modeling the hydromechanical responses of sandwich structure faults during underground fluid injection injection or withdrawal of fluid at depth may trigger felt seismicity at a site with natural faults or artificial fracturing cracks understanding how to avoid triggering felt earthquakes plays a crucial role in the success of underground anthropogenic activities such as subsurface energy storage and co2 geological sequestration with the application of a hydromechanical coupling finite element model we examine the potential of injection induced pore pressure change and the responding fault slippage along the fault surface in two injection scenarios multi layer injection mli and single layer injection sli the results indicate that fault slippage buildup is mainly located along the portion of the fault intersecting the reservoir mli leads to a relatively lower level of induced seismicity compared with sli in the fault zone the external zone and core of the fault exert a connection effect and barrier effect on the hydraulic connectivity respectively after 20 years of injection the pore pressure in the core presents a smaller change than that in the internal and external zones furthermore an uncertainty analysis was performed to evaluate the statistical significance of permeability in the caprock reservoir and fault core internal zone and external zone and only the permeability in the reservoir significantly affects the level of fault slippage based on the fault zone hydromechanics associated with subsurface energy storage and co2 geological sequestration

level te9c of sima del elefante sierra de atapuerca spain a comprehensive approach level te9c of the sima del elefante site sierra de atapuerca spain is one of the oldest sites with evidence of human occupation in western europe we began excavating level te9c in 2003 and the work there continues today the studies of the archaeology palaeontology and geology from this locality have provided an indispensable dataset with which to capture a picture in the scenario of the origin of humans in europe based on these data we raise and discuss several topics such as the possible origin of the lineage of the first hominins that inhabited western europe their capacity to have active hunting or scavenging abilities whether their subsistence strategies were successful and what the environment and habitats where these hominin groups settled was like the aim of this paper is to present the results and discussions obtained from the research team and to establish the primary features of early human occupations in southwestern europe tentatively we may conclude based on the events recorded at te9c that the first humans were in the iberian peninsula at around 1 2 ma they used the caves of the sierra de atapuerca as shelters probably during their hunting activities the cavities were surrounded by mediterranean forest rivers and water ponds and varied habitats as suggested by the rich and diverse assemblage of fossils of vertebrates fish amphibians and reptiles birds large and small mammals where humans possibly caught what they found in the surroundings

seismicity triggered by fluid injection induced aseismic slip anthropogenic fluid injections are known to induce earthquakes the mechanisms involved are poorly understood and our ability to assess the seismic hazard associated with geothermal energy or unconventional hydrocarbon production remains limited we directly measure fault slip and seismicity induced by fluid injection into a natural fault we observe highly dilatant and slow 4 micrometers per second m s aseismic slip associated with a 20 fold increase of permeability which transitions to faster slip 10 m s associated with reduced dilatancy and micro earthquakes most aseismic slip occurs within the fluid pressurized zone and obeys a rate strengthening friction law 0 67 0 045ln v v with v 0 1 m s fluid injection primarily triggers aseismic slip in this experiment with micro earthquakes being an indirect effect mediated by aseismic creep

triggered earthquakes and deep well activities earthquakes can be triggered by any significant perturbation of the hydrologic regime in areas where potentially active faults are already close to failure the increased pore pressure resulting from fluid injection or alternatively the massive extraction of fluid or gas can induce sufficient stress and or strain changes that with time can lead to sudden catastrophic failure in a major earthquake injection induced earthquakes typically result from the reduction in frictional strength along preexisting nearby faults caused by the increased formation fluid pressure earthquakes associated with production appear to respond to more complex mechanisms of subsidence crustal unloading and poroelastic changes in response to applied strains induced by the massive withdrawal of subsurface material as each of these different types of triggered events can occur up to several years after well activities have begun or even several years after all well activities have stopped this suggests that the actual triggering process may be a very complex combination of effects particularly if both fluid extraction and injection have taken place locally to date more than thirty cases of earthquakes triggered by well activities can be documented throughout the united states and canada based on these case histories it is evident that owing to preexisting stress conditions in the upper crust certain areas tend to have higher probabilities of exhibiting such induced seismicity

