microseismic event location for monitoring co2 injection using double difference tomography increases in pore pressure and volume caused by co2 injection will perturb the ambient stress field and possibly trigger brittle failure on small fractures or faults resulting in microearthquakes monitoring this induced seismicity can help understand the migration of the co2 front and the stress and strain changes in the reservoir miyazawa et al 2008 as part of research efforts of the southwest regional partnership on carbon sequestration supported by the u s department of energy and managed by the national energy technology laboratory a large number of microseismic events have been recorded since early 2008 using a geophone string cemented into a well for monitoring co2 injection at the aneth oil field in utah we use double difference tomography to obtain high precision locations of microseismic events and improve the spatial resolution of the velocity structure simultaneously the double difference seismic tomography method was developed for improving earthquake source locations and tomograph

automated seismic waveform location using multichannel coherency migration mcm ii application to induced and volcano tectonic seismicity locating microseismic events is essential for many areas of seismology including volcano and earthquake monitoring and reservoir engineering due to the large number of microseismic events in these settings an automated seismic location method is required to perform real time seismic monitoring the measurement environment requires a precise and noise resistant event location method for seismic monitoring in this paper we apply multichannel coherency migration mcm to automatically locate microseismic events of induced and volcano tectonic seismicity using sparse and irregular monitoring arrays compared to other migration based methods in spite of the often sparse and irregular distribution of the monitoring arrays the mcm can show better location performance and obtain more consistent location results with the catalogue obtained by manual picking our mcm method successfully locates many triggered volcano tectonic events with local magnitude smaller that 0 which demonstrates its applicability on locating very small earthquakes our synthetic event location example at a carbon capture and storage site shows that continuous and coherent drilling noise in industrial settings will pose great challenges for source imaging however automatic quality control techniques including filtering in the frequency domain and weighting are used to automatically select high quality data and can thus effectively reduce the effects of continuous drilling noise and improve source imaging quality the location performance of the mcm method for synthetic and real microseismic datasets demonstrates that the mcm method can perform as a reliable and automatic seismic waveform analysis tool to locate microseismic events

coseismic stress changes and crustal deformation on the reykjanes peninsula due to triggered earthquakes on 17 june 2000 1 a large mw 6 5 earthquake struck the south iceland seismic zone sisz on 17 june 2000 the 17 june main shock triggered increased seismicity over a large area and significant slip on at least three distinct faults on the reykjanes peninsula up to 87 km to the west of the event a second large mw 6 4 earthquake in the sisz occurred on 21 june 2000 about 17 km west of the 17 june main shock this event does not appear to have triggered as much activity on the reykjanes peninsula as the 17 june main shock although the epicenter was closer crustal deformation signals due to the june 2000 earthquakes on the reykjanes peninsula were observed with campaign and continuous gps and synthetic aperture radar interferometry with the largest coseismic deformation signal near lake kleifarvatn we model the faults using three uniform slip rectangular dislocations in an elastic half space best fit uniform slip models consistent with seismic and geodetic data indicate that all three faults trend n s and the motion on them was primarily right lateral strike slip our study suggests that the event near kleifarvatn had a significantly larger moment than seismic estimates indicating a component of aseismic slip on the fault lasting no more than several hours static coulomb failure stress change calculations indicate that the event at kleifarvatn increased the coulomb stress at the hypocenter of the nupshlidarhals event by 0 1 0 2 mpa as well as loading the hvalhnukur fault

spatial variations in fault friction related to lithology from rupture and afterslip of the 2014 south napa california earthquake following earthquakes faults are often observed to continue slipping aseismically it has been proposed that this afterslip occurs on parts of the fault with rate strengthening friction that are stressed by the main shock but our understanding has been limited by a lack of immediate high resolution observations here we show that the behavior of afterslip following the 2014 south napa earthquake in california varied over distances of only a few kilometers this variability cannot be explained by coseismic stress changes alone we present daily positions from continuous and survey gps sites that we remeasured within 12 h of the main shock and surface displacements from the new sentinel 1 radar mission this unique geodetic data set constrains the distribution and evolution of coseismic and postseismic fault slip with exceptional resolution in space and time we suggest that the observed heterogeneity in behavior is caused by lithological controls on the frictional properties of the fault plane

prediction of fracture induced permeability and fluid flow in the crust using experimental stress data the results of a series of two dimensional photoelastic experiments on dilational jogs cut in perspex sheets have been used to determine the second order fracture patterns and fluid flow network associated with tectonically loaded dilational jogs the stress data derived from these experiments orientation of the principal stresses and the distributions and magnitudes of the differential stress and the mean stress are used in combination with the theory of brittle failure to predict the orientation approximate distribution and likelihood of formation of second order brittle structures associated with the modeled jog geometries the orientations of brittle structures predicted compare favorably with those found in natural large scale dilational jogs such as the vienna basin the dead sea basin and the brawley jog california as well as in mesoscopic examples and those formed in analog models from the experimental results it is possible to determine areas of high and low mean stress and by assuming that fluid migration occurs in response to mean stress gradients it is possible to predict the fluid migration associated with the development of a dilational jog in this way it is possible to show that the intrajog region of underlapping and neutral dilational jogs will not be favorable sites for fluid ingress when the jog has an overlapping geometry the intrajog region is a mean stress low and consequently fluid ingress into the jog is more likely in addition as the fault overlap increases the distribution of mean stress and thus the associated fluid flow in the intrajog region increases in complexity by combining the predicted second order fracture patterns and the mean stress distribution derived from the photoelastic analysis it is possible to construct a map showing sites of likely fluid accumulation and mineralization these sites correspond to regions of low and high fluid throughflow respectively predictions based on this model compare well with natural examples of fluid accumulation and mineralization in dilational jogs

