statistical modeling and characterization of induced seismicity within the western canada sedimentary basin in western canada there has been an increase in seismic activity linked to anthropogenic energy related operations including conventional hydrocarbon production wastewater fluid injection and mor

influence of pore pressure on the event size distribution of induced earthquakes 1 during an enhanced geothermal system egs experiment fluid is injected at high pressure into crystalline rock to enhance its permeability and thus create a reservoir from which geothermal heat can be extracted the fracturing of the basement caused by these high pore pressures is associated with microseismicity however the relationship between the magnitudes of these induced seismic events and the applied fluid injection rates and thus pore pressure is unknown here we show how pore pressure can be linked to the seismic frequency magnitude distribution described by its slope theb value we evaluate the dataset of an egs in basel switzerland and compare the observed event size distribution with the outcome of a minimalistic model of pore pressure evolution that relates event sizes to the differential stress d we observe that the decrease of b values with increasing distance of the injection point is likely caused by a decrease in pore pressure this leads to an increase of the probability of a large magnitude event with distance and time

statistical analysis of the induced basel 2006 earthquake sequence introducing a probability based monitoring approach for enhanced geothermal systems summary geothermal energy is becoming an important clean energy source however the stimulation of a reservoir for an enhanced geothermal system egs is associated with seismic risk due to induced seismicity seismicity occurring due to the water injection at depth have to be well recorded and monitored to mitigate the seismic risk of a damaging event an appropriate alarm system needs to be in place for each individual experiment in recent experiments the so called traffic light alarm system based on public response local magnitude and peak ground velocity was used we aim to improve the pre defined alarm system by introducing a probability based approach we retrospectively model the ongoing seismicity in real time with multiple statistical forecast models and then translate the forecast to seismic hazard in terms of probabilities of exceeding a ground motion intensity level one class of models accounts for the water injection rate the main parameter that can be controlled by the operators during an experiment by translating the models into time varying probabilities of exceeding various intensity levels we provide tools which are well understood by the decision makers and can be used to determine thresholds non exceedance during a reservoir stimulation this however remains an entrepreneurial or political decision of the responsible project coordinators we introduce forecast models based on the data set of an egs experiment in the city of basel between 2006 december 2 and 8 approximately 11500 m 3 of water was injected into a 5km deep well at high pressures a six sensor borehole array was installed by the company geothermal explorers limited gel at depths between 300 and 2700 m around the well to monitor the induced seismicity the network recorded approximately 11200 events during the injection phase more than 3500 of which were located with the traffic light system actions where implemented after an ml 2 7 event the water injection was reduced and then stopped after another ml 2 5 event a few hours later an earthquake with ml 3 4 felt within the city occurred which led to bleed off of the well a risk study was later issued with the outcome that the experiment could not be resumed we analyse the statistical features of the sequence and show that the sequence is well modelled with the omori utsu law following the termination of water injection based on this model the sequence will last 31 29 14 years to reach the background level we introduce statistical models based on reasenberg and jones and epidemic type aftershock sequence etas models commonly used to model aftershock sequences we compare and test different model setups to simulate the sequences varying the number of fixed and free parameters for one class of the etas models we account for the flow rate at the injection borehole we test the models against the observed data with standard likelihood tests and find the etas model accounting for the on flow rate to perform best such a model may in future serve as a valuable tool for designing probabilistic alarm systems for egs experiments

a slow earthquake in the santa maria basin california an m l 3 5 earthquake near santa maria california was recorded by the southern california seismic network and a terrascope station at santa barbara sbc on 31 january 1991 the waveform of this event is dominated by 2 to 5 sec waves and is different from that of ordinary events with similar size inquiries into operations in several oil fields in the area revealed that hydro fracturing at a pressure of about 80 bars was being done at a depth of 100 to 300 m in the orcutt oil field in the santa maria basin from about 9 to 11 a m on 31 january and the earthquake occurred in the afternoon field evidence of 30 cm displacement to a depth of 300 m was reported the field evidence as well as the first motion data indicates that the event had a thrust mechanism with the p axis in the nne ssw direction which is in agreement with the regional stress field from the analysis of the sbc record and the field evidence we conclude that the source must be shallower than 1 km and the ratio of the radiated energy to the seismic moment is about 6 2 10 7 one to two orders of magnitude smaller than that of ordinary earthquakes the occurrence of this earthquake demonstrates that release of regional tectonic stress in shallow sediments can yield significant seismic radiation at periods of a few seconds the period range of engineering importance for large structures and has important implications for excitation of long period ground motions from large earthquakes in sedimentary basins

the tectonic crustal stress field and style of faulting along the pacific north america plate boundary in southern california we invert for the state of stress in the southern california crust using a catalogue of high quality earthquake focal mechanisms 1981 2010 the stress field is best resolved where seismicity rates are high and sufficient data are available to constrain the stress field across most of the region from the stress field we determine the maximum horizontal compressive stress s hmax orientations and the style of faulting across southern california the trend of s hmax exhibits significant regional and local spatial heterogeneities the regional trend of s hmax varies from north along the san andreas system to nne to the east in the eastern california shear zone as well as to the west within the continental borderland and the western transverse ranges the transition zones from one state of stress to the other occur over a distance of only a few kilometres following a trend from yucca valley to imperial valley to the east and the western edge of the peninsular ranges to the west the local scale heterogeneities in the s hmax trend include nnw trends along the san andreas fault near cajon pass tejon pass and the cucapah range as well as nne trends near the northern san jacinto fault and the wheeler ridge area the style of faulting exhibits similar complexity ranging from predominantly normal faulting in the high sierra nevada to strike slip faulting along the san andreas system to three consecutive bands of thrust faulting in the wheeler ridge area and the western transverse ranges the local variations in the style of faulting include normal faulting at the north end of the san jacinto fault and scattered areas of thrust faulting the regional variations in the shmax trends are very similar to the pattern of the gps measured maximum shortening axes of the surface strain rate tensor field although the strain field tends to be smoother and appears to capture some of the upper mantle deformation field the mean trend of s hmax departs about approximately 14 to the east from the trend of the maximum shortening directions derived from anisotropy in the upper mantle