frictional properties of sedimentary rocks and natural fault gouge from the longmen shan fault zone sichuan china in this paper we report friction experiments performed on samples collected from the region hit by the 2008 wenchuan earthquake in the longmen shan fault zone lfz of sichuan southwestern china the materials tested consisted of simulated gouges prepared from intact clay rich mudstone and sandstone a calcite limestone plus a natural fault gouge from a trenched surface rupture cutting the mudstone and sandstone the clay rich samples including the natural gouge were dominated by illite and quartz in our experiments we sheared 1 mm thick gouge layers between saw cut driver blocks using a triaxial testing machine at conditions corresponding to 2 km depth in the lfz temperature was varied from 25 c to 150 c and to investigate the velocity dependence of friction we stepped the shear displacement rate between 1 22 and 0 122 m s our results show that the natural gouge was more illite rich and much weaker than the protolith mudstone and sandstone and showed a steady state friction coefficient of 0 4 compared with 0 6 for the latter the limestone gouge displayed values of 0 6 0 7 all samples except the limestone showed stable velocity strengthening slip the limestone showed velocity strengthening at 25 c 50 c but quasi static oscillations at 100 c 150 c along with velocity weakening behavior at 150 c we apply our results to discuss the role of the sedimentary rocks studied during events such as the wenchuan earthquake and argue that the clay rich sediments of the region may have a damping effect upon ruptures propagating from depth whereas the limestone may accelerate propagation producing significant stress drops

isrm suggested methods for rock stress estimation part 3 hydraulic fracturing hf and or hydraulic testing of pre existing fractures htpf contents1 introduction 10112 summaryoftestmethods 10123 assumptionsandlimitations 10123 1 hydraulicfracturing hf 10123 2 hydraulictestsonpre existingfractures htpf 10124 apparatus 10134 1 fractureorientationdetectiondevices 10145 personnelquali cation 10146 equipmentperformance 10147 hydraulicfracturingtestprocedure 10148 hydraulic fracturedelineation 10168 1 impressionpackertechnique 10168 2 geophysicalimagingtechnique 10179 obtainingcriticalpressureparametersfromthepressure timerecords 101710 stresscalculations 101810 1 hydraulicfracturing hf 101810 2 hydraulictestsonpre existingfractures 101911 reportingofresults 1019references 1020

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

unloading induced rock fracture activation and maximum seismic moment prediction abstract hundreds of anthropogenic earthquakes have recently occurred worldwide due to underground space creation and energy extraction the mechanism behind the human induced geohazards is most likely associated with the reduction of normal stress on pre existing fractures and faults this study reports a series of laboratory experiments to investigate the mechanism of unloading induced fracture activation and proposes a simple approach to predict the maximum seismic moment for a critically stressed fracture the unloading driven shear test results exhibit that the unloading process induces the stress states of the sawcut and natural fractures to approach the mohr coulomb failure envelope and the normal stress unloading rate influences the peak slip rate the fracture instability is dependent on the relationship between the stiffness of the system and the slip weakening rate of the fracture and the shear dilation mainly occurs after the fracture activation the test results also show that the critical shear stress of the sawcut fracture during the unloading driven shear test is approximately equal to the residual shear strength after the displacement driven fracture slip this relationship inspires us to develop a new approach to estimate the maximum seismic moment our data demonstrate that the maximum seismic moments for both the fractures obtained from the unloading driven shear tests are all below the upper limit lines indicating that the proposed approach is reasonable the uncertainty analysis shows that the accurate estimation of fault size can improve the maximum seismic moment prediction

postseismic relaxation driven by brittle creep a possible mechanism to reconcile geodetic measurements and the decay rate of aftershocks application to the chi chi earthquake taiwan we evaluate the effect of coseismic stress changes on the fault slip at midcrustal depth assuming a velocity strengthening brittle creep rheology we show that this model can help reconcile the time evolution of afterslip as measured from geodesy with aftershocks decay we propose an analytical expression for slip of the brittle creeping fault zone bcfz that applies to any dynamic or static stress perturbation including shear stress and normal stress changes the model predicts an initial logarithmic increase of slip with time postseismic slip rate decays over a characteristic time t r a that does not depend on the amplitude of the stress perturbation and it asymptotically joins the long term creep imposed by interseismic stress buildup given that the seismicity rate might be considered proportional to the sliding velocity of the bcfz the model predicts a decay rate of aftershocks that follows omori s law with a mathematical formalism identical to that of dieterich 1994 although based on a different mechanical rationale our model also differs from dieterich s model in that it requires that aftershock sequences and deep afterslip as constrained from geodetic measurements should follow the same temporal evolution we test this for the 1999 chi chi earthquake m w 7 6 and find that both sets of data are consistent with a model of afterslip due to the response of the bcfz the inferred relaxation time t r 8 5 years requires a value for a log v being the coefficient of friction in the range between 1 3 10 3 and 10 2

