source processes of industrially induced earthquakes at the geysers geothermal area california microearthquake activity at the geysers geothermal area california mirrors the steam production rate suggesting that the earthquakes are industrially induced a 15 station network of digital three component seismic stations was operated for one month in 1991 and 3 900 earthquakes were recorded highly accurate moment tensors were derived for 30 of the best recorded earthquakes by tracing rays through tomographically derived 3 d vp and vp vs structures and inverting p and s wave polarities and amplitude ratios the orientations of the p and t axes are very scattered suggesting that there is no strong systematic deviatoric stress field in the reservoir which could explain why the earthquakes are not large most of the events had significant non double couple non dc components in their source mechanisms with volumetric components up to 30 of the total moment explosive and implosive sources were observed in approximately equal numbers and must be caused by cavity creation or expansion and c

the northeastern ohio earthquake of 31 january 1986 was it induced abstract on 31 january 1986 at 11 46 est an earthquake of m b 5 0 occurred about 40 km east of cleveland ohio and about 17 km south of the perry nuclear power plant the earthquake was felt over a broad area including 11 states the district of columbia and parts of ontario canada caused intensity vi vii at distances of 15 km and generated relatively high accelerations 0 18 g of short duration at the perry plant thirteen aftershocks were detected as of 15 april with six occurring within the first 8 days two of the aftershocks were felt magnitudes for the aftershocks ranged from about 0 5 to 2 5 focal depths for all of the earthquakes ranged from 2 to 6 km except for one small earthquake all of the aftershocks occurred in a very tight cluster with a north northeast orientation focal mechanisms of the aftershocks exhibit predominantly oblique right slip motion on nearly vertical nodal planes oriented n15 to 45 e with a nearly horizontal p axis north of east three deep waste disposal wells are currently operating within 15 km of the epicentral region and have been responsible for the injection of nearly 1 2 billion liters of fluid at pressures reaching 112 bars above ambient at a nominal depth of 1 8 km estimates of stress inferred from commercial hydrofracturing measurements suggest that the state of stress in northeastern ohio is close to the theoretical threshold for failure along favorably oriented preexisting fractures this implies that effective stress conditions near the bottom of the two most active wells may be at or near the critical level for incipient failure two and possibly three earthquakes have occurred within less than 5 km from the wells since 1983 the relative distance to the main shock epicenter and its aftershocks about 12 km the lack of large numbers of small earthquakes typical of many induced sequences the history of small to moderate earthquakes in the region prior to the initiation of injection and the attenuation of the pressure field with distance from the injection wells however all argue for a natural origin for the 1986 earthquakes in contrast the proximity to failure conditions at the bottom of the well and the probable spatial association of at least one earthquake suggest that triggering by well activities cannot be precluded

plate dynamics near divergent boundaries geophysical implications of postrifting crustal deformation in ne iceland the bulk of a tectonic plate is thought to move continously at a rate consistent with the geologic average on the other hand movements are highly episodic at plate boundaries we study the plate dynamics that relate to these two different modes by modelling the displacements observed using the global positioning system in northeast iceland 1987 1990 these observations were made about 10 years after an episodic divergent movement between the north american and eurasian plates 1975 1981 the horizontal displacement field fits well a two dimensional model of postrupture stress relaxation assuming a thin elastic layer overlying a layer of newtonian viscosity this analysis indicates values of about 10 m2 s for the stress diffusivity and 0 3 2 1018 pa s for the newtonian viscosity of the lower layer however no significant correlation exists between the observed and modeled vertical displacements probably because of the relative inaccuracy of the vertical component observations assuming that contemporary plate motion is the sum of many displacements that have diffused from boundaries where episodic displacements occur periodically we simulate the spatial transition from episodic to continuous plate movements the plate boundary zone where movements are episodic or quasi episodic is of the order of 100 km wide depending on the stress diffusivity which may be fairly uniform throughout the world and the frequency of episodic movements