stress heterogeneity and its impact on seismicity pattern along the equatorial bifurcation zone of the great sumatran fault indonesia abstract a stress transfer modeling revealed the stress heterogeneity and its correlation with the seismicity pattern in the bifurcation zone of the great sumatran fault gsf where the gsf is divided into two separate sub parallel segments up to 35 km apart i e angkola and barumun forming a strike slip duplex at around equator the analysis was conducted using a seismic catalog compiled from five institutions i e the indonesian agency for meteorology climatology and geophysics bmkg united states geological survey usgs harvard and global centroid moment tensor gcmt international seismological center isc and geoforschungzentrum gfz germany in the period of 1970 2017 our frequency magnitude distribution analysis indicates that this zone has a high stress regime especially at the tip of the bifurcation zone in which 20 clockwise rotation of principal stress and 20 increase in stress ratio are observed from the stress inversion consequently events with magnitude larger than 6 0 are concentrated in this area in addition our stress transfer modeling reveals the mechanism of the stress heterogeneity at the bifurcation zone and a possible interaction between segment ruptures two main internal structures perpendicular to the strike slip duplex are also observed from the displacement vector due to its high stress concentration and its close distance to populous cities of padang sidempuan bukittinggi solok and padang panjang in western sumatra the tip of gsf bifurcation poses a big potential of seismic hazard the information of the principal stress rotation and stress ratio obtained in this study can be used further to assess the potential of great ruptures and estimate the possible damage in the study area

homogeneity of small scale earthquake faulting stress and fault strength small scale faulting at seismogenic depths in the crust appears to be more homogeneous than previously thought i study three new high quality focal mechanism datasets of small m 3 earthquakes in southern california the east san francisco bay and the aftershock sequence of the 1989 loma prieta earthquake i quantify the degree of mechanism variability on a range of length scales by comparing the hypocentral distance between every pair of events and the angular difference between their focal mechanisms closely spaced earthquakes interhypocentral distance 2 km tend to have very similar focal mechanisms often identical to within the 1 sigma uncertainty of 25 this observed similarity implies that in small volumes of crust while faults of many orientations may or may not be present only similarly oriented fault planes produce earthquakes contemporaneously on these short length scales the crustal stress orientation and fault strength coefficient of friction are inferred to be homogeneous as well to produce such similar earthquakes over larger length scales 2 50 km focal mechanisms become more diverse with increasing interhypocentral distance differing on average by 40 70 mechanism variability on 2 to 50 km length scales can be explained by relatively small variations 30 in stress or fault strength it is possible that most of this small apparent heterogeneity in stress or strength comes from measurement error in the focal mechanisms as negligible variation in stress or fault strength 10 is needed if each earthquake is assigned the optimally oriented focal mechanism within the 1 sigma confidence region this local homogeneity in stress orientation and fault strength is encouraging implying it may be possible to measure these parameters with enough precision to be useful in studying and modeling large earthquakes

off fault damage and acoustic emission distributions during the evolution of structurally complex faults over series of stick slip events variations in fault structure for example surface roughness and deformation zone width influence the location and dynamics of large earthquakes as well as the distribution of small seismic events in nature changes in fault roughness and seismicity characteristics can rarely be studied simultaneously so that little is known about their interaction and evolution here we investigate the connection between fault structure and near fault distributions of seismic events over series of stick slip cycles in the laboratory we conducted a set of experiments on rough faults that developed from incipient fracture surfaces we monitored stress and seismic activity which occurred in the form of acoustic emissions aes we determined ae density distributions as a function of fault normal distance based on high accuracy hypocentre locations during subsequent interslip periods the characteristics of these distributions were closely connected to different structural units of the faults that is the fault core off fault and background damage zone the core deformation zone was characterized by consistently high seismic activity whereas the off fault damage zone displayed a power law decay of seismic activity with increasing distance from the fault core the exponents of the power law distributed off fault activity increased with successive stick slip events so that later interslip periods showed a more rapid spatial decay of seismic activity from the fault the increase in exponents was strongest during the first one to three interslip periods and reached approximately constant values thereafter the relatively rapid spatial decay of ae events during later interslip periods is likely an expression of decreasing fault zone complexity and roughness our results indicate a close relationship between fault structure stress and seismic off fault activity a more extensive mapping of seismic off fault activity decay has the potential to significantly advance the understanding of fault zone properties including variations in fault roughness and stress

stress tensor changes related to fluid injection at the geysers geothermal field california 1 studying variations of the stress field in reservoirs caused by massive fluid injection is important toward an improved understanding of geomechanical processes involved we report on spatio temporal variations of the local stress tensor orientation at the geysers geothermal field california we apply two stress inversion methods with detailed uncertainty assessments using a selection of events recorded between 2007 and 2012 our results clearly indicate variations in the orientation of the principal stress axes for the reservoir as a whole showing a normal faulting regime at the reservoir depth between 2 and 3 7 km bounded by a strike slip regime above and below analyzing the temporal evolution of the stress tensor orientation for a prominent seismicity cluster we observe a clear correlation of changes in orientation for 1 3 with the highest injection rates these results suggest that temporal changes in the stress tensor orientation could contribute to characterize reservoirs during stimulation