fault s hydraulic diffusivity enhancement during injection induced fault reactivation application of pore pressure diffusion inversions to laboratory injection experiments in situ observations of fluid induced fault slip reactivation as well as the analysis of induced seismicity have demonstrated the complexity of fluid fault interactions under geological conditions if fluid flow commonly reactivates faults in the form of aseismic slip or earthquakes the resulting shear deformation causes strong modifications of the hydraulic properties in this context the relationship between slip front and fluid front on deep faults remains not fully understood in this study we investigate shear induced fluid flow and hydraulic diffusivity enhancement during fracture shearing in the laboratory we use a series of injection reactivation tests conducted under triaxial conditions at different confining pressures 30 60 and 95 mpa the evolution of the fluid pressure along the saw cut andesite rock sample was monitored by two pressure sensors at two opposite locations of the experimental fault we estimate the history of the effective hydraulic diffusivity and its associated uncertainties governing the experimental fault using the pressure history at two points on the fault for this we develop a deterministic and a probabilistic inversion procedure which is able to reproduce the experimental data for a wide time range of the different experiments in this study the hydraulic diffusivity increases by one order of magnitude through the injection experiment hydraulic diffusivity changes are mainly governed by the reduction of the effective normal stress acting on the fault plane with a second order effect of the shear slip

evidence for coupled seismic and aseismic fault slip during water injection in the geothermal site of soultz france and implications for seismogenic transients summary we analysed the triggered seismicity recorded near 3 km in depth within granite during the 1993 water injection experiment at soultz france we selected all large multiplets associated with the largest dislocated fault identified on the borehole logging 4 3 cm slip and showed that each one consisted of the repeated rupture of a single asperity these asperities were forced to rupture due to fault creep around them so that their growing cumulative slip history reveals the creep history on the fault with a mean slip rate and a seismicity rate both decaying approximately as 1 t this is consistent with a rate strengthening friction law on the creeping faults and differs from the interpretation of omori type 1 t seismicity rates in terms of a weakening friction control of the unstable fault surfaces this model of asperity rupture forced by relaxation creep is easily generalized in seismogenic regions even for single ruptures of asperities on small creeping faults showing that a significant part of aftershock rates can be controlled by the strengthening friction properties and the creep of faults at all scales this lead us to introduce the concept of a critical asperity density above which dynamic interaction between neighbouring asperities can initiate large seismic ruptures at soultz the asperity density is subcritical but the neighbouring asperities nonetheless interact at distances up to a few source dimensions as revealed by their delayed cross triggering considering the seismic cloud in soultz at a global scale the strain produced by the injection experiment was mostly aseismic related to creep on major faults and causing large permeability changes influencing in turn the pore pressure diffusion and the creeping process our observations demonstrate that the identification and analysis of multiplets can provide accurate images of the geometry and kinematics of transient slip patches and indirectly detect pore pressure changes which should contribute to a better understanding of the coupling between transient fluid flow creep and microseismic activity in seismogenic regions

stable creeping fault segments can become destructive as a result of dynamic weakening faults in earth s crust accommodate slow relative motion between tectonic plates through either similarly slow slip or fast seismic wave producing rupture events perceived as earthquakes these types of behaviour are often assumed to be separated in space and to occur on two different types of fault segment one with stable rate strengthening friction and the other with rate weakening friction that leads to stick slip the 2011 tohoku oki earthquake with moment magnitude m w 9 0 challenged such assumptions by accumulating its largest seismic slip in the area that had been assumed to be creeping here we propose a model in which stable rate strengthening behaviour at low slip rates is combined with coseismic weakening due to rapid shear heating of pore fluids allowing unstable slip to occur in segments that can creep between events the model parameters are based on laboratory measurements on samples from the fault of the m w 7 6 1999 chi chi earthquake the long term slip behaviour of the model which we examine using a unique numerical approach that includes all wave effects reproduces and explains a number of both long term and coseismic observations some of them seemingly contradictory about the faults at which the tohoku oki and chi chi earthquakes occurred including there being more high frequency radiation from areas of lower slip the largest seismic slip in the tohoku oki earthquake having occurred in a potentially creeping segment the overall pattern of previous events in the area and the complexity of the tohoku oki rupture the implication that earthquake rupture may break through large portions of creeping segments which are at present considered to be barriers requires a re evaluation of seismic hazard in many areas