coseismic deformation from earthquake faulting on a layered spherical earth summary a method for calculating the static displacement field following earthquake faulting in a layered spherical earth is presented at shallow levels the earth s layering is characterized by sharp jumps in bulk and shear moduli at the conrad discontinuity and the moho and is therefore important to consider when evaluating crustal deformation the solution to the equations of static equilibrium is represented as a superposition of spheroidal and toroidal components that each depend on spherical harmonic degree and the moment tensor a method that has recently been applied to the problem of wave propagation on a layered spherical earth is here applied to the static deformation field by representing the point source in terms of discontinuities in the displacement stress vector the green s function for a particular source geometry is derived directly numerical tests are presented to verify the accuracy of the method and to illustrate the effects of sphericity and layering on the calculated deformation fields the effect of sphericity is generally less than about 2 per cent of maximum deformation within 100 km of an earthquake source at crustal depths comparisons between the deformation calculated on a spherical homogeneous earth and spherical layered earth show that up to 20 per cent errors would be introduced if the earth s layered structure were ignored the effect of layering is strongest for sources with a strong horizontal slip component

determining moho depth beneath sedimentary basins using regional pn multiples the study of the moho beneath thick sedimentary basins involving natural earthquakes is challenging as low velocity materials often cause strong reverberations that mask moho signals here we develop a method to determine the depth of the moho by taking advantage of the presence of the sediments the method utilizes the first pn crustal multiple from regional earthquakes pnpn and its differential travel time with respect to pn pnpn is usually weak in amplitude thus it is difficult to identify in regions without a sedimentary cover however pnpn is significantly amplified in the presence of low velocity sediments because of an increase in the near surface p to p reflection coefficient the arrival time amplitude and wave shape of pnpn if normalized by the reference pn are insensitive to earthquake source parameters such as focal mechanism and focal depth we demonstrate the potential of this method using both 1d and 2d waveform simulations synthetic waveforms suggest that pmppn and pnpmp one pn leg merges to pmp near the source or the receiver largely contribute to the pnpn amplitudes which depend on the near surface structure at their free surface p to p reflection points we further validate the method with two field examples in the imperial valley one is near the united states mexico border and the other is in oklahoma in the central united states both examples suggest that the method can be used to study the moho either near the source or the receiver

imaging fractures and sedimentary fabrics using shear wave splitting measurements made on passive seismic data summary the ability to detect aligned fractures using seismic anisotropy provides a valuable tool for exploiting hydrocarbon reservoirs better perhaps the most direct way of identifying anisotropy is by observing shear wave splitting however the interaction of shear waves with subsurface structure is often complicated although fractures in hydrocarbon reservoirs are usually subvertical shear waves recorded on downhole receivers from microseismic events in or near the reservoir are not likely to have travelled vertically as such interpreting splitting measurements made on such waves is a non trivial problem here we develop an approach to model the effects of subsurface structure on non vertically propagating shear waves rock physics theory is used to model the effects of sedimentary fabrics as well as fractures allowing us to use shear wave splitting measurements to invert for aligned fractures we use synthetic examples to demonstrate how it is possible to assess in advance how well splitting measurements will image structures and how this is highly dependent on the available range of ray coverage finally we demonstrate the inversion technique on a passive seismic data set collected during hydraulic fracture stimulation despite an unfavourable source receiver geometry the strike of an aligned fracture set is accurately identified

improved fracture network mapping through reducing overburden influence knowledge of the fracture network is indispensable for enhanced oil and gas production from naturally fractured reservoirs in theory the spatial distribution of subsurface fractures could be quantitatively predicted through seismic anisotropy but to improve a seismic guided fracture prediction the overburden effect needs to be reduced

a california statewide three dimensional seismic velocity model from both absolute and differential times abstract we obtain a seismic velocity model of the california crust and uppermost mantle using a regional scale double difference tomography algorithm we begin by using absolute arrival time picks to solve for a coarse three dimensional 3d p velocity v p model with a uniform 30 km horizontal node spacing which we then use as the starting model for a finer scale inversion using double difference tomography applied to absolute and differential pick times for computational reasons we split the state into 5 subregions with a grid spacing of 10 to 20 km and assemble our final statewide v p model by stitching together these local models we also solve for a statewide s wave model using s picks from both the southern california seismic network and usarray assuming a starting model based on the v p results and a v p v s ratio of 1 732 our new model has improved areal coverage compared with previous models extending 570 km in the sw ne direction and 1320 km in the nw se direction it also extends to greater depth due to the inclusion of substantial data at large epicentral distances our v p model generally agrees with previous separate regional models for northern and southern california but we also observe some new features such as high velocity anomalies at shallow depths in the klamath mountains and mount shasta area somewhat slow velocities in the northern coast ranges and slow anomalies beneath the sierra nevada at midcrustal and greater depths this model can be applied to a variety of regional scale studies in california such as developing a unified statewide earthquake location catalog and performing regional waveform modeling