the spatial footprint of injection wells in a global compilation of induced earthquake sequences fluid injection can cause extensive earthquake activity sometimes at unexpectedly large distances appropriately mitigating associated seismic hazards requires a better understanding of the zone of influence of injection we analyze spatial seismicity decay in a global dataset of 18 induced cases with clear association between isolated wells and earthquakes we distinguish two populations the first is characterized by near well seismicity density plateaus and abrupt decay dominated by square root space time migration and pressure diffusion injection at these sites occurs within the crystalline basement the second population exhibits larger spatial footprints and magnitudes as well as a power law like steady spatial decay over more than 10 kilometers potentially caused by poroelastic effects far reaching spatial effects during injection may increase event magnitudes and seismic hazard beyond expectations based on purely pressure driven seismicity

a comparison of seismicity rates and fluid injection operations in oklahoma and california implications for crustal stresses fluid injection into deep wellbores can increase pore pressure reduce effective stress and trigger earthquakes the extent of the seismogenic response to injection provides insight into how close faults are to failure in the injection affected area the seismogenic response to injection operations in hydrocarbon basins is examined in california and oklahoma changes in spatial and temporal seismicity rates are tested for significant variations and timing and location of such variations are determined based on nonparametric modeling of background seismicity rates oklahoma has experienced a recent surge of seismic events which exceeded the 95 confidence limit of poissonian background rates in c 2010 annual injection volumes in oklahoma increased systematically between 1998 and 2013 and have been connected to several earthquake sequences in california injection volumes increased monotonically between 2001 and 2009 however the seismogenic response was limited and was devoid of large scale background rate increase a detailed comparison of injection parameters in oklahoma and california included well density wellhead pressures peak and cumulative rates and injection depths no detectable difference was found that could readily explain the observed changes in seismicity rate in oklahoma and the lack thereof in california a strongly different seismogenic response to similar pressure perturbations indicates that the injection parameters considered are only of secondary influence on the resulting earthquake activity the primary controls on injection induced earthquakes might be the specific geologic setting and the stress state on nearby faults

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

seismicity in deep gold mines of south africa implications for tectonic earthquakes we have studied induced seismicity associated with five deep gold mines located in the far west rand district republic of south africa focusing on the digital data recorded from january 1994 until february 2000 by in mine arrays of three component geophones the observed seismicity which exceeds 1 000 events per day can be divided into two kinds of events designated as types a and b type a events are tightly clustered in time and space and generally occur within 100 m of an active mining face or development tunnel their spectra are comparatively enriched in high frequencies and they have an upper moment magnitude cutoff at mmax 1 we associate these events with the fracture dominated rupture of com petent rock induced by dynamic stresses during blasting and quasi static stress per turbations from the excavation and closure of individual stopes in contrast type b events are temporally and spatially distributed throughout the active mining region they represent friction dominated slip in existing shear zones such as faults or dikes and have source scaling properties that agree well with extrapolations from tectonic earthquakes in the far west rand region type b events can have large magnitudes m 3 but they show a distinct lower magnitude cutoff at mmin 0 we interpret this cutoff in terms of a critical patch size for nucleation of shear failure and we show that the data are consistent with a rate and state dependent friction model in which the critical slip distance d c 10 4 m both the spectral predictions of this model and accelerograms of type b events agree that f max 200 hz

earthquakes induced by water injection at 3 km depth within the rongchang gas field chongqing china 1 unwanted water amounting more than 1 million m 3 has been injected intermittently at a pumping pressure of 2 1 2 9 mpa over hydrostatic at 2 6 2 9 km depth within the rongchang gas field western chongqing china since july 1988 the injections have induced more than 32 000 surface recorded earthquakes including 2 of m l 5 14 of m l 4 and more than 100 of m l 3 up until the end of 2006 we examined the epidemic type aftershock sequence etas model and the temporal evolution of several statistics of the earthquake sequence the random component in the etas model can be considered as an indicator of fluid driven activity while the rate of omori law type aftershocks indicates stress triggering of the preceding earthquakes we observed three distinct seismic activity phases that were coincident with different injection periods phase i corresponds to initial injections with low injection rate long time interval and shows a vast majority of omori type events and only 8 externally forced activity phase ii corresponds to a period of high injection rates and shows intense earthquake activity characterized by a high percentage 45 of fluid driven earthquakes phase iii demonstrates decreasing seismicity as a result of a lowering in the injection rate more than 70 of earthquakes occurred randomly over time indicating the dominance of external triggering the physical mechanism that induced the rongchang earthquake sequence was a change in coulomb s failure stress caused by pore pressure diffusion and transformed from seismic slip of preceding earthquakes a pressure increase of 2 1 2 9 mpa at the injection well is more than sufficient to induce failure within the local formations

why large earthquakes do not nucleate at shallow depths although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer because frictional strength and stress drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth we have examined spontaneous rupture models with gradients of both average stress drop and average strength relative to ambient stress to determine whether this explains the observed phenomena we report here that ruptures that are nucleated with in the low stress drop shallow regions of the model are inhibited from propagating but those that are nucleated within the high stress drop regions can propagate over the entire fault plane