an induced seismicity experiment across a creeping segment of the philippine fault the location of seismicity induced by forced fluid flow provides information about domains of pore pressure variation while changes in fluid content are identified through changes in seismic velocity these effects have been investigated in the geothermal field of rrongona n which lies on a creeping portion of the philippine falllt on levte island t ocally the left lateral strike slip philippine fa ult branches out into three subparallel segments eastern central and western fault lines in june jnly 1997 a water stinlulation was undertaken in a well that intersects the central f ault line l980 m below grour tl surface 36 000 m3 were iriject ed between the casing shoe at 1308 m and the well bottom at 2177 m the seismicity was monitored with a surface station nctwork of 18 sta iions more than 400 events induced by the injection cxpcrimcrlt as well as by r outine injections associated with the gcothcrrnal field exploitation were recnrded in the vicinity of the well they have been located througlr 3 sim llt aneol s t hr e dimensior d 3 d velocity hypocent er invervior procedure none of the microearthquakes are locateti along the central fault line they all occurred below the casing shoe to the east of thr farllt line i e within thc gcothcrmal reservoir and m ostly below thr bottom of the well results from the injcct ion experiment and the 18 rnonths of seismic monitoring along thc c erlt ral and west fa uit lirics suggest an aseismic behavior of this major continental fa llt at this localion the 3 d velocily n odel determirled lrorr thc lravel lirrle ir versioll fur seismic erents observed during iruecliu s is cun pared lo lhat ub1ai11t d rrom seismic monitoring conducted prior to any injection activities an increase of p wave velocity is observed during the water injection this vclocitj irlcrease is localized a ilhin the seisrr icily cloud and is interpreted as an increase in liquid content within the initial liquid vapor multiphase part of the reservoir 3philipplne yalional oil c e l cor duct ed l r ross

location of microseismic swarms induced by salt solution mining ground failures caving processes and collapses of large natural or man made underground cavities can produce significant socio economic damages and represent a serious risk envisaged by the mine managements and municipalities in order to improve our understanding of the mechanisms governing such a geohazard and to test the potential of geophysical methods to prevent them the development and collapse of a salt solution mining cavity was monitored in the lorraine basin in northeastern france during the experiment a huge microseismic data set similar to 50 000 event files was recorded by a local microseismic network 80 per cent of the data comprised unusual swarming sequences with complex clusters of superimposed microseismic events which could not be processed through standard automatic detection and location routines here we present two probabilistic methods which provide a powerful tool to assess the spatio temporal characteristics of these swarming sequences in an automatic manner both methods take advantage of strong attenuation effects and significantly polarized p wave energies at higher frequencies 100 hz the first location approach uses simple signal amplitude estimates for different frequency bands and an attenuation model to constrain the hypocentre locations the second approach was designed to identify significantly polarized p wave energies and the associated polarization angles which provide very valuable information on the hypocentre location both methods are applied to a microseismic data set recorded during an important step of the development of the cavity that is before its collapse from our results systematic spatio temporal epicentre migration trends are observed in the order of seconds to minutes and several tens of meters which are partially associated with cyclic behaviours in addition from spatio temporal distribution of epicentre clusters we observed similar epicentre migration in the order of hours and days all together we suggest that the recorded microseismicity mainly represents detachment and block breakage processes acting at the cavity s roof indicating a zone of critical state of stress and where partial fractures cause chain reaction failures as a result of stress redistribution processes

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

fluid induced seismicity guided by a continental fault injection experiment of 2004 2005 at the german deep drilling site ktb 1 recent hydraulic experiments at the ktb site have shown that seismicity induced by long term fluid injection directly into a continental crustal fault remains guided by this fault the seismicity is triggered by pressure perturbations as low as 0 01 1 bars at the hypocenters a combination of sequential one year fluid extraction 2002 2003 and one year fluid injection 2004 2005 experiments has shown that only positive pore pressure perturbation i e injections was able to induce the seismicity moreover the onset of seismicity roughly coincides with the time of compensation of the extracted fluid volume by the following injection this confirms that the pressure diffusion is a dominant mechanism of seismicity triggering by fluid injections the probed fault shows a significant anisotropy and non linearity of its hydraulic behaviour its hydraulic diffusivity is up to one order of magnitude larger than that of surrounding rocks