impact of co2 injection through horizontal and vertical wells on the caprock mechanical stability abstract subsurface injection of carbon dioxide co2 may induce large overpressures that may put caprock integrity at risk this overpressure may induce microseismic events if the rock yields yielding of the caprock could open up a leakage path for co2 rock mechanical integrity is strongly related to fluid pressure evolution which is significantly different for vertical and horizontal wells while co2 pressure builds up sharply at the beginning of injection but afterwards drops when injecting a constant co2 mass flow rate through a vertical well a horizontal well leads to a continuous co2 pressure increase thus for a vertical well the less stable situation in the saline aquifer occurs at the beginning of injection however the changes induced in the effective stress field are small so unstable conditions are unlikely both in a normal faulting and a reverse faulting stress regimes in extensive saline aquifers such as the one considered in this study by contrast fluid pressure becomes larger than that of a vertical well for a common length of horizontal wells around 2 km which causes a significant increase in horizontal total stresses that improves the reservoir and caprock mechanical stability in a nf stress regime but worsens it in a rf stress regime though in general the caprock integrity is unlikely to be compromised fluid pressure evolution should be always monitored and mitigation measures should be carried out if it deviates from its expected evolution

microscale modeling of fluid flow geomechanics seismicity relationship between permeability and seismic source response in deformed rock joints studying rock joint deformation including both slippage and opening mechanisms provides an opportunity to investigate the connection between the permeability and seismic source mechanisms a microscale fluid flow geomechanics seismicity model was built to evaluate the transport response and failure mechanism of microcracks developed along a joint in berea sandstone samples during deformation the modeling method considers comprehensive grain cement interactions fluid flow behavior is obtained through a realistic network model of the pore space in the compacted assembly the geometric description of the complex pore structure is characterized to predict permeability of the rock sample as a function of rock deformation by using a dynamic pore network model as a result of microcracks development forces and displacements in grains involved in bond breakage are measured to determine seismic moment tensor shear and nonshear displacements are applied to the joint samples to investigate their effects on permeability evolution and failure mechanism of microcracks during joint deformation in addition the effect of joint roughness is analyzed by performing numerical compression tests we also investigate how confining pressure affects volumetric deformation leading to opening or closure of developed microcracks and permeability changes of samples with joints

comparison of geomechanical deformation induced by megatonne scale co2 storage at sleipner weyburn and in salah geological storage of co2 that has been captured at large point source emitters represents a key potential method for reduction of anthropogenic greenhouse gas emissions however this technology will only be viable if it can be guaranteed that injected co2 will remain trapped in the subsurface for thousands of years or more a significant issue for storage security is the geomechanical response of the reservoir concerns have been raised that geomechanical deformation induced by co2 injection will create or reactivate fracture networks in the sealing caprocks providing a pathway for co2 leakage in this paper we examine three large scale sites where co2 is injected at rates of 1 megatonne y or more sleipner weyburn and in salah we compare and contrast the observed geomechanical behavior of each site with particular focus on the risks to storage security posed by geomechanical deformation at sleipner the large high permeability storage aquifer has experienced little pore pressure increase over 15 y of injection implying little possibility of geomechanical deformation at weyburn 45 y of oil production has depleted pore pressures before increases associated with co2 injection the long history of the field has led to complicated sometimes nonintuitive geomechanical deformation at in salah injection into the water leg of a gas reservoir has increased pore pressures leading to uplift and substantial microseismic activity the differences in the geomechanical responses of these sites emphasize the need for systematic geomechanical appraisal before injection in any potential storage site

seismic rupture and ground accelerations induced by co2 injection in the shallow crust seismic rupture and ground accelerations induced by co 2 injection in the shallow crust f rederic cappa 1 2 and jonny rutqvist 1 lawrence berkeley national laboratory earth sciences division university of california berkeley usa geoazur university of nice sophia antipolis cote d azur observatory france e mail addresses cappa geoazur unice fr f cappa jrutqvist lbl gov j rutqvist july 23 2012

plateau versus fissure ridge travertines from quaternary geothermal springs of italy and turkey interactions and feedbacks between fluid discharge paleoclimate and tectonics abstract morphologically different deposits of thermal travertines are known worldwide but what factors controlled their morphology volume and growth for tens of thousands of years is only partially understood two main morphotypes of quaternary thermal travertines are reconsidered here to understand the reasons for their differential growth the fissure ridge travertines of denizli basin western turkey and the travertine plateau of tivoli central italy for comparable longevities and average vertical deposition rates the main differences between the studied travertines are as follows 1 volume of the travertine plateau is about one hundred times larger than each fissure ridge 2 despite a larger volume the travertine plateau does not produce relief whereas the fissure ridges produce a characteristic prominent topography 3 the travertine plateau grew primarily through lateral progradation whereas the fissure ridges through vertical aggradation 4 travertine deposition occurred in different environments principally low energy flat or shallow environments at tivoli and high energy inclined environments at denizli 5 the growth of the tivoli plateau occurred in a subsiding basin whereas the fissure ridges were not influenced by significant subsidence 6 c and o isotope signatures from the two studied travertines are different 7 despite similar annual precipitations the present water discharge in the tivoli area is about ten times greater than that of the denizli basin u series ages from the two deposits are correlated with paleoclimate oscillations at regional and global scales geological field evidence together with paleoclimate correlations suggest that in both the study cases the main body of travertine deposits the bedded travertine grew preferentially when the water table was high warm and or humid periods conversely when the water table was depressed cold and or dry periods the tivoli travertine underwent partial erosion and the denizli ridges were cut by axial veins and lateral sill like structures filled by banded sparitic travertine a comparative model is proposed where the main factor driving the difference in the morphostratigraphic architecture of fissure ridges and travertine plateaus is the volume of water discharge a high discharge rate resulted in the precipitation of caco3 far away from the springs hence driving the lateral progradation of the tivoli plateau a reduced discharge rate caused travertine precipitation close to the springs thus causing the vertical aggradation of the denizli fissure ridges paleoclimate oscillations must have controlled the amount of fluid discharge which in turn must have influenced the opening of the feeding fractures by an increased pore pressure

