracturing of an asperity is characterized by a dense spatial clustering of ae events and a changing b value b hereinafter which is manifest in three typical stages of failure as follows 1 foreshocks exhibiting a decrease in b 2 a period of mainshocks corresponding to a minimum in b and 3 aftershocks of increasing b the progressive fracture of several coupled asperities results in short term precursory fluctuations in both b and ae rate furthermore some ae events possess similar dynamic rupture features to those of earthquakes having an initial phase associated with the transition from quasi dynamic to dynamic rupture we conclude based on these experimental observations that fault rupture has hierarchical characteristics quasi static nucleation of fault rupture represents dynamic fracture of the asperities on the fault plane likewise a quasi static nucleation process characterized by dynamic microfracturing precedes the fracture of an asperity since dynamic motions are easier to detect remotely than static deformations understanding the hierarchical processes underlying fault rupture may thus be helpful for elucidating quasi static nucleation at larger scales in terms of the dynamic rupture of the asperities at smaller scales careful studies of asperity failure in the lab may guide future seismic studies of large asperities on natural faults potentially making it possible to recognize the final preparation stage before a large earthquake

discrete element modeling of subglacial sediment deformation 1 the discrete element method dem is used in this study to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to those at the bed of warm based glaciers complementary to analog experiments the numerical approach allows a detailed analysis of the material dynamics and the shear zone development during progressive shear strain the geometry of the heterogeneous stress network is visible in the form of force carrying grain bridges and adjacent volumetrically dominant inactive zones we demonstrate how the shear zone thickness and dilation depend on the level of normal overburden stress and we show how high normal stress can mobilize material to great depths the particle rotational axes tend to align with progressive shear strain with rotations both along and reverse to the shear direction the results from successive laboratory ring shear experiments on simple granular materials are compared to results from similar numerical experiments the simulated dem material and all tested laboratory materials deform by an elastoplastic rheology under the applied effective normal stress these results demonstrate that the dem is a viable alternative to continuum models for small scale analysis of sediment deformation it can be used to simulate the macromechanical behavior of simple granular sediments and it provides an opportunity to study how microstructures in subglacial sediments are formed during progressive shear strain

3d numerical simulations of fault gouge evolution during shear grain size reduction and strain localization abstract strain localization has important implications for the mechanical strength and stability of evolving fault zones structural fabrics interpreted as strain localization textures are common in natural and laboratory faults however the dynamic microscale processes controlling localization and delocalization are difficult to observe directly discrete numerical models of faulting allow a degree of dynamic visualization at the grain scale not easily afforded in nature when combined with laboratory validation experiments and field observations they become a powerful tool for investigating the dynamics of fault zone evolution we present a method that implements realistic gouge evolution in 3d simulations of granular shear the particle based model includes breakable bonds between individual particles allowing fracture of aggregate grains that are composed of many bonded particles during faulting simulations particle motions and interactions as well as the mechanical behavior of the entire system are continuously monitored we show that a model fault gouge initially characterized by mono disperse spherical aggregate grains gradually evolves with accumulated strain to a wide size distribution the comminution process yields a highly heterogeneous textural signature that is qualitatively comparable to natural and laboratory produced fault gouges mechanical behavior is comparable to a first order with relevant laboratory data simulations also reveal a strong correlation between regions of enhanced grain size reduction and localized strain thus in addition to producing realistic fault gouge textures the model offers the possibility to explore direct links between strain partitioning and structural development in fault zones this could permit investigation of subtle interactions between high and low strain regions that may trigger localization delocalization events and therefore control macroscopic frictional stability and hence the seismic potential of evolving fault zones

fracture surface energy of the punchbowl fault san andreas system the energetics of earthquake rupture slip weakening and rupture surface formation are poorly understood by measuring the energy associated with fracture damage along the punchbowl fault in the devil s punchbowl los angeles county park part of the san andreas system chester et al show that the fracture surface energy for rupture along a mature fault of this type accounts for only a small fraction of the energy released during large earthquakes as determined by seismologists this suggests that mechanisms such as melt lubrication and thermal pressurization are responsible for slip weakening fracture energy is a form of latent heat required to create an earthquake rupture surface and is related to parameters governing rupture propagation and processes of slip weakening1 2 3 fracture energy has been estimated from seismological and experimental rock deformation data4 5 6 7 8 yet its magnitude mechanisms of rupture surface formation and processes leading to slip weakening are not well defined8 9 10 here we quantify structural observations of the punchbowl fault a large displacement exhumed fault11 12 in the san andreas fault system and show that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake if fracture energy is attributed entirely to the production of fracture surfaces then all of the fracture surface area in the punchbowl fault could have been produced by earthquake displacements totalling 1 km but this would only account for a small fraction of the total energy budget and therefore additional processes probably contributed to slip weakening during earthquake rupture

rock fragmentation and particle size in crushed zones by faulting abstract the size distribution of fragments in the crushed zone adjacent to a fault and that of grains in a zone of very fine grained particles the fault gouge was determined for two different faults in central japan one fault has a wide ca 100 m zone of rock fragmentation between the fault gouge zone and the basement rock formation dimensions of the 50 largest rock fragments were measured in the field at locations within a continuous outcrop of this fault at some places bulk samples were also collected and the particle fragment and grain size distributions were determined in the laboratory the relationship between the logarithmic mean of the sizes of the largest 50 fragments and their distances from the fault suggests that size reduction of fragments is due to bulk crushing away from the fault and to surface grinding near the fault the other fault has no crushed zone of basement rocks but does have a 5 m wide zone of fault gouge ten samples of ca 200 g each and three bulk samples of ca