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

deep fault plane geometry inferred from multiplet relative relocation beneath the south flank of kilauea dense microearthquake swarms occur in the upper south flank of kilauea providing multiplets composed of hundreds of events the similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters a simple and efficient method has been developed which allowed the relative relocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically relocation of these events greatly improves the definition of the seismic image of the fault that generates them indeed relative relocations define a plane dipping about 6 northward although corresponding absolute locations are widely dispersed in the swarm a composite focal mechanism built from events providing a correct spatial sampling of the multiplet also gives a well constrained northward dip of about 5 to the near horizontal plane this technique thus collapses the clouds of hypocenters of single event locations to a plane coinciding with the slip plane revealed by previous focal mechanism studies we cannot conclude that all south flank earthquakes collapse to a single plane there may locally be several planes perhaps with different dips and depths throughout the south flank volume the 6 northward dipping plane we found is too steep to represent the overall flexure of the oceanic crust under the load of the island of hawaii this plane is probably an important feature that characterizes the basal slip layer below the upper south flank of kilauea volcano differences in seismicity rate and surface deformations between the upper and lower south flank could be related to the geometry of this deep fault plane the present work illustrates how high precision relative relocations of similar events in dense swarms combined with the analysis of geodetic measurements can help to describe deep fault plane geometry systematic selection and extensive relative relocation of similar earthquakes could be attempted in other well instrumented highly seismic areas to provide reliable basic information especially useful for understanding of earthquake generation processes

abnormal reverse faulting above a depleting reservoir subsurface deformation is observed during pumping of some hydrocarbon fields deformation features include subsidence centered on the field and subsidence related centripetal horizontal displacements and faulting focal mechanisms yield reverse movements on steeply dipping faults in our sand silicone analogue model the reservoir is represented by a latex balloon or by undercompacted ground sand deflation of the reservoir results in formation of steeply dipping reverse faults bounding a downward opened cone the cone moves downward to follow the reservoir contraction faults along the cone are straight beneath a thick reservoir cover and tend to curve upwards with decreasing cover our results similar to natural structures observed around magma chambers allow us to reinterpret paul segall9s numerical model of poroelastic stresses caused by changes in the distribution of pore fluids and draw a new pattern of active faults

nucleation process of an m2 earthquake in a deep gold mine in south africa inferred from on fault foreshock activity using a network of sensitive high frequency acoustic emission sensors we observed foreshock activity of an mw 2 2 earthquake main shock in a deep gold mine in south africa foreshock activity which selectively occurred on a part of the rupture plane of the forthcoming main shock lasted for at least 6 months until the main shock rock samples recovered from the main shock source region showed evidence of ancient hydrothermal alteration on the main shock rupture plane suggesting that the foreshock activity occurred on a preexisting weakness the foreshocks during 3 months before the main shock were concentrated in three clusters f1 f3 which we interpret as representing localized preslip at multiple sites while the location of mining area the source of stress perturbations changed with time the locations of foreshock clusters did not change suggesting that the preslip patches were controlled by strength heterogeneity rather than stress distribution activity over the entire foreshock area was generally constant but the largest cluster f2 showed accelerated activity starting at least 7 days before the main shock while mining stress did not increase in this period the main shock initiated at a point close to f1 away from f2 all the six foreshocks during the final 41 h occurred in f1 and f2 and in between these suggest that in the last stage of the preparation process of the main shock preslip patches interacted with each other through the stress concentration ahead of the expanding preslip patch f2 which should be the only driving force of the preparation process under the constant external load

slow deformation and lower seismic hazard at the new madrid seismic zone global positioning system gps measurements across the new madrid seismic zone nmsz in the central united states show little if any motion these data are consistent with platewide continuous gps data away from the nmsz which show no motion within uncertainties both these data and the frequency magnitude relation for seismicity imply that had the largest shocks in the series of earthquakes that occurred in 1811 and 1812 been magnitude 8 their recurrence interval should well exceed 2500 years longer than has been assumed alternatively the largest 1811 and 1812 earthquakes and those in the paleoseismic record may have been much smaller than typically assumed hence the hazard posed by great earthquakes in the nmsz appears to be overestimated