rocks under strain strain induced time lapse time shifts are observed for depleting reservoirs pressure depletion as a result of oil and gas production creates changes in the stress and strain fields of the rock material both inside and outside the reservoir seismic waves propagating through these rocks have different traveltimes before and after depletion because the layer thicknesses change and the seismic velocity is altered by the changes in the stress and strain fields the cumulative change in two way traveltimes with depth is referred to as time lapse time shift

gps observed horizontal ground extension at the hutubi china underground gas storage facility and its application to geomechanical modeling for induced seismicity abstract induced earthquakes and ground displacements were seldom reported in relation to gas injection or extraction the hutubi underground gas storage hugs facility is the largest one in china and is also a unique case with both earthquakes and ground displacements detected during multiple cycles of injection and extraction since 9 june 2013 unlike previous studies with a primarily seismological focus here we conducted quantitative analysis on the geomechanics of seismicity induced by the hugs through developing a hydrogeologic framework which systematically integrated geodetic geophysical and geological data first we measured horizontal ground extension and shortening on the order of cm in response to gas injection and extraction of the hugs at depth using a local gps network which was not reported in other regions with induced seismicity second we synthesized a variety of data including seismic reflection profiles a newly acquired local velocity model rock physics measurements well drilling and logging data to build up a 2d geomechanical model for the hugs third based on fully coupled poroelasticity we proposed two methods to optimize the permeability of the upper aquifers as well as the reservoir porosity and permeability with constraints from well level gps and well pressure data numerical simulations using the calibrated 2d model revealed that the horizontal extension due to the reservoir dilation is larger than ground uplift the observed seismicity on faults without hydraulic connections to the gas repository was probably induced by the poroelastic effect of reservoir dilation our study provided a prototype scheme for detecting and characterizing the geomechanical behavior of cyclic fluid injection and extraction in a deep reservoir which would be applicable to other ugs facilities

exploring trends in microcrack properties of sedimentary rocks an audit of dry core velocity stress measurements rock physics models are used increasingly to link fluid and mechanical deformation parameters for dynamic elastic modeling we explore the input parameters of an analytical stress dependent rock physics model to do this we invert for the stressdependent microcrack parameters of more than 150 sedimentary rock velocity stress core measurements taken from a literature survey theinversionschemeisbasedonamicrostructuraleffective medium formulation defined by a second rank crack density tensor scalar crack model or by a second and fourth rank crack density tensor joint inversion model then the inversion results are used to explore and predict the stress dependent elastic behavior of various sedimentary rock lithologies using an analytical microstructural rock physics model via the initial model inputparameters initialcrackaspectratioandinitialcrackdensity estimates of initial crack aspect ratio are consistent among most lithologies with a mean of 0 0004 but for shales they differ uptoseveraltimesinmagnitudewithameanof0 001 estimates of initial aspect ratio are relatively insensitive to the inversion method although the scalar crack inversion becomes less reliableatlowvaluesofnormal to tangentialcrackcomplianceratio bn bt initialcrackdensityissensitivetothedegreeofdamage as well as the inversion procedure an important implication is thatthefourth rankcrack densitytermisnotnecessarilynegligible for most sedimentary rocks and evaluation of this term or bn bt is necessary for accurate prediction of initial crack density this is especially important because recent studies suggest thatbn btcanindicatefluidcontentincracks

three dimensional structure influences on the strong motion wavefield of the 1989 loma prieta earthquake abstract the accurate calculation of ground motions for future large earthquakes depends on detailed knowledge of three dimensional 3d geologic structure and the earthquake source process as well as sufficient computational resources in this article we describe the results of finite difference simulations for the 1989 loma prieta ca earthquake with a 3d seismic velocity model for the san francisco bay region and a heterogeneous slip model of the source additionally we explore the sensitivity of the synthetics to the major geologic structures in the velocity model the san francisco bay region particularly the loma prieta region is a unique area for the study of 3d wave propagation because of the pronounced lateral velocity contrasts across the strike slip faults of the region understanding the effects of such long wavelength structure is doubly important when considering a source located close to such a contrast our simulations show that the lateral velocity contrast across the san andreas fault saf would be expected to substantially affect the propagation of elastic waves radiated from a source in the loma prieta region indeed we find that the refraction of energy by the saf serves to reduce ground motions at stations located along the san francisco peninsula and the quaternary and tertiary alluvial basins of the san francisco bay region are found to amplify and extend the duration of ground motions in santa clara valley livermore valley and san pablo bay we find that the 3d model as currently defined accurately describes the spatial variation of peak ground velocity for frequencies less than 0 5 hz which suggests that this model may be used to estimate ground motions for future earthquake scenarios

seismogeodesy of the 2014 mw6 1 napa earthquake california rapid response and modeling of fast rupture on a dipping strike slip fault real time high rate geodetic data have been shown to be useful for rapid earthquake response systems during medium to large events the 2014 mw6 1 napa california earthquake is important because it provides an opportunity to study an event at the lower threshold of what can be detected with gps we show the results of gps only earthquake source products such as peak ground displacement magnitude scaling centroid moment tensor cmt solution and static slip inversion we also highlight the retrospective real time combination of gps and strong motion data to produce seismogeodetic waveforms that have higher precision and longer period information than gps only or seismic only measurements of ground motion we show their utility for rapid kinematic slip inversion and conclude that it would have been possible with current real time infrastructure to determine the basic features of the earthquake source we supplement the analysis with strong motion data collected close to the source to obtain an improved postevent image of the source process the model reveals unilateral fast propagation of slip to the north of the hypocenter with a delayed onset of shallow slip the source model suggests that the multiple strands of observed surface rupture are controlled by the shallow soft sediments of napa valley and do not necessarily represent the intersection of the main faulting surface and the free surface we conclude that the main dislocation plane is westward dipping and should intersect the surface to the east either where the easternmost strand of surface rupture is observed or at the location where the west napa fault has been mapped in the past