characteristic features of local breakdown near a crack tip in the transition zone from nucleation to unstable rupture during stick slip shear failure abstract characteristic features of local breakdown near a propagating crack tip during slip failure nucleation and its transition process to unstable rupture under mode ii conditions are experimentally studied using a rock sample with a simulated fault the size of which is large compared with the size of the breakdown zone in addition distinctive features in the nucleation zone are compared with those in the zone of steady dynamic rupture propagation it is found that the local breakdown stress drop the local dynamic stress drop the local stress increase immediately before the slip weakening process the dimensionless parameter s defined by das and aki and the shear fracture energy are functions of crack length in the transition zone from nucleation to unstable rupture i e they all increase with crack growth in the nucleation zone this is contrasted with the observations in the zone of steady dynamic rupture propagation where these physical parameters do not depend on crack length an increase in the magnitudes of these parameters with crack growth in the nucleation zone results in increasing resistance to crack extension with crack growth in the nucleation zone the reason for the increase in crack growth resistance in this zone is that a slip failure nucleus is formed at a point or zone where the local strength and the shear fracture energy are at a minimum this basic physical information can provide the key to earthquake prediction since the nucleation itself is a precursory phenomenon the ratio of the size of the breakdown zone to the crack length decreases abruptly beyond the critical crack length rupture velocity v accelerates with crack growth and reaches the shear wave velocity or super shear and v is found to be expressed empirically as a function of crack length in the nucleation zone a positive correlation is found between s and v in the nucleation zone v increases with increasing s however no such correlation was observed in the zone of steady dynamic rupture propagation where v seems to have reached the shear wave velocity of super shear for s 0 5 this is consistent with das and aki s numerical result the interrelationships between shear stress slip displacement slip velocity and acceleration near the crack tip during the breakdown in the nucleation zone are revealed and they are compared with those in the zone of dynamic rupture propagation the present experiments suggest that the relationship between shear stress and slip displacement is more fundamental during the breakdown process than the relationship between shear stress and slip velocity a model is presented to describe the breakdown process in the nucleation zone

application of open source photogrammetric software micmac for monitoring surface deformation in laboratory models quantifying deformation is essential in modern laboratory models of geological systems this paper presents a new laboratory monitoring method through the implementation of the open source software micmac which efficiently implements photogrammetry in structure from motion algorithms critical evaluation is provided using results from two example laboratory geodesy scenarios magma emplacement and strike slip faulting micmac automatically processes images from synchronized cameras to compute time series of digital elevation models dems and orthorectified images of model surfaces micmac also implements digital image correlation to produce high resolution displacements maps the resolution of dems and displacement maps corresponds to the pixel size of the processed images using 24 mp cameras the precision of dems and displacements is 0 05 mm on a 40 40 cm surface processing displacement maps with matlab scripts allows automatic fracture mapping on the monitored surfaces micmac also offers the possibility to integrate 3 d models of excavated structures with the corresponding surface deformation data the high resolution and high precision of micmac results and the ability to generate virtual 3 d models of complex structures make it a very promising tool for quantitative monitoring in laboratory models of geological systems

volcanic avalanche fault zone with pseudotachylite and gouge in french massif central abstract structures and textures with sedimentological variations at different scales of the lithofacies assemblage help us to constrain the basal kinematic transition from non depositional to depositional conditions during volcanic avalanche emplacement in the well exposed impact sheared contact along volcanic avalanche fault zone in the french massif central we observe how the granular textures of the pseudotachylite and fault gouge have recorded the propagation of shock wave with granular oscillatory stress sequential events of basal aggradation along avalanche fault zone have been established related to fractal d values temperature pressure regime and oscillatory stress during slow wave velocity a typical lithofacies assemblage with a reverse grading shows the pseudotachylite and fault gouge a cataclastic gradient is characterised by the fractal d values from 2 7 in jigsaw breccias with pseudotachylite partial melt to 2 6 in the polymodal gouge shock brecciation and comminution produce cataclastic shear bands in the pseudotachylite and quartz microstructures along the basal contact of the volcanic debris avalanche deposit gouge microstructures show granular segregation cataclasis with antithetic rotational riedel shear and an arching effect between the riedel shear bands x ray microtomography provided 3d microfabrics along the clastic vein in the sandy gouge from the available statistical dataset a few equations have been developed implicating the same cataclastic origin with a co genetic evolution of lithofacies an impact wave during primary shear propagation may contribute to produce hydroclastic matrix pseudotachylite partial melt and proximal gouge thixotropy with v 50m s and a t

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

from sub rayleigh to supershear ruptures during stick slip experiments on crustal rocks supershear earthquake ruptures propagate faster than the shear wave velocity although there is evidence that this occurs in nature it has not been experimentally demonstrated with the use of crustal rocks we performed stick slip experiments with westerly granite under controlled upper crustal stress conditions supershear ruptures systematically occur when the normal stress exceeds 43 megapascals mpa with resulting stress drops on the order of 3 to 25 mpa comparable to the stress drops inferred by seismology for crustal earthquakes in our experiments the sub rayleigh to supershear transition length is a few centimeters at most suggesting that the rupture of asperities along a fault may propagate locally at supershear velocities in turn these sudden accelerations and decelerations could play an important role in the generation of high frequency radiation and the overall rupture energy budget

discussion on palaeoseismic structures in quaternary sediments related to an assumed fault zone north of the permian peissen gnutz salt structure nw germany neotectonic activity and earthquakes from the saalian to the holocene grube 2019 abstract we discuss the significance of deformation structures in quaternary sediments observed by grube 2019 in the peissen quarries nw germany in light of the geological context evidence for polygonal patterns visible in aerial images in the study area shows that the wedge structures interpreted by grube 2019 as earthquake induced sand blows may rather correspond to thermal contraction cracks filled with aeolian sand in a permafrost environment in the study sites brittle deformations caused by i the rise of a salt diapir ii salt dissolution iii the development of pleistocene permafrost and iv possibly water circulation under pressure in the scandinavian ice sheet margin may have coexisted we support the idea that while the morphology of deformation generally makes it possible to determine the stress state to which the sediments have been subjected and the quantity of water available in the system at the time of deformation the nature of the factors causing the stresses remains difficult to identify in the end we highlight other useful criteria that should be privileged for palaeoseismic research in such complex geological settings