laboratory micro seismic signature of shear faulting and fault slip in shale this article reports the results of a triaxial deformation experiment conducted on a transversely isotropic shale specimen this specimen was instrumented with ultrasonic transducers to monitor the evolution of the micro seismic activity induced by shear faulting triaxial failure and subsequent fault slip at two different rates the strain data demonstrate the anisotropy of the mechanical quasi static compliance of the shale the p wave velocity data demonstrate the anisotropy of the elastic dynamic compliance of the shale the spatio temporal evolution of the micro seismic activity suggests the development of two distinct but overlapping shear faults a feature similar to relay ramps observed in large scale structural geology the shear faulting of the shale specimen appears quasi aseismic at least in the 0 5 mhz range of sensitivity of the ultrasonic transducers used in the experiment concomitantly the rate of micro seismic activity is strongly correlated with the imposed slip rate and the evolution of the axial stress the moment tensor inversion of the focal mechanism of the high quality micro seismic events recorded suggests a transition from a non shear dominated to a shear dominated micro seismic activity when the rock evolves from initial failure to larger and faster slip along the fault the frictional behaviour of the shear faults highlights the possible interactions between small asperities and slow slip of a velocity strengthening fault which could be considered as a realistic experimental analogue of natural observations of non volcanic tremors and very low frequency earthquakes triggered by slow slip events

intermittent slip along the alto tiberina low angle normal fault in central italy the alto tiberina normal fault atf in central italy is a 50 km long crustal structure that deepens at a low angle 15 20 events on the fault plane are about ten times less than those located i

long period long duration seismic events during hydraulic fracture stimulation of a shale gas reservoir we report here a series of long period and long duration lpld seismic events observed during hydraulic fracturing in a shale gas reservoir these unusual events 10 100 s in duration are observed most clearly in the frequency band of 10 80 hz and are remarkably similar in appearance to tectonic tremor sequences first observed in subduction zones these complex but coherent wave trains have finite moveouts obtained from cross correlation the moveout direction of the events confirms that they originate in the reservoir from the area where the fracturing is going on clear p and s wave arrivals cannot be resolved within the lpld episodes but in some cases small micro earthquakes occur in the sequences whether these micro earthquakes are causal or coincidental is not known it has also been observed that in three contiguous frac stages all lpld events appear to come from two distinct places along one of two hypothetical fracture planes interestingly the stages which have the largest number of lpld

status of co2 storage in deep saline aquifers with emphasis on modeling approaches and practical simulations carbon capture and storage ccs is the only viable technology to mitigate carbon emissions while allowing continued large scale use of fossil fuels the storage part of ccs involves injection of carbon dioxide captured from large stationary sources into deep geological formations deep saline aquifers have the largest identified storage potential with estimated storage capacity sufficient to store emissions from large stationary sources for at least a century this makes ccs a potentially important bridging technology in the transition to carbon free energy sources injection of co2 into deep saline aquifers leads to a multicomponent multiphase flow system in which geomechanics geochemistry and nonisothermal effects may be important while the general system can be highly complex and involve many coupled nonlinear partial differential equations the underlying physics can sometimes lead to important simplifications for example the large density difference between injected co2 and brine may lead to relatively fast buoyant segregation making an assumption of vertical equilibrium reasonable such simplifying assumptions lead to a range of simplified governing equations whose solutions have provided significant practical insights into system behavior including improved estimates of storage capacity easy to compute estimates of co2 spatial migration and pressure response and quantitative estimates of leakage risk when these modeling studies are coupled with observations from well characterized injection operations understanding of the overall system behavior is enhanced significantly this improved understanding shows that while economic and policy challenges remain co2 storage in deep saline aquifers appears to be a viable technology and can contribute substantially to climate change solutions

on the evolution of elastic properties during laboratory stick slip experiments spanning the transition from slow slip to dynamic rupture the physical mechanisms governing slow earthquakes remain unknown as does the relationship between slow and regular earthquakes to investigate the mechanism s of slow earthquakes and related quasi dynamic modes of fault slip we performed laboratory experiments on simulated fault gouge in the double direct shear configuration we reproduced the full spectrum of slip behavior from slow to fast stick slip by altering the elastic stiffness of the loading apparatus k to match the critical rheologic stiffness of fault gouge kc our experiments show an evolution from stable sliding when k kc to quasi dynamic transients when k kc to dynamic instabilities when k kc to evaluate the microphysical processes of fault weakening we monitored variations of elastic properties we find systematic changes in p wave velocity vp for laboratory seismic cycles during the coseismic stress drop seismic velocity drops abruptly consistent with observations on natural faults in the preparatory phase preceding failure we find that accelerated fault creep causes a vp reduction for the complete spectrum of slip behaviors our results suggest that the mechanics of slow and fast ruptures share key features and that they can occur on same faults depending on frictional properties in agreement with seismic surveys on tectonic faults our data show that their state of stress can be monitored by vp changes during the seismic cycle the observed reduction in vp during the earthquake preparatory phase suggests that if similar mechanisms are confirmed in nature high resolution monitoring of fault zone properties may be a promising avenue for reliable detection of earthquake precursors