the 2016 mw5 1 fairview oklahoma earthquakes evidence for long range poroelastic triggering at 40 km from fluid disposal wells abstract wastewater disposal in the central u s is likely responsible for an unprecedented surge in earthquake activity much of this activity is thought to be driven by induced pore pressure changes and slip on pre stressed faults which requires a hydraulic connection between faults and injection wells however direct pressure effects and hydraulic connectivity are questionable for earthquakes located at large distances and depths from the injectors here we examine triggering mechanisms of induced earthquakes which occurred at more than 40 km from wastewater disposal wells in the greater fairview region northwest oklahoma employing numerical and semi analytical poroelastic models the region exhibited few earthquakes before 2013 when background seismicity started to accelerate rapidly culminating in the mw5 1 fairview earthquake in february 2016 injection rates in the 2 2 5 km deep arbuckle formation started to increase rapidly in 2012 about two years before the start of seismicity increase most of the injection activity was concentrated toward the northeast of the study region generating a relatively cohesive zone of pressure perturbations between 0 1 and 1 mpa much of the near injection seismicity was likely triggered by pressure effects and fault assisted pressure diffusion to seismogenic depth outside of the high pressure zone we observed two remarkably detached linear seismicity clusters which occurred at 20 to 50 km distance from the initial seismicity and 10 to 40 km from the nearest high rate injector semi analytical models reveal that poroelastically induced coulomb stress changes surpass pore pressure changes at these distances providing a plausible triggering mechanism in the far field of injection wells these results indicate that both pore pressures and poroelastic stresses can play a significant role in triggering deep and distant earthquakes by fluid injection and should be considered for seismic hazard assessment beyond the targeted reservoir

induced seismicity associated with fluid injection into a deep well in youngstown ohio 1 over 109 small earthquakes mw 0 4 3 9 were detected during january 2011 to february 2012 in the youngstown ohio area where there were no known earthquakes in the past these shocks were close to a deep fluid injection well the 14 month seismicity included six felt earthquakes and culminated with a mw 3 9 shock on 31 december 2011 among the 109 shocks 12 events greater than mw 1 8 were detected by regional network and accurately relocated whereas 97 small earthquakes 0 4 mw 1 8 were detected by the waveform correlation detector accurately located earthquakes were along a subsurface fault trending ene wsw consistent with the focal mechanism of the main shock and occurred at depths 3 5 4 0 km in the precambrian basement we conclude that the recent earthquakes in youngstown ohio were induced by the fluid injection at a deep injection well due to increased pore pressure along the preexisting subsurface faults located close to the wellbore we found that the seismicity initiated at the eastern end of the subsurface fault close to the injection point and migrated toward the west away from the wellbore indicating that the expanding high fluid pressure front increased the pore pressure along its path and progressively triggered the earthquakes we observe that several periods of quiescence of seismicity follow the minima in injection volumes and pressure which may indicate that the earthquakes were directly caused by the pressure buildup and stopped when pressure dropped

human impact on the environment in the ethiopian and eritrean highlands a state of the art abstract this review analyses the environmental evolution of the ethiopian highlands in the late quaternary the late pleistocene 20 000 12 000 14c years bp was cold and dry with 1 low lake levels in the rift valley 2 large debris fans on the flanks of lake abhe basin and 3 the blue nile transporting coarse bedload then a period with abundant and less seasonal rains existed between 11 500 and 4800 14c years bp as suggested by increased arboreal pollen high river and lake levels low river turbidities and soil formation around 5000 4800 14c years bp there was a shift to more arid conditions and more soil erosion many phenomena that were previously interpreted as climate driven might however well be of anthropic origin thick sediment deposits on pediments as well as an increase of secondary forest scrub and ruderal species in pollen diagrams are witnesses of this human impact one important aspect of the late quaternary palaeoenvironment is unclear changes in nile flow discharges and rift valley lake levels have been linked to changes in precipitation depth most authors do not take into account changes in land use in the highlands nor changes in the seasonality of rain both of which can lead to a change in runoff coefficients tufa and speleothem deposition around 14 000 years ago tend to indicate that at the end of the last glacial maximum lgm conditions might have been wetter than generally accepted the most important present day geomorphic processes are sheet and rill erosion throughout the country gullying in the highlands and wind erosion in the rift valley and the peripheral lowlands based on existing sediment yield data for catchments draining the central and northern ethiopian highlands an equation was developed allowing to assess area specific sediment yields sy 2595a 0 29 n 20 r 2 0 59 where sy area specific sediment yield t km 2 year 1 and a drainage area km2 with respect to recent environmental changes temporal rain patterns apart from the catastrophic impact of dry years on the degraded environment cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere causes are changing land use and land cover which are expressions of human impact on the environment deforestation over the last 2000 3000 years was probably not a linear process in ethiopia studies on land use and land cover change show however a tendency over the last decades of increasing removal of remnant vegetation which is slowed down or reversed in northern ethiopia by a set aside policy ongoing land degradation requires urgent action at different levels of society soil and water conservation swc structures are now widely implemented local knowledge and farmer s initiatives are integrated with introduced swc techniques at various degrees impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion in high rain areas runoff management requires greater emphasis during the design of soil conservation structures in such areas investment in swc might not be profitable at farm level although benefits for society are positive this pleads in favour of public support the present land degradation in the ethiopian highlands has a particular origin which includes poverty and lack of agricultural intensification causes of these are to be found in the nature of past and present regional social relations as well as in international unequal development this review strengthens our belief that under improved socio economic conditions land husbandry can be made sustainable leading to a reversal of the present desertification and land degradation of the ethiopian highlands