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

shear heating in granular layers heat flow measurements imply that the san andreas fault operates at lower shear stresses than generally predicted from laboratory friction data this suggests that a dramatic weakening effect or reduced heat production occur during dynamic slip numerical studies intimate that grain rolling or localization may cause weakening or reduced heating however laboratory evidence for these effects are sparse we directly measure frictional resistance m shear heating and microstructural evolution with accumulated strain in layers of quartz powder sheared at a range of effective stresses sn5 70 mpa and sliding velocities v0 01 10 mm s tests conducted at sn 25 mpa show strong evidence for shear localization due to intense grain fracture in contrast tests conducted at low effective stress sn 5 mpa show no preferential fabric development and minimal grain fracture hence we conclude that non destructive processes such as grain rolling sliding distributed throughout the layer dominate deformation temperature measured close to the fault increases systematically with sn and v consistent with a one dimensional heat flow solution for frictional heating in a finite width layer mechanical results indicate stable sliding m0 6 for all tests irrespective of deformation regime and show no evidence for reduced frictional resistance at rapid slip or high effective stresses our measurements verify that the heat production equation qmsnv holds regardless of localization state or fracture regime thus for quasistatic velocities v510 mm s and effective stresses relevant to earthquake rupture neither grain rolling sliding or shear localization appear to be a viable mechanism for the dramatic weakening or reduced heating required to explain the heat flow paradox

the micromechanics of friction in a granular layer a grain bridge model is used to provide a physical interpretation of the rate and state dependent friction parameters for the simple shear of a granular layer this model differs from the simpler asperity model in that it recognizes the difference between the fracture of a grain and the fracture of an adhesion between grains and it explicitly accounts for dilation in the granular layer the model provides an explanation for the observed differences in the friction of granular layers deformed between rough surfaces and those deformed between smooth surfaces and for the evolution of the friction parameters with displacement the observed evolution from velocity strengthening to velocity weakening with displacement is interpreted as being due to the change in the micromechanics of strain accommodation from grain crushing to slip between adjacent grains this change is associated with the observed evolution of a fractal grain structure

observed source parameters for dynamic rupture with non uniform initial stress and relatively high fracture energy we have conducted dynamic rupture propagation experiments to establish the relations between insource stress drop fracture energy and the resulting particle velocity during slip of an unconfined 2 m long laboratory fault at normal stresses between 4 and 8 mpa to produce high fracture energy in the source we use a rough fault that has a large slip weakening distance an artifact of the high fracture energy is that the nucleation zone is large such that precursory slip reduces fault strength over a large fraction of the total fault length prior to dynamic rupture making the initial stress non uniform shear stress particle velocity fault slip and acceleration were recorded coseismically at multiple locations along strike and at small fault normal distances stress drop increases weakly with normal stress average slip rate depends linearly on the fault strength loss and on static stress drop both with a nonzero intercept a minimum fracture energy of 1 8 j m 2 and a linear slip weakening distance of 33 mm are inferred from the intercept the large slip weakening distance also affects the average slip rate which is reduced by in source energy dissipation from on fault fracture energy because of the low normal stress and small per event slip w86 mm no thermal weakening such as melting or pore fluid pressurization occurs in these experiments despite the relatively high fracture energy and the very low heat production energy partitioning during these laboratory earthquakes is very similar to typical earthquake source properties the product of fracture energy and fault area is larger than the radiated energy seismic efficiency is low at w2 the ratio of apparent stress to static stress drop is w27 consistent with measured overshoot the fracture efficiency is w33 the static and dynamic stress drops when extrapolated to crustal stresses are 2e7 3 mpa and in the range of typical earthquake stress drops as the relatively high fracture energy reduces the slip velocities in these experiments the extrapolated average particle velocities for crustal stresses are 0 18e0 6 m s that these experiments are consistent with typical earthquake source properties suggests albeit indirectly that thermal weakening mechanisms such as thermal pressurization and melting which lead to near complete stress drops dominate earthquake source properties only for exceptional events unless crustal stresses are low published by elsevier ltd

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

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

scaling of earthquake rupture growth in the parkfield area self similar growth and suppression by the finite seismogenic layer 1 we propose a new framework on the scaling of earthquake rupture growth time history and we scale the moment rate and the cumulative moment functions of earthquakes over a wide magnitude range mw 1 7 6 0 in parkfield california the moment rate and the cumulative moment functions of the small and medium earthquakes mw 1 7 4 6 are derived by slip inversion analyses with the empirical green s function technique the moment rate functions of the investigated earthquakes except the mw 6 0 event are similar to each other increasing rapidly in the first half growth stage and decelerating in the latter half decline stage in the growth stage the cumulative moment functions are approximated by mo t nm 2 1017 t s 3 independent of the final size of the earthquakes the proportionality of the cumulative moment to the cube of time implies self similarity during earthquake rupture growth in the decline stage the cumulative moment function veers off the common rupture curve the mw 6 0 event also grows along the same rupture curve until 1 s after which the cumulative moment function is proportional to time from the onset itself this is because the finite seismogenic layer limits the vertical extent of dynamic rupture our method and results contribute to our understanding of earthquake source physics especially on earthquake rupture growth processes which may help to improve earthquake early warning techniques