geochemistry of reversible hydratable tephra from the trans mexican volcanic belt rhyolitic glass of high reversible adsorption water to 12 63 wt occurs in pyroclastic rocks from the la malinche stratovolcano in the mexican volcanic belt the glass constitutes 98 vol of the pyroclastics it is a heterogeneous glass that dehydrates reversibly at 72 c composed of sodic and non sodic glasses of surface activity caused by iv al substituting in q 4 1a1 and q 4 2a1 positions minor v a1 tetrahedra terminating in nbos and insufficient na and ca to charge balance al in the glass network adsorption is of molecular water h 2 o m in interstitial sites h bonded to silanol groups to the silica network and to other h 2 o m molecules sodic glasses contain 71 80 77 77 wt sio 2 are partially devitrified to crystallites 5 nm size of na plagioclase and clinopyroxene and exhibit minor low grade metamorphism to 1 vol crystals of mazzite 10 m size sodium free glasses are more siliceous with 74 84 83 88 wt sio 2 show partial devitrification to crystallites 5 nm size of ca plagioclase and clinopyroxene with minor low grade metamorphism of glass and plagioclase to 1 vol crystals of laumontite 10 m size

a numerical model for fluid injection induced seismicity at soultz sous forets abstract during fluid injection experiments at the geothermal site of soultz sous forets france more than 114 000 induced seismic events with magnitudes between 2 0 and 2 9 were detected by a local downhole monitoring network of these 35 039 events are sufficiently constrained to be located hypocenters align along a sub vertical planar structure with the apparent width being dominated by data scattering indicating that seismic activity predominantly occurs along a pre existing larger scale fault structure for this scenario we present a numerical model to simulate hydraulic overpressures and induced seismicity during hydraulic injection the numerical model is based on the physical processes of fluid pressure and stress diffusion with triggering of the induced seismicity being controlled by coulomb friction even in its simplest form of a fault zone without any structural heterogeneity the numerical model reproduces typical observations at soultz sous forets such as number and magnitude of induced events hypocenter locations including the kaiser effect occurrence of post injection seismicity and the largest magnitude event occurring several days after shut in

plate boundary deformation and man made subsidence around geothermal fields on the reykjanes peninsula iceland abstract we present interferometric synthetic aperture radar insar data from 1992 1999 and 2003 2008 as well as gps data from 2000 2009 for the active plate boundary on the reykjanes peninsula southwest iceland the geodetic data reveal deformation mainly due to plate spreading anthropogenic subsidence caused by geothermal fluid extraction and possibly increasing pressure in a geothermal system subsidence of around 10 cm is observed during the first 2 years of production at the reykjanes geothermal power plant which started operating in may 2006 we model the surface subsidence around the new power plant using point and ellipsoidal pressure sources in an elastic halfspace short lived swarms of micro earthquakes as well as aseismic fault movement are observed near the geothermal field following the start of production possibly triggered by the stresses induced by geothermal fluid extraction

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 events with non shear component ii rock bursts with implosive source component the rock burst mechanism of an implosive character is introduced and discussed it is based on the idea that the compressional component of the stress field around the loaded stress concentrator can participate in the rock burst event for easier classification of the rock burst from this viewpoint a set of p wave radiation patterns of the rock burst with a varying portion of the implosive component and varying propagation velocity of the displacements is presented and discussed to support the credibility of the rock burst mechanism of an implosive character the seismic signals of the rock bursts from the mid bohemian coal district with negative signs of the p wave first onsets are presented

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

full moment tensor retrieval from waveform inversion in the larderello geothermal area the area is characterized by numerous events with small magnitude the duration magnitude m d of the events which occurred from 1977 to 1994 varies from 0 5 to 3 5 the algorithm developed by and allows us to retrieve the full source moment tensor from waveform inversion the factorization of the moment tensor rate functions into a source mechanism and a source time function and the further decomposition into a double couple a compensated linear vector dipole and a volumetric component allows us to determine if there are phenomena in the source that can be related to the circulation of fluids a quite common event in geothermal areas

moment tensors of ten witwatersrand mine tremors ground motions recorded both underground and on the surface in two of the south african gold mining districts were inverted to determine complete moment tensors for 10 mining induced tremors in the magnitude range 1 9 to 3 3 the resulting moment tensors fall into two separate categories seven of the events involve substantial coseismic volumetric reduction v together with normal faulting entailing shear deformation ad where the summation is over fault planes of areaa and average slipd for these events the ratio v ad ranges from 0 58 to 0 92 with an average value of 0 71 for the remaining three events v is not significantly different from zero these events are largely double couple sources involving normal faulting surprisingly the two types of source mechanism appear to be very distinct in that there is not a continuous distribution of the source mix from v 0 to v ad presumably the coseismic closure indicates substantial interaction between a mine stope and adjacent shear failure in the surrounding rock under the influence of an ambient stress for which the maximum principal stress is oriented vertically