optimizing multi station earthquake template matching through re examination of the youngstown ohio sequence abstract a series of earthquakes in 2011 near youngstown oh has been a focal point for discussions of seismicity induced by a nearby wastewater disposal well utilizing an efficient waveform template matching procedure the optimal correlation template to study the youngstown sequence was identified by varying parameters such as the stations utilized frequency passband and seismogram length a catalog composed of 566 events was identified between january 2011 and february 2014 double difference relocation refines seismicity to a 800 m linear streak from the northstar 1 injection well to the wsw along the same strike as the fault plane of the largest event calculated gutenberg richter b values are consistent with trends observed in other regions with seismicity induced by fluid injection

the absence of remotely triggered seismicity in japan remote dynamic triggering occurred in the western united states and greece following the following the denali hector mine landers and izmit main shocks in seismically active largely geothermal areas that experienced shaking from the mainshock typically exceeding 0 2 cm sec because triggered earthquakes are small often below mw 2 investigations of whether remote triggering is a generally occurring phenomena must be concentrated in well instrumented locations as a seismically active geothermal well instrumented location japan constitutes an ideal location for such study we systematically examine whether remote dynamic trig gering occurs following mainshocks in japan producing amplitudes in the study region above 0 2 cm sec at distances greater than two estimated fault lengths from 1998 to 2004 we use both catalog data and filtered waveforms to search for seis micity increases one significant regional increase follows a mw 7 6 mainshock in march 2000 largely due to a volcanic swarm that began the day before and increased in intensity in the following days two localized increases in cataloged seismicity occur in kyushu which is the only area in the study region with onshore tectonic extension two factors possibly inhibit regional triggering in japan 1 compres sional tectonics and 2 the frequent occurrence of large mainshocks the ambient seismicity in japan combined with the lack of observable widespread triggering ren ders rate state friction and stress corrosion to be improbable triggering mechanisms fracture unclogging as a triggering mechanism is consistent with the observations

an objective method for the assessment of fluid injection induced seismicity and application to tectonically active regions in central california changes in seismicity rates whether of tectonic or of induced origin can readily be identified in regions where background rates are low but are difficult to detect in seismically active regions we present a novel method to identify likely induced seismicity in tectonically active regions based on short range spatiotemporal correlations between changes in fluid injection and seismicity rates the method searches through the entire parameter space of injection rate thresholds and determines the statistical significance of correlated changes in injection and seismicity rates applying our method to kern county central california we find that most earthquakes within the region are tectonic however fluid injection contributes to seismicity in four different cases three of these are connected to earthquake sequences with events above m4 each of these sequences followed an abrupt increase in monthly injection rates of at least 15 000 m 3 the probability that the seismicity sequences and the abrupt changes in injection rates in kern county coincide by chance is only 4 the identified earthquake sequences display low gutenberg richter b values of 0 6 0 7 and at times systematic migration patterns characteristic for a diffusive process our results show that injection induced pressure perturbations can influence seismic activity at distances of 10 km or more triggering of earthquakes at these large distances may be facilitated by complex local geology and faults in tectonically active regions our study provides the first comprehensive statistically robust assessment of likely injection induced seismicity within a large tectonically active region

poroelastic stressing and induced seismicity near the lacq gas field southwestern france o hundreds of shallow small to moderate earthquakes have occurred near the lacq deep gas field in southwestern france since 1969 these earthquakes are clearly separated from tectonic seismicity occurring in the pyrenees 25 km to the southwest the induced seismicity began when the reservoir pressure had declined by 30 mpa repeated levehng over the field shows localized subsidence reaching a maximum of 60 mm in 1989 segall 1989 suggested that poroelastic stressing associated with volumetric contraction of the reservoir rocks is responsible for induced seismicity associated with fluid extraction to test this model we compare the observed subsidence and hypocentral distributions with the predicted displacement and stress fields we find that the relationship between average reservoir pressure drop and subsidence is remarkably linear lending support to the linear poroelastic model displacements and stresses are computed based on a priori knowledge of the reservoir geometry materim properties and reservoir pressure changes the computed vertical displacements are found to be in excellent agreement with the subsidence observed from leveling stress perturbations accompanying gas extraction computed using the same parameters are found to be 0 2 mpa or less changes in coulomb failure stress are computed assuming that shp occurs on optimally oriented planes the predicted failure zones correlate very well with the spatial distribution of earthquakes if the perturbing stresses are small in comparison to the ambient regional deviatoric stresses and if the minimum regional compresslye stress axis is vertical accurate determination of focal mechanisms of the induced events would allow a more rigorous test of the poroelastic model and could lead to important inferences about the crustal stress state

two year survey of earthquakes and injection production wells in the eagle ford shale texas prior to the mw4 8 20 october 2011 earthquake abstract between november 2009 and september 2011 the earthscope usarray program deployed 25 temporary seismograph stations on a 70 km grid in south central texas between 27 n 31 n and 96 w 101 w this area includes the eagle ford shale for decades this geographic region has produced gas and oil from other strata using conventional methods but recent developments in hydrofracturing technology has allowed extensive development of natural gas resources from within the eagle ford our study surveys small magnitude seismic events and evaluates their correlation with fluid extraction and injection in the eagle ford identifying and locating 62 probable earthquakes including 58 not reported by the u s geological survey the 62 probable earthquakes occur singly or in clusters at 14 foci of these foci two were situated near wells injecting recently increased volumes of water eight were situated near wells extracting recently increased volumes of oil and or water and four were not situated near wells reporting significant injection extraction increases thus in this region while the majority of small earthquakes may be triggered induced by human activity they are more often associated with fluid extraction than with injection we also investigated the m w 4 8 20 october 2011 fashing earthquake the largest historically reported earthquake in south central texas that occurred two weeks after the removal of the temporary usarray stations a field study indicated that the highest intensity mmi vi region was about 10 km south of 2010 2011 foreshock activity and that there were no high volume injection wells within 20 km of the mmi v vi region or the foreshocks however the 20 october 2011 earthquake did coincide with a significant increase in oil water extraction volumes at wells within the mmi v vi region and this was also true for previous earthquakes felt at fashing in 1973 and 1983 in contrast our study found significant increases in injection prior to an mblg3 6 20 july 1991 earthquake near falls city texas thus the eagle ford geographic region with seismic activity associated both with extraction and injection appears to be more complex than the barnett shale of northeast texas where a similar survey found possible correlations only with fluid injection