frictional heterogeneities on carbonate bearing normal faults insights from the monte maggio fault italy observations of heterogeneous and complex fault slip are often attributed to the complexity of fault structure and or spatial heterogeneity of fault frictional behavior such complex slip patterns have been observed for earthquakes on normal faults throughout central italy where many of the mw 6 to 7 earthquakes in the apennines nucleate at depths where the lithology is dominated by carbonate rocks to explore the relationship between fault structure and heterogeneous frictional properties we studied the exhumed monte maggio fault located in the northern apennines we collected intact specimens of the fault zone including the principal slip surface and hanging wall cataclasite and performed experiments at a normal stress of 10 mpa under saturated conditions experiments designed to reactivate slip between the cemented principal slip surface and cataclasite show a 3 mpa stress drop as the fault surface fails then velocity neutral frictional behavior and significant frictional healing overall our results suggest that 1 earthquakes may readily nucleate in areas of the fault where the slip surface separates massive limestone and are likely to propagate in areas where fault gouge is in contact with the slip surface 2 postseismic slip is more likely to occur in areas of the fault where gouge is present and 3 high rates of frictional healing and low creep relaxation observed between solid fault surfaces could lead to significant aftershocks in areas of low stress drop

stress and fluid transfer in a fault zone due to overpressures in the seismogenic crust 1 stress and fluid transfers were analyzed on the surface of a 30 m thick dextral strike slip fault zone subjected to an overpressure of 63 kpa pressure strain measurements taken during the pressurization indicated a strain state primarily controlled by the hydromechanical behavior of permeable fractures and then by the fluid diffusion in the matrix using thm modeling we simulated an extension of these hydromechanical effects to a 25 km depth in the seismogenic crust applying a lithostatic pressure at the base of the fault the simulations indicate that a significant strain in the damage zone greater than that in the protolith along with the differences in hydraulic diffusivity between the damage zone and protolith may induce high static stress and fluid accumulations in the core under this stress this core is projected to exhibit deformity as much as a 12 m shear slip distributed over a 175 m long active zone

seismicity controlled by a frictional afterslip during a small magnitude seismic sequence m l 5 on the chihshang fault taiwan we report evidence for frictional afterslip at shallow depths about 5 to 7 km during a small magnitude seismic sequence with ml 5 along the chihshang fault a main active structure of the longitudinal valley in southeast taiwan the afterslip which was recorded by a nearby borehole dilatometer lasted about a month with a cumulative geodetic moment magnitude of 4 8 0 2 the afterslip comprised two stages and controlled the aftershock sequence the first postseismic stage which followed a ml 4 6 earthquake lasted about 6 h and mostly controlled the ruptures of neighboring asperities e g multiplets near the hypocenter then a 4 week duration large afterslip event following a ml 4 9 earthquake controlled the rate of aftershocks during its first 2 days through brittle creep the study presents a rare case of simultaneous seismological and geodetic observations for afterslip following earthquakes with magnitude lower than 5 furthermore the geodetic moment of the postseismic phase is at least equivalent to the coseismic moment of the sequence

late cenozoic ge si record of marine biogenic opal implications for variations of riverine fluxes to the ocean we have determined germanium silicon ratios in purified diatoms and radiolarians from siliceous sediments in holocene core tops one late pleistocene piston core and four high latitude southern ocean deep sea drilling project dsdp sites ranging in age from holocene to oligocene high latitude holocene diatoms in the 10 to 38 m size fraction ge si opal 0 693 0 039 10 6 faithfully record the present oceanic ratio ge si seawater 0 699 0 004 10 6 this confirms that diatoms from siliceous oozes are monitors of seawater ge si and may record past changes in rates of delivery of weathering products to the sea from continental and seafloor weathering large diatoms and radiolarians display ratios much lower than ge si seawater suggesting that ge si opal to some extent displays the effects of biological fractionation from 0 to 2 5 m y b p ge si opal in diatoms from dsdp sites ranges from about 0 54 to 0 70 10 6 similar to the range observed in one late pleistocene piston core over the last 25 kyr from 2 5 to 6 m y b p the ratio is more variable while prior to 10 m y b p ratios are clearly higher averaging about 0 9 10 6 in the early to middle miocene low values of the ratio are consistent with global weathering regimes dominated by river silica input to the sea while higher ratios suggest periods of enhanced hydrothermal input or reduced fluvial contribution nevertheless an oceanic one box model with variable river and hydrothermal inputs and biogenic opal output cannot adequately account for the magnitude or the timing of changes observed in the record suggesting the existence of other sources sinks for inorganic germanium and or silicon to or from the sea