earthquake source scaling relationships from 1 to 5 ml using seismograms recorded at 2 5 km depth the scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades studies of such events have shown a tendency for the constant stress drop self similarity of larger earthquakes to breakdown at small magnitudes and an apparent minimum source dimension of about 100 m has been observed other studies showed that this apparent breakdown in scaling could be an artifact of severe near surface attenuation limiting the spatial resolution of surface data in this study source parameters are determined for over 100 nearby tectonic earthquakes from recordings at a depth of 2 5 km in granite in the cajon pass scientific drill hole southern california comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the earth s crust source parameters are calculated by spectral modeling of three component p and s waves assuming four source models based on the brune 2 n 2 model in model l n 2 is fixed and q of p and s waves is determined to be 912 581 1433 and 1078 879 1323 respectively the numbers in parentheses are 1 standard deviation in model 2 qp qs 1000 is assumed and n is allowed to vary the 2 model is a good average for the data set but there is some real scatter supported by the data in model 3 qp qs 1000 is also assumed and 2 is constrained and in model 4 attenuation is ignored and n is allowed to vary source dimensions of less than 10m are observed for all four models 10 times smaller than the proposed minimum no breakdown in constant stress drop scaling is seen in the downhole data approximately ml 1 to 5 5 m0 109 1016 nm the ratio between radiated seismic energy estimated by integrating the velocity squared spectra with adequate signal bandwidth and seismic moment appears to decrease gradually with decreasing moment in the magnitude range 1 to 7 this is not incompatible with a constant stress drop but could result from errors in calculating energy the ratio of the s wave energy to that radiated by the p waves is about 14 after correction for attenuation this low value is consistent with the corner frequency shift of about 1 3 this corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled

extension fracture propagation in rocks with veins insight into the crack seal process using discrete element method modeling 1 we investigate how existing veins interact with extension fractures in rocks using 3 d discrete element method models with a geometry inspired by tension tests with notched samples in a sensitivity study we varied a the angle between the vein and the bulk extension direction and b the strength ratio between host rock and vein material results show a range of vein fracture interactions which fall into different robust structural styles veins which are weaker than the host rock tend to localize fracturing into the vein even at high misorientation angles veins which are stronger than the host rock cause deflection of the fracture tip along the vein host rock interface fractures are arrested at the interface from weak to stronger material when propagating from a stronger to a weaker material macroscopic bifurcation of the fracture is common complex interactions are favored by a low angle between the vein and the fracture and by high strength contrast the structural styles in the models show good agreement with microstructures and mesostructures of crack seal veins found in natural systems we propose that these structural styles form the basis for criteria to recognize strength contrasts and stress of crack seal systems in nature

experimental study of thermal field evolution in the short impending stage before earthquakes phenomena at critical points are vital for identifying the short impending stage prior to earthquakes the peak stress is a critical point when stress is converted from predominantly accumulation to predominantly release we call the duration between the peak stress and instability the meta instability stage which refers to the short impending stage of earthquakes the meta instability stage consists of a steady releasing quasi static stage and an accelerated releasing quasi dynamic stage the turning point of the above two stages is the remaining critical point to identify the two critical points in the field it is necessary to study the characteristic phenomena of various physical fields in the meta instability stage in the laboratory and the strain and displacement variations were studied considering that stress and relative displacement can be detected by thermal variations and peculiarities in the full field observations we employed a cooled thermal infrared imaging system to record thermal variations in the meta instability stage of stick slip events generated along a simulated precut planer strike slip fault in a granodiorite block on a horizontally bilateral servo controlled press machine the experimental results demonstrate the following 1 a large area of decreasing temperatures in wall rocks and increasing temperatures in sporadic sections of the fault indicate entrance into the meta instability stage 2 the rapid expansion of regions of increasing temperatures on the fault and the enhancement of temperature increase amplitude correspond to the turning point from the quasi static stage to the quasi dynamic stage our results reveal thermal indicators for the critical points prior to earthquakes that provide clues for identifying the short impending stage of earthquakes

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

rupture preparation process controlled by surface roughness on meter scale laboratory fault abstract we investigate the effect of fault surface roughness on rupture preparation characteristics using meter scale metagabbro specimens we repeatedly conducted the experiments with the same pair of rock specimens to make the fault surface rough we obtained three experimental results under the same experimental conditions 6 7 mpa of normal stress and 0 01 mm s of loading rate but at different roughness conditions smooth moderately roughened and heavily roughened during each experiment we observed many stick slip events preceded by precursory slow slip we investigated when and where slow slip initiated by using the strain gauge data processed by the kalman filter algorithm the observed rupture preparation processes on the smooth fault i e the first experiment among the three showed high repeatability of the spatiotemporal distributions of slow slip initiation local stress measurements revealed that slow slip initiated around the region where the ratio of shear to normal stress was the highest as expected from finite element method fem modeling however the exact location of slow slip initiation was where became locally minimum probably due to the frictional heterogeneity in the experiment on the moderately roughened fault some irregular events were observed though the basic characteristics of other regular events were similar to those on the smooth fault local stress data revealed that the spatiotemporal characteristics of slow slip initiation and the resulting drop for irregular events were different from those for regular ones even under similar stress conditions on the heavily roughened fault the location of slow slip initiation was not consistent with anymore because of the highly heterogeneous static friction on the fault which also decreased the repeatability of spatiotemporal distributions of slow slip initiation these results suggest that fault surface roughness strongly controls the rupture preparation process and generally increases its complexity with the degree of roughness

grain fracture in 3d numerical simulations of granular shear we present a new method to implement realistic grain fracture in 3d numerical simulations of granular shear we use a particle based model that includes breakable bonds between individual particles allowing the simulation of fracture of large aggregate grains during shear grain fracture simulations produce a comminuted granular material that is texturally comparable to natural and laboratory produced fault gouge our model is initially characterized by monodisperse large aggregate grains and gradually evolves toward a fractal distribution of grain sizes with accumulated strain comminution rate and survival of large grains is sensitive to applied normal stress the fractal dimension of the resultant grain size distributions 2 3 0 3 and 2 9 0 5 agree well with observations of natural gouges and theoretical results that predict a fractal dimension of 2 58 copyright 2005 by the american geophysical union