the impact of injection on seismicity at the geysers california geothermal field water injection into geothermal systems has often become a required strategy to extend and sustain production of geothermal resources to reduce a trend of declining pressures and increasing non condensable gas concentrations in steam produced from the geysers operators have been injecting steam condensate local rain and stream waters and most recently treated wastewater piped to the field from neighboring communities if geothermal energy is to provide a significant increase in energy in the united states the us department of energy goal is 40 000 mw by 2040 injection must play a larger role in the overall strategy i e enhanced geothermal systems egs presented in this paper are the results of monitoring microseismicity during an increase in injection at the geysers field in california using data from a high density digital microearthquake array although seismicity has increased due to increased injection it has been found to be somewhat predictable thus implying that intelligent injection control may be able to control large increases in seismicity

influence of anthropogenic groundwater unloading in indo gangetic plains on the 25 april 2015 mw 7 8 gorkha nepal earthquake groundwater usage in the indo gangetic plains exceeds replenishment of aquifers leading to substantial reduction in the mass such anthropogenic crustal unloading may promote long term fault slip or may modulate seismic activity in the adjoining himalayan region our simulation using gravity recovery and climate experiment data and hydrological models of such a process indicates that the thrust earthquakes on the main himalayan thrust mht including the recent 25 april 2015 mw 7 8 gorkha nepal earthquake are probably influenced by the anthropogenic groundwater unloading process in the gangetic plains the groundwater withdrawal leading to crustal unloading in the gangetic plains causes a significant component of horizontal compression which adds to the secular interseimic compression at the seismogenic depth 5 20 km on the mht beneath the himalayan arc and at hypocentral depth of the 2015 gorkha nepal earthquake this effect enhances the coulomb stress on the locked zone of mht

geogas transport in fractured hard rock correlations with mining seismicity at 3 54 km depth tautona gold mine south africa abstract an on site gas monitoring study has been conducted in the framework of an earthquake laboratory the international nelsam dafgas projects at the tautona gold mine south africa five boreholes up to 60 m long were drilled at 3 54 km depth into the highly fractured pretorius fault zone and instruments for chemical and seismic monitoring installed therein over the span of 4 years sensitive gas monitoring devices were continuously improved to enable the direct observation of geogas concentration variations in the dafgas borehole the major gas concentrations are constant and air like with about 78 n 2 21 o 2 1 ar the geogas components co 2 ch 4 he and h 2 show the most interesting trends and variations on the minute by minute basis and significantly correlate with seismic data while the 222 rn activity remains constant time series and cross correlation analysis allow the identification of different gas components geogas and tunnel air and the identification of two processes influencing the borehole gas composition 1 pumping induced tunnel air breakthrough through networks of initially water saturated fault fractures and 2 seismicity induced permeability enhancement of fault fractures to above 5 10 10 m 2 the current set up of the gas monitoring system is sensitive enough to quantify the resulting geogas transport during periods of intense blasting activities including recorded blasts with seismic moment 1 10 9 nm located within 1000 m of the cubby and it is suggested also during induced earthquakes a final goal of the project

a review of mining induced seismicity in china active seismicity and rockbursting have been an emerging problem in chinese mines the distribution and characteristics of mining induced seismicity in china and its monitoring mitigation and research are reviewed in this paper mining at depth and the activity of current tectonic stress field are the two major factors leading to rockburst hazards three critical depths i e critical initiation depth roof upper bound depth and floor lower bound depth have been identified based on hypocenter data of seismic events in coalmines a strong correlation between rockbursts and gas outbursts in coalmines has also been established and it is recommended to use this correlation for rockburst and gas outburst hazard assessment and warning interchangeably we find that the key problems of rockburst hazard mitigation in china are the lack of mine seismicity monitoring networks in most mines and the need for improvement of the accuracy of the monitoring systems for mines that have been equipped with such systems because the demand for minerals resources is extremely high and the mining activities are progressing deeper and deeper an increasing trend of mining induced seismicity hazards in china may be anticipated for the near future mining induced seismicities are hazards but at the same time they have been found useful for studying geophysical problems in deep ground particularly in the field of earthquake prediction with the enforcement of relevant laws for the mining industry and the continuous effort to study rockburst problems using rock mechanics and geophysics principles and methods it is believed that new approaches for rockburst hazard control and mitigation can be developed