modelling carbon dioxide accumulation at sleipner implications for underground carbon storage an analytical solution to the equations describing the flow of a buoyant fluid released into a porous medium below a horizontal impermeable boundary is used to model the growth of co2 accumulations beneath thin mudstone beds in the utsira sand reservoir at sleipner in the north sea here supercritical co2 has been injected at a rate of 1 mt yr since 1996 and imaged by time lapse seismic data in 1999 2001 and 2002 the co2 rises as a narrow plume and is partially trapped by a number of thin mudstones before reaching the caprock to the reservoir the radii of the individual layers of trapped co2 increase as the square root of time since initiation as predicted by the modelling for constant input flux however apparent negative initiation times for horizons low in the reservoir suggests that net input fluxes for these layers have decreased with time most probably as the spreading layers have increased their leakage rates accumulation of co2 in the layers higher in the reservoir was initiated up to 3 yr after injection started modelling of the thickness profiles across three of the higher layers suggests that their net input fluxes have increased with time the observation that the central thicknesses of the deeper layers have remained approximately constant or have slightly decreased since first imaged in 1999 is consistent with the model predictions that the central thickness is directly proportional to net input flux however estimates of the permeability of the reservoir from the rate of increase of the radii of the co2 accumulations are an order of magnitude less than measured permeabilities on the reservoir sandstone permeabilities estimated from the modelling of layer thickness changes scatter in the same range these discrepancies may arise from 1 approximations in the model not being valid 2 the measured permeabilities not being representative of the permeability for two phase flow on the scale of the reservoir or considered less likely 3 that much less co2 is being stored in the imaged co2 accumulations than estimated from the seismic reflection profiles the most probable cause of the discrepancy is that the relative permeability for the co2 phase is significantly reduced at lower co2 saturations

aftershocks of the 2014 south napa california earthquake complex faulting on secondary faults abstract we investigate the aftershock sequence of the 2014 m w 6 0 south napa california earthquake low magnitude aftershocks missing from the network catalog are detected by applying a matched filter approach to continuous seismic data with the catalog earthquakes serving as the waveform templates we measure precise differential arrival times between events which we use for double difference event relocation in a 3d seismic velocity model most aftershocks are deeper than the mainshock slip and most occur on the west of the mapped surface rupture although the mainshock coseismic and postseismic slip appears to have occurred on the near vertical strike slip west napa fault many of the aftershocks occur in a complex zone of secondary faulting earthquake locations in the main aftershock zone near the mainshock hypocenter delineate multiple dipping secondary faults composite focal mechanisms indicate strike slip and oblique reverse faulting on the secondary features the secondary faults were moved toward failure by coulomb stress changes from the mainshock slip clusters of aftershocks north and south of the main aftershock zone exhibit vertical strike slip faulting more consistent with the west napa fault the northern aftershocks correspond to the area of the largest mainshock coseismic slip whereas the main aftershock zone is adjacent to the fault area that has primarily slipped postseismically unlike most creeping faults the zone of postseismic slip does not appear to contain embedded stick slip patches that would have produced on fault aftershocks the lack of stick slip patches along this portion of the fault may contribute to the low productivity of the south napa aftershock sequence

stress induced temporal variations in seismic anisotropy observed in microseismic data summary in situ stress monitoring of crustal rocks is desirable as it yields insights into earthquake mechanisms volcanic eruptions and changes in hydrocarbon reservoirs shear wave splitting induced by stress controlled cracks in the shallow crust provides a way to infer this stress temporal variations in these observations can be difficult to quantify due to scatter in the data and discontinuous observations here we present evidence of temporal variations in shear wave splitting from a continuous time series with a high occurrence of microseismic events recorded in a borehole we interpret these observations in terms of stress controlled cracks and are able to infer changes in stress and via modelling suggest the cause of the anisotropy possible origins of the temporal variation in per cent anisotropy are tidal or oil field production processes our results suggest that shear wave splitting is a viable probe for inferring changes in crustal stress in cracked rock

seismic and aseismic deformations and impact on reservoir permeability the case of egs stimulation at the geysers california usa author s jeanne p rutqvist j rinaldi ap dobson pf walters m hartline c garcia j abstract 2015 american geophysical union all rights reserved in this paper we use the seismicity based reservoir characterization approach to study the spatiotemporal dynamics of an injection induced microseismic cloud monitored during the stimulation of an enhanced geothermal system and associated with the northwest geysers enhanced geothermal system egs demonstration project california we identified the development of a seismically quiet domain around the injection well surrounded by a seismically active domain then we compare these observations with the results of 3 d thermo hydro mechanical simulations of the egs which accounts for changes in permeability as a function of the effective normal stress and the plastic strain the results of our modeling show that 1 the aseismic domain is caused by both the presence of the injected cold water and by thermal processes these thermal processes cause a cooling stress reduction which prevent shear reactivation and favors fracture opening by reducing effective normal stress and locally increasing the permeability this process is accompanied by aseismic plastic shear strain 2 in the seismic domain microseismicity is caused by the reactivation of the preexisting fractures resulting from an increase in injection induced pore pressure our modeling indicates that in this domain permeability evolves according to the effective normal stress acting on the shear zones whereas shearing of preexisting fractures may have a low impact on permeability we attribute this lack of permeability gain to the fact that the initial permeabilities of these preexisting fractures are already high up to 2 orders of magnitude higher than the host rock and may already be fully dilated by past tectonic straining