scaling of rock friction constitutive parameters the effects of surface roughness and cumulative offset on friction of gabbro we describe experiments in which large 14 40 cm nominal contact area blocks of gabbro were sheared in a direct shear apparatus at room temperature 5 mpa normal stress and slip velocities from 0 1 to 10 m s the apparatus was servocontrolled using a displacement feedback measurement made directly between the gabbro blocks two surface roughnesses were studied rough produced by sandblasting and smooth produced by lapping with 60 grit and accumulated displacements reached 60 mm measurements of surface topography were used to characterize roughness and asperity dimensions step changes in loading velocity were used to interrogate friction constitutive properties both rough and smooth surfaces showed appreciable displacement hardening the coefficient of friction for rough surfaces was about 0 45 for initial slip and 0 7 after sliding 50 mm smooth surfaces exhibited higher and a greater tendency for unstable slip the velocity dependence of frictiona b and the characteristic friction distanced c show systematic variations with accumulated displacement for rough surfacesa b started out positive and became negative after about 50 mm displacement andd c increased from 1 to 4 m over the same interval for smooth surfaces a b began negative and decreased slightly with displacement andd c was about 2 m independent of displacement for displacements 30 mm rough surfaces exhibit a second state variable with characteristic distance about 20 m the decrease ina b with displacement is associated with disappearance of the second state variable our data indicate thatd c is controlled by surface roughness in a complex way including but not limited to the effect of roughness on contact junction dimensions for bare rock surfaces the data show that simple descriptions of roughness such as rms and peak to trough are not sufficient to inferd c our observations are consistent with a model in whichd c scales with gouge thickness

runout of the socompa volcanic debris avalanche chile a mechanical explanation for low basal shear resistance we propose a mechanical explanation for the low basal shear resistance about 50 kpa previously used to simulate successfully the complex well documented deposit morphology and lithological distribution produced by emplacement of the 25 km3 socompa volcanic debris avalanche deposit chile stratigraphic evidence for intense basal comminution indicates the occurrence of dynamic rock fragmentation in the basal region of this large granular mass flow and we show that such fragmentation generates a basal shear stress retarding motion of the avalanche that is a function of the flow thickness and intact rock strength the topography of the socompa deposit is realistically simulated using this fragmentation derived resistance function basal fragmentation is also compatible with the evidence from the deposit that reflection of the avalanche from topography caused a secondary wave that interacted with the primary flow

large nucleation before large earthquakes is sometimes skipped due to cascade up implications from a rate and state simulation of faults with hierarchical asperities 1 does a large earthquake have a large quasi static preparation a hierarchical asperity model in which a large tough patch patch l contains smaller fragile patches patch s enables a large earthquake to start with only small preparation because of cascade up rupture growth we realized such a model in a rate and state framework by heterogeneous distributions of the state evolution distance and its consequences are investigated by earthquake sequence simulations focus is put on elementary processes the interaction between two scales one patch l containing one patch s is simulated with their size ratio as a parameter if patch s is larger than the nucleation size of patch l the system falls into a limit cycle consisting of only one earthquake that starts with patch s nucleation and grows dynamically to rupture entire patch l if patch s is considerably smaller than the nucleation size of patch l small earthquakes never dynamically cascade up and the large earthquakes are initiated by large quasi static nucleation in between large earthquakes start in various ways by large nucleation dynamic cascade up or delayed cascade up in the final stage of quasi static nucleation the preseismic moment release rate increases roughly inversely proportional to the time to failure tf with its amplitude depending on the nucleation size for a patch s rupture to cascade up strength in the adjacent region must have been reduced manifested by a higher creep velocity before the patch s nucleation starts following 1 tf acceleration large nucleation sometimes has a precursory small earthquake characterized by larger afterslip than nonprecursory ones

nature of stress accommodation in sheared granular material insights from 3d numerical modeling active faults often contain distinct accumulations of granular wear material during shear this granular material accommodates stress and strain in a heterogeneous manner that may influence fault stability we present new work to visualize the nature of contact force distributions during 3d granular shear our 3d discrete numerical models consist of granular layers subjected to normal loading and direct shear where gouge particles are simulated by individual spheres interacting at points of contact according to simple laws during shear we observe the transient microscopic processes and resulting macroscopic mechanical behavior that emerge from interactions of thousands of particles we track particle translations and contact forces to determine the nature of internal stress accommodation with accumulated slip for different initial configurations we view model outputs using novel 3d visualization techniques our results highlight the prevalence of transient directed contact force networks that preferentially transmit enhanced stresses across our granular layers we demonstrate that particle size distribution psd controls the nature of the force networks models having a narrow i e relatively uniform psd exhibit discrete pipe like force clusters with a dominant and focussed orientation oblique to but in the plane of shear wider psd models e g power law size distributions d 2 6 also show a directed contact force network oblique to shear but enjoy a wider range of orientations and show more out of plane linkages perpendicular to shear macroscopic friction level is insensitive to these distinct force network morphologies however force network evolution appears to be linked to fluctuations in macroscopic friction our results are consistent with predictions based on recent laboratory observations that force network morphologies are sensitive to grain characteristics such as particle size distribution of a sheared granular layer our numerical approach offers the potential to investigate correlations between contact force geometry evolution and resulting macroscopic friction thus allowing us to explore ideas that heterogeneous force distributions in gouge material may exert an important control on fault stability and hence the seismic potential of active faults 2007 elsevier b v all rights reserved

evidence for aseismic deformation rate changes prior to earthquakes abstractfor ten earthquakes in nonmagmatic settings there are credible published accounts of pre earthquake deformation rate changes lasting hundreds of seconds to more than a decade although most m 7 5 earthquakes have occurred without detectable pre earthquake deformation the detection threshold for aseismic deformation remains high in that aseismic slip with moment equivalent to an m5 earthquake would in most although not all cases have been missed interseismic deformation rates vary without being followed by earthquakes and a strain rate change prior to the 1989 mw6 9 loma prieta california earthquake is shown to be similar in size to many other rate changes that have occurred since that time most examples of pre earthquake aseismic deformation lasting hundreds of seconds or more probably originate adjacent to or downdip of the seismic rupture plane rather than within the zone that undergoes seismic failure