induced earthquake magnitudes are as large as statistically expected a major question for the hazard posed by injection induced seismicity is how large induced earthquakes can be are their maximum magnitudes determined by injection parameters or by tectonics deterministic limits on induced earthquake magnitudes have been proposed based on the size of the reservoir or the volume of fluid injected however if induced earthquakes occur on tectonic faults oriented favorably with respect to the tectonic stress field then they may be limited only by the regional tectonics and connectivity of the fault network in this study we show that the largest magnitudes observed at fluid injection sites are consistent with the sampling statistics of the gutenberg richter distribution for tectonic earthquakes assuming no upper magnitude bound the data pass three specific tests 1 the largest observed earthquake at each site scales with the log of the total number of induced earthquakes 2 the order of occurrence of the largest event is random within the induced sequence and 3 the injected volume controls the total number of earthquakes rather than the total seismic moment all three tests point to an injection control on earthquake nucleation but a tectonic control on earthquake magnitude given that the largest observed earthquakes are exactly as large as expected from the sampling statistics we should not conclude that these are the largest earthquakes possible instead the results imply that induced earthquake magnitudes should be treated with the same maximum magnitude bound that is currently used to treat seismic hazard from tectonic earthquakes

change of stress regime during geothermal reservoir stimulation earthquakes are induced by man made changes of the stress field by injection or withdrawal of fluids in hydrocarbon production geothermal exploitation and wastewater disposal however the actual perturbation of the stress field and stress release by injection induced seismicity remains largely unknown we provide evidence for currently not understood hydromechanical processes after shut in of the well we invert earthquake focal mechanisms from a massive stimulation to invert for stress resolved in time and depth to obtain changes of the stress orientation and magnitude prior information about fracture orientations from well logs is taken into account comparison with independent stress measures reveals that stresses obtained from inversion of fluid induced seismicity are highly perturbed and not representative of the initial stress field the horizontal stresses change by tens of megapascals turning the stress regime from transitional normal faulting strike slip faulting to pure normal faulting the observed stress changes are attributed to large scale aseismic deformation

identifying potentially induced seismicity and assessing statistical significance in oklahoma and california we develop a statistical method for identifying induced seismicity from large datasets and apply the method to decades of wastewater disposal and seismicity data in california and oklahoma the study regions are divided into gridblocks we use a longitudinal study design seeking associations between seismicity and wastewater injection volume along time series within each gridblock in each gridblock we find the maximum likelihood estimate for a model parameter that relates induced seismicity hazard to total volume of wastewater injected each year to assess significance we compute likelihood ratio test statistics in each gridblock and each state california and oklahoma resampling with permutation and random temporal offset of injection data is used to estimate p values from the likelihood ratio statistics we focus on assessing whether observed associations between injection and seismicity occur more often than would be expected by chance we do not attempt to quantify the overall incidence of induced seismicity the study is designed so that under reasonable assumptions the associations can be formally interpreted as demonstrating causality wastewater disposal is associated with other activities that can induce seismicity such as reservoir depletion therefore our results should be interpreted as finding seismicity induced by wastewater disposal and all other associated activities in oklahoma the analysis finds with extremely high confidence that seismicity associated with wastewater disposal has occurred in california the analysis finds moderate evidence that seismicity associated with wastewater disposal has occurred but the result is not strong enough to be conclusive

geochemistry geochronology and sr nd isotopic compositions of permian volcanic rocks in the northern margin of the north china block implications for the tectonic setting of the southeastern central asian orogenic belt the southeastern part of the central asian orogenic belt caob which records the collision of the north china block ncb with the south mongolian microcontinent is a key area for reconstructing the tectonic history of the caob controversy persists regarding the timing of the final structural amalgamation of the region therefore it remains unclear whether the late paleozoic thick volcanic successions were generated in a subduction or post orogenic environment redefining the age of the formation and analyzing the geochemical compositions of these volcanic rocks can provide clues regarding the regional tectonic evolution during the late paleozoic and place constraints on the closure time of the paleo asian ocean in this study we present geochemical geochronologic and sr nd isotopic data for 29 volcanic rock samples from the elitu formation in xianghuangqi central inner mongolia the elitu volcanic rocks have latest early to middle permian ages between 272 and 268 ma most of the mafic intermediate and felsic rocks show k normal and high k calc alkaline characteristics melting is considered to be due to large scale upwelling of the metasomatic lithospheric mantle and different degrees of melting of the thickened lower crust the northern margin of the ncb which represents the southeastern boundary of the caob records transtensional and subsequently extensional tectonics associated with late carboniferous to middle permian volcanic activity