ground motion modeling of hayward fault scenario earthquakes part ii simulation of long period and broadband ground motions abstract we simulate long period t 1 0 2 0 s and broadband t 0 1 s ground motions for 39 scenario earthquakes m w 6 7 7 2 involving the hayward calaveras and rodgers creek faults for rupture on the hayward fault we consider the effects of creep on coseismic slip using two different approaches both of which reduce the ground motions compared with neglecting the influence of creep nevertheless the scenario earthquakes generate strong shaking throughout the san francisco bay area with about 50 of the urban area experiencing modified mercalli intensity vii or greater for the magnitude 7 0 scenario events long period simulations of the 2007 m w 4 18 oakland earthquake and the 2007 m w 5 45 alum rock earthquake show that the u s geological survey s bay area velocity model version 08 3 0 permits simulation of the amplitude and duration of shaking throughout the san francisco bay area for hayward fault earthquakes with the greatest accuracy in the santa clara valley san jose area the ground motions for the suite of scenarios exhibit a strong sensitivity to the rupture length or magnitude hypocenter or rupture directivity and slip distribution the ground motions display a much weaker sensitivity to the rise time and rupture speed peak velocities peak accelerations and spectral accelerations from the synthetic broadband ground motions are on average slightly higher than the next generation attenuation nga ground motion prediction equations we attribute much of this difference to the seismic velocity structure in the san francisco bay area and how the nga models account for basin amplification the nga relations may underpredict amplification in shallow sedimentary basins the simulations also suggest that the spudich and chiou 2008 directivity corrections to the nga relations could be improved by increasing the areal extent of rupture directivity with period

physical modeling of overburden effects reservoir properties can be inferred from the amount of anisotropy estimated from seismic data unfortunately irregularities in the formations above the reservoir unit can mask or overprint the true seismic anisotropy of the reservoir unit this overburden effect subjects the measured reservoir seismic anisotropy to a high degree of uncertainty we investigate this overburden effect on p waves with a three layer ultrasonic laboratory scale model whose middle layer contains localized gas filled vertical fractures we analyze the reflection amplitudes and traveltimes of a p wave reflection event from below the overburden to understand the overburden effect on anisotropy analysis and imaging our study shows that steps must be taken to reduce the p wave overburden effect when significant irregularities occur in the formations above the reservoir unit

linking microseismic event observations with geomechanical models to minimise the risks of storing co2 in geological formations abstract for carbon capture and storage ccs in geological formations to be scientifically viable we must be able to model and monitor the effects of geomechanical deformation on the integrity of the caprock excess deformation may open fractures providing pathways for co2 leakage from the reservoir an acceptable geomechanical model must provide a good match with field observations microseismic activity is a direct manifestation of mechanical deformation so it can be used to constrain geomechanical models the aim of this paper is to develop the concept of using observations of microseismic activity to help ground truth geomechanical models microseismic monitoring has been ongoing at the weyburn co2 storage and monitoring project since 2003 we begin this paper by presenting these microseismic observations less than 100 events have been recorded documenting a low rate of seismicity most of the events are located close to nearby producing wells rather than the injection well a pattern that is difficult to interpret within the conventional framework for injection induced seismicity many events are located in the overburden without geomechanical simulation it is difficult to assess what these observations mean for the integrity of the storage formation to address these uncertainties we generate numerical geomechanical models to simulate the changes in stress induced by co2 injection and use these models to predict the generation of microseismic events and seismic anisotropy the initial geomechanical model that we generate using material properties based on laboratory core measurements does not provide a good match with either event locations or s wave splitting measurements made on the microseismic events we find that an alternative model whose reservoir is an order of magnitude softer than lab core sample measurements provides a much better match with observation as it leads shear stresses to increase above the production wells promoting microseismicity in these areas and generates changes in effective horizontal stresses that match well with s wave splitting observations this agreement between geophysical observations and a softer than lab measurements reservoir model highlights the difficulties encountered in upscaling lab scale results there is a strong need to link geomechanical models with observable manifestations of deformation in the field such as induced seismicity for calibration only then can we accurately assess the risks of leakage generated by mechanical deformation

source process of the 1995 kobe earthquake determination of spatio temporal slip distribution by bayesian modeling the kobe hyogo ken nanbu earthquake ms 6 8 occurred on 16 january 1995 in western japan bringing about a tremendous disaster we constructed its source model by determining a spatio temporal slip distribution using strong motion seismograms recorded at a number of stations near the hypocenter the source area is expressed using both a rough and a detailed subfault discretization system and a source time function expanded with a finite number of basis functions at each subfault is determined to perform stable inversions with many parameters we construct a bayesian model with unknown hyperparameters incorporating prior constraints on the smoothness of spatio temporal slip distribution in the observation equation using akaike s bayesian information criterion abic the optimal set of hyperparameters are objectively determined from observed data which enables us to uniquely determine a spatio temporal slip distribution after rough analysis in which the actual rupture area is estimated as 45 20 km we carried out a detailed analysis with a finer subfault discretization system 5 5 km in our final source model most of the slip exists beneath awaji island and the rupture below the city of kobe where the damage was most serious is relatively smaller the large slips of about 2 m in the shallowest subfaults located southwest of the hypocenter correspond to the surface offsets observed along the nojima fault the overall focal mechanism of the source is right lateral strike slip with a small thrust component while total seismic moment is 2 0 1019 nm the source time functions at shallow subfaults show a longer duration than those at deeper ones being hypothesized to represent a possible combined effect of low rupture propagation velocity and varying friction behavior in shallow regions of crust