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

effects of physical fault properties on frictional instabilities produced on simulated faults laboratory studies of large scale simulated faults show that physical properties of the fault specifically normal stress and fault roughness significantly influence the unstable shear failure behavior of the fault in addition the experiments provide insights into important length scaling effects that are useful for assessing concepts such as critical crack length or rupture nucleation dimension stick slip shear failures have been generated along a 2 m long simulated fault in a block of sierra white granite experiments were conducted at normal stresses between 0 6 and 4 mpa with two different prepared roughnesses of the simulated fault a smooth fault with a profilometer measured roughness of 0 2 m and a rough fault with a measured roughness of 80 m high speed records of shear strains and slip velocities clearly show both the propagation of the stick slip failures and details of the local deformations and motions from which slip weakening of the fault at the onset of sliding can be resolved both the dynamic stress drops of the stick slip events and the apparent fracture energies of the events determined from the records of the fault slip weakening increase with increasing normal stress although critical slip weakening displacements are insensitive to normal stress they increase with increased fault roughness such that the apparent fracture energies are also larger on the rough fault than on the smooth fault slip velocities in these experiments are influenced by roughness and are proportional to stress drop although the onset of slip is sharply defined the termination of slip is more gradual without a clear stopping phase measured rupture velocities on the rough fault are lower than those on the smooth fault when a length scale based on rupture nucleation dimensions directly proportional to critical displacements is introduced the data suggest that at fault lengths close to the nucleation lengths rupture velocity and slip velocity increase with increasing fault length

observations of premonitory acoustic emission and slip nucleation during a stick slip experiment in smooth faulted westerly granite 1 to investigate laboratory earthquakes stick slip events were induced on a saw cut westerly granite sample by triaxial loading at 150 mpa confining pressure acoustic emissions ae were monitored using an innovative continuous waveform recorder the first motion of each stick slip was recorded as a large amplitude ae signal these events source locate onto the saw cut fault plane implying that they represent the nucleation sites of the dynamic failure stick slip events the precise location of nucleation varied between events and was probably controlled by heterogeneity of stress or surface conditions on the fault the initial nucleation diameter of each dynamic instability was inferred to be less than 3 mm a small number of ae were recorded prior to each macro slip event for the second and third slip events premonitory ae source mechanisms mimic the large scale fault plane geometry

the evolution of crack seal vein and fracture networks in an evolving stress field insights from discrete element models of fracture sealing veins are ubiquitous in upper and middle crustal rocks due to strength and stiffness contrast to the host rock veins can influence crack propagation here we present discrete element models to investigate crack vein interactions by simulating cycles of fracturing of a rock mass sealing the cracks to form veins and refracturing the rock mass after rotating the stress field we observe different styles of interaction between new fractures and existing veins depending on the strength ratio between vein and host rock and on the changes in the stress field between the different deformation stages if the orientation of stress field does not change between deformation stages ataxial crack seal veins are produced if the veins are weak and a bundle of subparallel microveins if the veins are strong if the stress field is rotated between deformation stages the interactions include reactivation fracture deflection and crosscutting reactivation of weak veins occurs even if the vein orientation is highly unfavorable relative to the stress field relays of fractures between reactivated veins form at a higher angle to the veins than expected this demonstrates that the orientation of secondary veins does not reflect the regional stress field in a simple manner and that veins can strongly influence fracture connectivity with implications for paleostress analysis and basin modeling simulation results compare well with field examples of multiphase vein networks in carbonates from jebel akhdar oman

properties of the seismic nucleation phase abstract near source observations show that earthquakes begin abruptly at the p wave arrival but that this beginning is weak with a low moment rate relative to the rest of the main shock we term this initial phase of low moment rate the seismic nucleation phase we have observed the seismic nucleation phase for a set of 48 earthquakes ranging in magnitude from 1 1 8 1 the size and duration of the seismic nucleation phase scale with the total seismic moment of the earthquake suggesting that the process responsible for the seismic nucleation phase carries information about the eventual size of the earthquake the seismic nucleation phase is characteristically followed by quadratic growth in the moment rate consistent with self similar rupture at constant stress drop in this paper we quantify the properties of the seismic nucleation phase and offer several possible explanations for it

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

simulation of the frictional stick slip instability a lattice solid model capable of simulating rock friction fracture and the associated seismic wave radiation is developed in order to study the origin of the stick slip instability that is responsible for earthquakes the model consists of a lattice of interacting particles in order to study the effect of surface roughness on the frictional behavior of elastic blocks being rubbed past one another the simplest possible particle interactions were specified corresponding to radially dependent elastic brittle bonds the model material can therefore be considered as round elastic grains with negligible friction between their surfaces although breaking of the bonds can occur fracturing energy is not considered stick slip behavior is observed in a numerical experiment involving 2d blocks with rough surfaces being rubbed past one another at a constant rate slip is initiated when two interlocking asperities push past one another exciting a slip pulse the pulse fronts propagate with speeds ranging from the rayleigh wave speed up to a value between the shear and compressional wave speeds in agreement with field observations and theoretical analyses of mode ii rupture slip rates are comparable to seismic rates in the initial part of one slip pulse whose front propagates at the rayleigh wave speed however the slip rate is an order of magnitude higher in the main part of pulses possibly because of the simplified model description that neglected intrinsic friction and the high rates at which the blocks were driven or alternatively uncertainty in slip rates obtained through the inversion of seismograms particle trajectories during slip have motions normal to the fault indicating that the fault surfaces jump apart during the passage of the slip pulse normal motion is expected as the asperities on the two surfaces ride over one another the form of the particle trajectories is similar to those observed in stick slip experiments involving foam rubber blocks bruneet al 1993 additional work is required to determine whether the slip pulses relate to the interface waves proposed by brune and co workers to explain the heat flow paradox and whether they are capable of inducing a significant local reduction in the normal stress it is hoped that the progressive development of the lattice solid model will lead to realistic simulations of earthquake dynamics and ultimately provide clues as to whether or not earthquakes are predictable