soil landscape relationships in the basalt dominated highlands of tigray ethiopia article i nfo though knowledge about the distribution and properties of soils is a key issue to support sustainable land management existing knowledge of the soils in tigray northern ethiopian highlands is limited to either maps with a small scale or with a small scope the goal of this study is to establish a model that explains the spatial soil variability found in the may leiba catchment and to open the scope for extrapolating this information to the surrounding basalt dominated uplands a semi detailed scale 1 40000 soil survey was conducted in the catchment profile pits were described and subjected to physico chemical analysis and augerings were conducted this information was combined with information from aerial photographs and geological and geomorphologic observations the main driving factors that define the variability in soil types found were 1 geology through soil parent material and the occurrence of harder layers often acting as aquitards or aquicludes 2 different types of mass movements that occupy large areas of the catchment and 3 severe human induced soil erosion and deposition these factors lead to red black skeletic cambisol pellic vertisol catenas on basalt and calcaric regosol colluvic calcaric cambisols calcaric vertisol catenas on limestone the driving factors can be derived from aerial photographs this creates the possibility to extrapolate information and predict the soil distribution in nearby regions with a comparable geology a model was elaborated which enables the user to predict soil types using topography geomorphology geology and soil colours all of which can be derived from aerial photographs this derived model was later applied to other catchments and validated in the field

waveform relocated earthquake catalog for southern california 1981 to june 2011 we determine a new relocated catalog hys catalog 2011 for southern california from 1981 through june 2011 about 75 3 of the hypocenters are calculated with absolute and differential travel time picks and 24 7 could be relocated only by using absolute travel time picks with 3d or 1d velocity models the total catalog consists of more than 502 000 earthquakes in the region extending from baja california in the south to coalinga and owens valley in the north the catalog consists of three m 7 1 m 7 2 and m 7 3 mainshocks their foreshocks and aftershocks and background seismicity caused by tectonic and other processes in the southern california crust hypocenters in the new relocated catalog exhibit tighter spatial clustering of seismicity than does the routinely generated catalog and the depth distribution is tighter and reflects the thickness of the seismogenic zone more accurately compared to the standard catalog the relocated hypocenters are more easily related to other data sets such as mapped late quaternary faults

observations of static coulomb stress triggering of the november 2011 m5 7 oklahoma earthquake sequence in november 2011 a m5 0 earthquake occurred less than a day before a m5 7 earthquake near prague oklahoma which may have promoted failure of the mainshock and thousands of aftershocks along the wilzetta fault including a m5 0 aftershock the m5 0 foreshock occurred in close proximity to active fluid injection wells fluid injection can cause a buildup of pore fluid pressure decrease the fault strength and may induce earthquakes keranen et al 2013 links the m5 0 foreshock with fluid injection but the relationship between the foreshock and successive events has not been investigated here we examine the role of coseismic coulomb stress transfer on earthquakes that follow the m5 0 foreshock including the m5 7 mainshock we resolve the static coulomb stress change onto the focal mechanism nodal plane that is most consistent with the rupture geometry of the three m 5 0 earthquakes as well as specified receiver fault planes that reflect the regional stress orientation we find that coulomb stress is increased e g fault failure is promoted on the nodal planes of 60 of the events that have focal mechanism solutions and more specifically that the m5 0 foreshock promoted failure on the rupture plane of the m5 7 mainshock we test our results over a range of effective coefficient of friction values hence we argue that the m5 0 foreshock induced by fluid injection potentially triggered a cascading failure of earthquakes along the complex wilzetta fault system

testing the stress shadow hypothesis 1 a fundamental question in earthquake physics is whether aftershocks are predominantly triggered by static stress changes permanent stress changes associated with fault displacement or dynamic stresses temporary stress changes associated with earthquake shaking both classes of models provide plausible explanations for earthquake triggering of aftershocks but only the static stress model predicts stress shadows or regions in which activity is decreased by a nearby earthquake to test for whether a main shock has produced a stress shadow we calculate time ratios defined as the ratio of the time between the main shock and the first earthquake to follow it and the time between the last earthquake to precede the main shock and the first earthquake to follow it a single value of the time ratio is calculated for each 10 10 km bin within 1 5 fault lengths of the main shock epicenter large values of the time ratio indicate a long wait for the first earthquake to follow the main shock and thus a potential stress shadow whereas small values indicate the presence of aftershocks simulations indicate that the time ratio test should have sufficient sensitivity to detect stress shadows if they are produced in accordance with the rate and state friction model we evaluate the 1989 mw 7 0 loma prieta 1992 mw 7 3 landers 1994 mw 6 7 northridge and 1999 mw 7 1 hector mine main shocks for each main shock there is a pronounced concentration of small time ratios indicating the presence of aftershocks but the number of large time ratios is less than at other times in the catalog this suggests that stress shadows are not present by comparing our results to simulations we estimate that we can be at least 98 confident that the loma prieta and landers main shocks did not produce stress shadows and 91 and 84 confident that stress shadows were not generated by the hector mine and northridge main shocks respectively we also investigate the long hypothesized existence of a stress shadow following the 1906 san francisco bay area earthquake we find that while bay area catalog seismicity rates are lower in the first half of the twentieth century than in the last half of the nineteenth this seismicity contrast is also true outside of the bay area in regions not expected to contain a stress shadow this suggests that the rate change is due to a more system wide effect such as errors in the historical catalog or the decay of aftershocks of the larger 1857 fort tejon earthquake