multi scale simulation of thermal pressurization of fault fluid under co2 injection for storage and utilization purposes abstract we investigated the impact of thermal pressurization tp on temperature and pore pressure changes within the shear zone of a slipping fault under supercritical co 2 injection as a weakening mechanism tp of fault fluid during slip tends to reduce frictional resistance by increasing pore pressures along the fault shear zone we employed a multi scale modeling approach in which the co 2 injection was simulated in a large scale model of reservoir and the results were then exported into a micro scale model of the fault shear zone which incorporates the tp effect a poroelasticity finite element model was created in the large scale model to determine the stress and pressure changes in the reservoir and within the fault zone during co 2 injection the fault slip pressure and stresses obtained from the large scale model were fed into the small scale model to define hydraulic and mechanical boundary conditions of the shear zone of the fault two small scale models were developed to compare the response of the fault shear zone with and without considering the tp for the model with the tp effect a simple constitutive law was implemented to couple pore pressure changes with temperature rise due to frictional heating the results indicated that in general tp can lead to a significant increase of pore pressure build up due to co 2 injection the effects of hydraulic diffusivity slip velocity shear zone thickness as well as injection rate on pore pressure temperature and fracture energy prediction were evaluated it was found that temperature and pore pressure rise decrease significantly by decreasing the slip velocity and increasing the shear zone thickness for the values which were examined hydraulic diffusivity was found to have insignificant effect on pore pressure and temperature changes which can be attributed to high compressibility of co 2 findings of this study can be used and further expanded in future studies for more accurate assessment of induced seismicity potential due to co 2 injection

geological co2 sequestration in multi compartment reservoirs geomechanical challenges 1 sequestration of large amounts of co2 within deep underground reservoirs has been proposed as a potential approach for reducing atmospheric emissions of greenhouse gases a co2 sequestration project should address the associated environmental and safety issues and in this respect the importance of geomechanics has recently been widely recognized geomechanics is even more important when fluid injection is planned in faulted reservoirs how much co2 can be safely injected into multi compartment reservoirs are geomechanical constraints more restrictive than flow dynamic constraints these and other questions are addressed in the present study using a three dimensional finite element interface element geomechanical model we simulate the possible mechanical failure in both the injected formation and the caprock the fault thrust reactivation and the ground surface displacement in a faulted reservoir of the offshore northern italy where seismic surveys provided an accurate characterization of the faulted geological structure based on reliable petrophysical geomechanical properties from well logs and pore overpressure as predicted by a fluid dynamic model the results show that the injection of 1 106 ton a of co2 may be performed over a few years only thereafter part of the injected formation fails by shear stress a number of parametric scenarios are investigated to address the major uncertainties on the geomechanical response to co2 injection the modeling outcome suggests that shear failure and faults thrusts reactivation can occur much before attaining the hydraulic fracturing pressure hence representing two major constraints for a safe and permanent containment

the iquique earthquake sequence of april 2014 bayesian modeling accounting for prediction uncertainty the subduction zone in northern chile is a well identified seismic gap that last ruptured in 1877 on 1 april 2014 this region was struck by a large earthquake following a two week long series of foreshocks this study combines a wide range of observations including geodetic tsunami and seismic data to produce a reliable kinematic slip model of the mw 8 1 main shock and a static slip model of the mw 7 7 aftershock we use a novel bayesian modeling approach that accounts for uncertainty in the green s functions both static and dynamic while avoiding nonphysical regularization the results reveal a sharp slip zone more compact than previously thought located downdip of the foreshock sequence and updip of high frequency sources inferred by back projection analysis both the main shock and the mw 7 7 aftershock did not rupture to the trench and left most of the seismic gap unbroken leaving the possibility of a future large earthquake in the region

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

modelling microseismicity of a producing reservoir from coupled fluid flow and geomechanical simulation in this paper we investigate production induced microseismicity based on modelling material failure from coupled fluid flow and geomechanical simulation the model is a graben style reservoir characterized by two normal faults subdividing a sandstone reservoir into three compartments the results are analysed in terms of spatial and temporal variations in distribution of material failure we observe that material failure and hence potentially microseismicity is sensitive to not only fault movement but also fluid movement across faults for sealing faults failure is confined to the volume in and around the well compartment with shear failure localized along the boundaries of the compartment and shear enhanced compaction failure widespread throughout the reservoir compartment for non sealing faults failure is observed within and surrounding all three reservoir compartments as well as a significant distribution located near the surface of the overburden all shear enhanced compaction failures are localized within the reservoir compartments fault movement leads to an increase in shear enhanced compaction events within the reservoir as well as shear events located within the side burden adjacent to the fault we also evaluate the associated moment tensor mechanisms to estimate the pseudo scalar seismic moment of failure based on the assumption that failure is not aseismic the shear enhanced compaction events display a relatively normal and tight pseudo scalar seismic moment distribution centred about 106 pa whereas the shear events have pseudo scalar seismic moments that vary over three orders of magnitude overall the results from the study indicate that it may be possible to identify compartment boundaries based on the results of microseismic monitoring