particle mechanics model for the effects of shear on solute retardation coefficient in rock fractures abstract damage on rock fracture surfaces during shear process changes the mechanical and hydrological properties of the fractures and therefore affects solute migration in fractured rocks laboratory experiments on this issue are rarely reported in the literature due to technical difficulties in measuring asperity damage and gouge generation to conceptually investigate the effects of rock fracture surface damage on solute sorption during shear we present a retardation coefficient model considering the effect of wear the particle mechanics model was employed to investigate the effects of gouge generation abrasive wear and microcrack development in the damaged zones on the solute retardation coefficient in rock fractures the results from demonstration examples show that the shear process significantly increases the retardation coefficients by offering more sorption surfaces in the factures due to gouge generation wear microcracking and crushing of gouge particles conceptually three damage zones are classified to characterize the various wear impacts on the solute transport in single fractures outstanding issues of the present model and suggestions for future study are also presented

time dependent compaction as a mechanism for regular stick slips owing to their destructive potential earthquakes receive considerable attention from laboratory studies in friction experiments stick slips are studied as the laboratory equivalent of natural earthquakes and numerous attempts have been made to simulate stick slips numerically using the discrete element method dem however while laboratory stick slips commonly exhibit regular stress drops and recurrence times stick slips generated in dem simulations are highly irregular this discrepancy highlights a gap in our understanding of stick slip mechanics which propagates into our understanding of earthquakes in this work we show that regular stick slips emerge in dem when time dependent compaction by pressure solution is considered we further show that the stress drop and recurrence time of stick slips is directly controlled by the kinetics of pressure solution since compaction is known to operate in faults this mechanism for frictional instabilities directly relates to natural seismicity

mechanical origin of power law scaling in fault zone rock 1 a nearest neighbor fragmentation model previously developed to explain observations of power law particle distributions in 3d with mass dimension d 3 2 6 d 2 2 6 in 2d section in low strain fault gouge and breccia is extended to the case of large strains to explain recent observations of d 3 3 0 d 2 2 0 in 2d section in the highly strained cores of many exhumed fault zones at low strains the elimination of same sized nearest neighbors has been shown to produce a power law distribution which is characterized by a mass dimension near d 3 2 6 with increasing shear strain these isolated same size neighbors can collide in which case one of them fractures the probability of two same size neighbors colliding and fragmenting in a simple shear flow is a function of the size and density of the two particles only for a power law distribution with d 3 3 0 is this collision probability independent of the size of the particles citation sammis

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

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

variability of earthquake nucleation in continuum models of rate and state faults and implications for aftershock rates using two continuum models of rate and state faults one with a weaker patch and the other with rheological transition from steady state velocity weakening to velocity strengthening friction we simulate several scenarios of spontaneous earthquake nucleation plausible for natural faults investigate their response to static shear stress steps and infer the corresponding aftershock rates overall nucleation processes at weaker patches behave similarly to theories based on spring slider models with some notable deviations in particular nucleation and aftershock rates are affected by normal stress heterogeneity in the nucleation zone nucleation processes at rheological transitions behave differently producing complex slip velocity histories nonmonotonic responses to static stress changes and aftershock rates with pronounced peaks and seismic quiescence for such processes positive stress steps sometimes delay nucleation of seismic events by inducing aseismic transients that relieve stress and postpone seismic slip superposition of the complex aftershock response for spatially variable stress changes results in omori s law for a period of time followed by seismic quiescence such behavior was observed at the base of the seismogenic zone near the 1984 morgan hill earthquake we show that the computed aftershock rates are linked to unperturbed slip velocity evolution in the nucleation zone and construct simplified analytical scenarios that explain some features of the response the qualitative differences that we find between the two nucleation models indicate that aftershock response of rate and state faults to static stress changes would depend on the conditions under which nucleation occurs on natural faults and may be different from predictions based on spring slider models

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

slow and fast ruptures on a laboratory fault controlled by loading characteristics recent geodetic observations indicate that a single fault may slip slowly and silently or fast and seismically in different circumstances we report laboratory experiments that demonstrate how the mode of faulting can alternate between fast stick slip events 70 to 500 mm s sliding rates and slow silent events 0 001 to 30 mm s as a result of loading conditions rather than friction properties or stiffness of the loading machine the 760 mm long granite sample is close to the critical nucleation length scale for unstable slip so small variations in nucleation properties result in measurable differences in slip events slow events occur when instability cannot fully nucleate before reaching the sample ends dynamic events occur after long healing times or abrupt increases in loading rate which suggests that these factors shrink the spatial and temporal extents of the nucleation zone arrays of slip strain and ground motion sensors installed on the sample allow us to quantify seismic coupling and study details of premonitory slip and afterslip we find that seismic coupling decreases when slip rates fall below about 70 mm s the slow slip events we observe are primarily aseismic less than 1 of the seismic coupling of faster events and produce swarms of very small m 6 to m 8 events these mechanical and seismic interactions suggest that faults with transitional behavior where creep small earthquakes and tremor are often observed could become seismically coupled if loaded rapidly either by a slow slip front or dynamic rupture of an earthquake that nucleated elsewhere

the relationship between particle characteristics and frictional strength in basal fault breccias implications for fault rock evolution and rockslide susceptibility abstract microstructural analysis of fault rock breccias along the basal shear zone bsz of potential rock slope failures in norway demonstrates that particle size distribution curves show a spectrum from particle to matrix supported we demonstrate that variation in residual shear strength from 450 to 700 kpa is directly related to particle size and shape characteristics we also show that these characteristics are dependent on the fault rock evolution and breakdown mechanism and demonstrate this with simulated examples particle supported fault rocks have higher residual strength than matrix supported samples as the amount of fine material increases the residual shear strength of the fault material decreases particle aspect ratio is variable suggesting different comminution mechanisms and fault zone evolution as particle size decreases below 10 20 m particle aspect ratio approaches 1 1 suggesting that as the fault rock evolves the particles become rounder as aspect ratio increases fault strength decreases we contend that the susceptibility of potential rock slope failures is fundamentally linked to and depends partly on the microscopic properties of the underlying fault rocks and is linked to their comminution history