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

mapping the frequency magnitude distribution in asperities an improved technique to calculate recurrence times we hypothesize that highly stressed asperities may be defined by mapping anomalously low b values along the san andreas fault near parkfield the asperity under middle mountain with its b 0 46 can be distinguished from all other parts of the fault surface likewise along the calaveras fault the northern asperity of the morgan hill 1984 m6 2 rupture can be identified by its low b of 0 5 as a high stress patch along the fault we add further evidence to the observations that the b value of the frequency magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the parkfield segment of the san andreas and along the calaveras fault in both of these areas b values above and below 5 km depth are 1 2 and 0 8 respectively we propose that probabilistic recurrence times tr based on the seismicity parameters a and b should be calculated from their values within asperities only instead of from the values of the entire rupture area of the maximum expected earthquake the strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone which slips along passively when an asperity breaks therefore no information on tr is contained in the passive fault segments only in the asperities at parkfield the probabilistic estimates of tr derived from the data in the whole rupture and in the asperity only are 72 18 24 and 23 12 18 years respectively compared to the historically observed repeat time of 22 years at morgan hill the tr estimates are 122 46 76 and 78 47 110 years respectively compared to the observed repeat time of 72 years

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

age petrogenesis and tectonic implications of early devonian bimodal volcanic rocks in the south altyn nw china abstract in this paper we report zircon u pb dating hf isotopes geochemical and sr nd isotopic data with the aim the petrogenesis and regional tectonic evolution of early devonian bimodal volcanic rock in the south altyn nw china new la icpms zircon u pb isotopic data constrained them at ca 406 ma the mafic samples are characterized by high fe cr and ni contents low ti and mg contents slightly enriched lree patterns and low la yb n la nb and la ta ratios and positive e nd t values 3 3 to 3 4 indicating that they were likely derived from strong batch melting of the asthenosphere in the spinel facies field the felsic rocks show an a type affinity with high alkalis fe ga zr nb ce and y contents low mg sr content high rb sr and ga al ratios enrichment in lile e g rb k th u and lree and depletion in ba sr nb ta p and ti and fractionated ree patterns with very strong negative eu anomalies these features along with distinct e nd t values 0 5 to 2 3 and mostly positive e hf t 0 29 to 5 18 indicate that the felsic rocks were mainly generated by partial melting of the crust in low pressure and high temperature conditions and simultaneously underwent slight magma mixing of such melts with mantle magma according to the petrogenetic schemes and geological background of the early devonian bimodal volcanic rocks tholeiite and a type dacite rhyolite they should have formed in a post collisional extensional setting moreover on the basis of spatial and temporal distribution and formation mechanism the tectonic magmatic evolution of the early paleozoic south altyn tagh could be divided into three stages 505 472 ma continental collision the magmatite formed under high pressure conditions due to the deep subduction and initial tearing of continental slab 467 450 ma continental slab break off the magmatite formed at high temperature and low pressure in virtue of felsic upper crust uplifting and mantle magma underplating and 432 385 ma post collisional extension the magmatite consists of a type granites and bimodal volcanic rocks which are the products of the interaction between mantle and crust and asthenosphere upwelling

triggering of the 1999 mw 7 1 hector mine earthquake by aftershocks of the 1992 mw 7 3 landers earthquake 1 there is strong observational evidence that the 1999 mw 7 1 hector mine earthquake in the mojave desert california was triggered by the nearby 1992 mw 7 3 landers earthquake many authors have proposed that the landers earthquake directly stressed the hector mine fault our model of the landers aftershock sequence however suggests there is an 85 chance that the hector mine hypocenter was actually triggered by a chain of smaller earthquakes that was initiated by the landers main shock we perform our model simulations using the monte carlo method based on the gutenberg richter relationship omori s law bath s law and assumptions that all earthquakes including aftershocks are capable of producing aftershocks and that aftershocks produce their own aftershocks at the same rate that other earthquakes do in general our simulations show that if it has been more than several days since an m 7 main shock most new aftershocks will be the result of secondary triggering these secondary aftershocks are not physically constrained to occur where the original main shock increased stress this may explain the significant fraction of aftershocks that have been found to occur in main shock stress shadows in static coulomb stress triggering studies

acoustic emissions document stress changes over many seismic cycles in stick slip experiments 1 the statistics of large earthquakes commonly involve large uncertainties due to the lack of long term robust earthquake recordings small scale seismic events are abundant and can be used to examine variations in fault structure and stress we report on the connection between stress and microseismic event statistics prior to the possibly smallest earthquakes those generated in the laboratory we investigate variations in seismic b value of acoustic emission events during the stress buildup and release on laboratory created fault zones we show that b values mirror periodic stress changes that occur during series of stick slip events and are correlated with stress over many seismic cycles moreover the amount of b value increase associated with slip events indicates the extent of the corresponding stress drop consequently b value variations can be used to approximate the stress state on a fault a possible tool for the advancement of time dependent seismic hazard assessment citation goebel t h w d schorlemmer t w becker g dresen and c g sammis 2013 acoustic emissions document stress changes over many seismic cycles in stick slip experiments geophys res lett 40 2049 2054 doi 10 1002 grl 50507

anthropogenic seismicity rates and operational parameters at the salton sea geothermal field geothermal power is a growing energy source however efforts to increase production are tempered by concern over induced earthquakes although increased seismicity commonly accompanies geothermal production induced earthquake rate cannot currently be forecast on the basis of fluid injection volumes or any other operational parameters we show that at the salton sea geothermal field the total volume of fluid extracted or injected tracks the long term evolution of seismicity after correcting for the aftershock rate the net fluid volume extracted injected provides the best correlation with seismicity in recent years we model the background earthquake rate with a linear combination of injection and net production rates that allows us to track the secular development of the field as the number of earthquakes per fluid volume injected decreases over time