rupture history of 2014 mw 6 0 south napa earthquake inferred from near fault strong motion data and its impact to the practice of ground strong motion prediction the spatiotemporal rupture history of the 2014 south napa earthquake is constrained using near fault strong motion records an aggressive source parameterization with 1372 subfaults is adopted to match the signals in the transverse components up to 4 hz the result reveals that the rupture of the napa earthquake initiated at a depth of 9 84 km and propagated mainly to north northwest nnw and updip on a 13 km long fault patch a gradual increase in average rise time when the rupture propagates to shallower depth is observed however it is the rupture of a small mw 4 9 isolated and high stress drop fault patch that excited the largest ground acceleration at stations south of the epicenter such fine scale rupture heterogeneity shall be considered during seismic hazard analysis

simulation of seismic events induced by co2 injection at in salah algeria a b s t r a c t article history carbon capture and storage technology has the potential to reduce anthropogenic co2 emissions however the geomechanical response of the reservoir and sealing caprocks must be modelled and monitored to ensure that injected co2 is safely stored to ensure confidence in model results there is a clear need to develop ways of comparing model predictions with observations from the field in this paper we develop an approach to simulate microseismic activity induced by injection which allows us to compare geomechanical model predictions with observed microseismic activity we apply this method to the in salah ccs project algeria a geomechanical reconstruction is used to simulate the locations orientations and sizes of pre existing fractures in the in salah reservoir the initial stress conditions in combination with a history matched reservoir flow model are used to determine when and where these fractures exceed mohr coulomb limits triggering failure the sizes and orientations of fractures and the stress conditions thereon are used to determine the resulting micro earthquake focal mechanisms and magnitudes we compare our simulated event population with observations made at in salah finding good agreement between model and observations in terms of event locations rates of seismicity and event magnitudes 2015 the authors published by elsevier b v this is an open access article under the cc by license

detection of multiple fracture sets using observations of shear wave splitting in microseismic data the hydrocarbon industry is moving increasingly towards tight sandstone and shale gas resources reservoirs that require fractures to be produced economically therefore techniques that can identify sets of aligned fractures are becoming more important fracture identification is also important in the areas of coal bed methane production carbon capture and storage ccs geothermal energy nuclear waste storage and mining in all these settings stress and pore pressure changes induced by engineering activity can generate or reactivate faults and fractures p and s waves are emitted by such microseismic events which can be recorded on downhole geophones the presence of aligned fracture sets generates seismic anisotropy which can be identified by measuring the splitting of the s waves emitted by microseismic events the raypaths of the s waves will have an arbitrary orientation controlled by the event and geophone locations meaning that the anisotropy system may only be partly illuminated by the available arrivals therefore to reliably interpret such splitting measurements it is necessary to construct models that compare splitting observations with modelled values allowing the best fitting rock physics parameters to be determined commonly splitting measurements are inverted for one fracture set and rock fabrics with a vertical axis of symmetry in this paper we address the challenge of identifying multiple aligned fracture sets using splitting measured on microseismic events we analyse data from the weyburn ccs eor reservoir which is known to have multiple fracture sets and from a hydraulic fracture stimulation where it is believed that only one set is present we make splitting measurements on microseismic data recorded on downhole geophone arrays our inversion technique successfully discriminates between the single and multiple fracture cases and in all cases accurately identifies the strikes of fracture sets previously imaged using independent methods borehole image logs core samples microseismic event locations we also generate a synthetic example to highlight the pitfalls that can be encountered if it is assumed that only one fracture set is present when splitting data are interpreted when in fact more than one fracture set is contributing to the anisotropy

the effect of microstructure and nonlinear stress on anisotropic seismic velocities recent work in hydrocarbon reservoir monitoring has focused on developing coupled geomechanical fluid flow simulations to allow production related geomechanical effects such as compaction and subsidence to be included in reservoir models to predict realistic time lapse seismic signatures generation of appropriate elastic models from geomechanical output is required these elastic models should include not only the fluid saturation effects of intrinsic shape induced and stress induced anisotropy but also should incorporate nonlinear stress dependent elasticity to model nonlinear elasticity we use a microstructural effective medium approach in which elasticity is considered as a function of mineral stiffness and additional compliance is caused by the presence of low aspect ratio displacement discontinuities by jointly inverting observed ultrasonic p and s wave velocities to determine the distribution of such discontinuities we assessed the appropriateness of modeling them as simple planar penny shaped features by using this approximation we developed a simple analytical approach to predict how seismic velocities will vary with stress we tested our approach by analyzing the elasticity of various sandstone samples from a united kingdom continental shelf ukcs reservoir some of which display significant anisotropy as well as two data sets taken from the literature

a comparison of passive seismic monitoring of fracture stimulation from water and co2 injection hydraulic fracturing is used to create pathways for fluid migrationandtostimulateproduction usually wateristheinjected fluid although alternative fluids such as carbon dioxide co2 have been used recently the amount of fracturing that co2 can induce is also of interest for the security of carbon capture and storage hydraulic fracturing is usually monitored using passive seismic arrays detecting microseismic events generated by the fracturing it is of interest to compare the amount of seismicity that co2 injection can generate in comparison with water with this in mind we have analyzed a passive seismic data set monitoring the injection of water and supercritical co2 under very similar conditions allowing us to make a direct comparison between the fluids we examined event locations and event magnitudes and we used shear wave splitting to image the fractures that are generated for both fluids the event locations map the formation of fractures moving away from the injection well with normalsparalleltotheminimumprincipalstress theeventsduring water injection are limited to the injection depth while during co2 injection activity migrates above the injection depth eventmagnitudesaresimilarinbothcases andlargereventmagnitudes appear to correlate with higher injection pressures shear wave splitting suggests that water injection generates more fractures though the data quality is not good enough to make a robust conclusion about this the comparability between water and co2 injection means that lessons can be learned from theabundantexperienceofconventionalwaterinjection