numerical simulation of earthquake faults with gouge toward a comprehensive explanation for the heat flow paradox the particle based lattice solid model is used to simulate transform faults with and without fault gouge stick slip frictional behavior is observed in two dimensional numerical experiments of model faults both with and without gouge when no gouge is present the fault is strong and the heat generation and stress drops are correspondingly high in disaccord with observations surrounding the heat flow paradox in contrast when a gouge is specified the fault is weak and the heat generation as well as stress drops are low in quantitative agreement with observational constraints the heat flow is low on average and during short periods of aseismic creep seismic efficiencies are compatible with observationally based bounds counter intuitively the fault strength decreases as the intrinsic friction between particles is increased beyond a given threshold the mechanism for low fault strength and heat is rolling and jostling of fault gouge grains during slip this allows macroscopic movement of the fault with only minimal slip between surfaces of the gouge grains as this dynamical mechanism operates during seismic and aseismic slip it provides an explanation for the lack of a heat flow anomaly in both the seismic and creeping parts of the san andreas fault the simulation results provide the first comprehensive and quantitative possible explanation of the heat flow paradox and suggest that fault gouge plays a fundamental role on the dynamics of earthquake faults whether rolling and jostling of fault gouge particles provides the explanation for the heat flow paradox in nature remains to be validated by observation evidence

origin and behavior of clay minerals in the bogd fault gouge mongolia we analyzed twelve fault gouge samples from the bogd fault in south western mongolia to understand the origin and behavior of clay minerals the investigation relies on x ray powder diffraction xrd scanning electron microscopy sem transmission electron microscopy tem and high energy synchrotron x ray diffraction methods to investigate microstructure and preferred orientation smectite montmorillonite illite smectite mixed layers illite mica and kaolinite are the major clay components in addition to quartz and feldspars which are present in all samples the observations suggest that the protoliths and the fault rocks were highly altered by fluids the fluid rock interactions allow clay minerals to form as well as alter feldspars to precipitate kaolinite and montmorillonite thus newly formed clay minerals are heterogeneously distributed in the fault zone the decrease of montmorillonite component of some of the highly deformed samples also suggests that dehydration processes during deformation were leading to illite precipitation based on synchrotron x ray diffraction data the degree of preferred orientation of constituent clay minerals is weak with maxima for 001 ranging from 1 3 to 2 6 multiples of a random distribution m r d co existing quartz and feldspars have random orientation distributions microstructure and texture observations of the gouges from the foliated microscopic zone alternating with micrometric isotropic clay rich area also indicate that the bogd fault experienced brittle and ductile deformation episodes the clay minerals may contribute to a slip weakening behavior of the fault

3 d numerical modelling of the influence of pre existing faults and boundary conditions on the distribution of deformation example of north western ghana abstract high strain zones bound and separate the high grade tectono metamorphic terranes from low grade greenstone belts in the north western parts of ghana these belts are bounded by granitoid domains characterized by two main episodic pulses of magmatic intrusive events which occurred between 2213 ma and 2086 ma high strain zones are thought to play a significant role in creating fluid pathways particularly for partially molten material from lower crustal sources to the upper crust in this study a three dimensional thermo mechanical model has been used to explore the evolution of high strain zones and relief under compressional and simple shear boundary conditions different orientations of a system of branched strike slip faults were tested the effects of the frictional angle and density contrast on the evolution of relief were also compared in this study the resulting model indicates domains of tensile vs compressional strain as well as shear zones this shows that the internal fault zones as well as the host rock in between the faults behave as relatively weaker domains than the external regions under both applied compressive and simple shear boundary conditions these weakened domains constitute preferential zones of tensile and shear strain accommodations in the upper crust which may favour infilling by deeper partially molten rocks this processes is suggested by the authors as the most likely processes to have occurred in pre existing branched shear zones systems in north western ghana during the eburnean orogeny around 2 20 2 10 ga the orientations of faults in these models play an important role in controlling the evolution of relief and localized deformation in particular greatest the largest relief is obtained when faults dip parallel to each other and when they are inclined at depth as they thus facilitate strain rotation and material transfer from depth the host rock density does not play a primary role in producing relief compared to variations in friction angle at crustal scale of our model relief increases by 200 300 m when the host rock density is increased by 200 kg m3 whereas relief reduces by about 1200 m when decreasing the host rock friction from 20 to 10 this study suggests a model for interpreting the evolution and locus of exhumation of partially molten rocks in north western ghana

three dimensional boundary integral modeling of spontaneous earthquake sequences and aseismic slip 1 fault processes involve complex patterns of seismic events and aseismic slip this work develops a three dimensional 3 d methodology for simulating long term history of spontaneous seismic and aseismic slip on a vertical planar strike slip fault subjected to slow tectonic loading our approach reproduces all stages of earthquake cycles from accelerating slip before dynamic instability to rapid dynamic propagation of earthquake rupture to postseismic slip and to interseismic creep including aseismic transients we use the developed 3 d methodology to study interaction of fault slip with a small patch of higher normal stress over long term slip history for uniform initial prestress dynamic rupture is significantly affected by the stronger patch in the first simulated event but not in subsequent ones the change in behavior is due to redistribution of shear stress by prior slip which demonstrates that distributions of fault strength and stress are related and illustrates the importance of simulating long slip histories even in studies of dynamic rupture despite no long term effect on dynamic rupture the small patch of higher normal stress influences nucleation processes and hence long term slip patterns in the model comparison of the fully dynamic simulations and a widely used quasi dynamic approach shows that the quasi dynamic approach modifies long term slip patterns in addition to resulting in much smaller slip velocities and rupture speeds during dynamic events we show that the response of quasi dynamic formulations with reduced radiation damping terms can be scaled to match the results of the standard quasi dynamic formulation and hence cannot improve the comparison