a detailed view of the injection induced seismicity in a natural gas reservoir in zigong southwestern sichuan basin china 1 seismicity at a gas reservoir located in the relatively stable sichuan basin china mirrors the injection pressure of unwanted water suggesting that the seismicity is injection induced injection under high pressure on a routine basis began on 9 january 2009 and continued to july 2011 during the injection period over 120 000 m3 of water was pumped under a wellhead pressure of up to 6 2 mpa into the limestone formation of permian 2 45 to 2 55 km beneath the surface the injection induced more than 7000 surface recorded earthquakes including 2 m4 the largest one was ml4 4 20 m3 and more than 100 m2 events data observed by a nearby local seismic network and five temporal stations provide a detailed view of the spatiotemporal distribution of the induced earthquakes most events were limited to depths ranging from 2 5 to 4 km which is consistent with the limestone formation of permian in a map view hypocenters are concentrated in a nnw extended ellipsoidal zone approximately 6 km long and approximately 2 km wide centered approximately at the injection well multisources of evidence such as the shear mechanism pattern of hypocenter distribution and small elevated pore pressure as compared with the least principal stress in the region show that the induced earthquakes occurred as a result of lowering of the effective normal stress on known or unknown preexisting blind faults which are critically loaded under the regional stress field epidemic type aftershock sequence modeling results indicate that injection inducing and earthquake triggering are both important during earlier periods of injection while later periods are dominated by forced injection induced seismicity

a century of induced earthquakes in oklahoma seismicity rates have increased sharply since 2009 in the central and eastern united states with especially high rates of activity in the state of oklahoma growing evidence indicates that many of these events are induced primarily by injection of wastewater in deep disposal wells the upsurge in activity has raised two questions what is the background rate of tectonic earthquakes in oklahoma how much has the rate varied throughout historical and early instrumental times in this article we show that 1 seismicity rates since 2009 surpass previously observed rates throughout the twentieth century 2 several lines of evidence suggest that most of the significant earthquakes in oklahoma during the twentieth century were likely induced by oil production activities as they exhibit statistically significant temporal and spatial correspondence with disposal wells and intensity measurements for the 1952 el reno earthquake and possibly the 1956 tulsa county earthquake follow the pattern observed in other induced earthquakes and 3 there is evidence for a low level of tectonic seismicity in southeastern oklahoma associated with the ouachita structural belt the 22 october 1882 choctaw nation earthquake for which we estimate m w 4 8 occurred in this zone online material a discussion of historical earthquakes and populations in oklahoma figures of intensity and earthquake location maps and a cumulative count of earthquakes in oklahoma and tables of accounts and felt reports

an implosive component in the seismic moment tensor of a mining induced tremor in early 1988 a special study in one of the major south african gold fields yielded seismograms that indicate seismic moment tensors having substantial implosive components the moment tensor resulting from the inversion of the ground motion data from the best recorded event was decomposed into isotropic and deviatoric components from which both the coseismic volumetric closure v and the total shear deformation ad where a is fault area and d is average slip could be estimated the finding here that v ad is consistent with earlier analyses of how the tabular mine stopes interact with the surrounding rock mass to produce seismic deformation

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

source analysis of the crandall canyon utah mine collapse analysis of seismograms from a magnitude 3 9 seismic event on 6 august 2007 in central utah reveals an anomalous radiation pattern that is contrary to that expected for a tectonic earthquake and which is dominated by an implosive component the results show that the seismic event is best modeled as a shallow underground collapse interestingly large transverse surface waves require a smaller additional noncollapse source component that might represent either faulting in the rocks above the mine workings or deformation of the medium surrounding the mine seismic moment tensor results for nuclear explosions explosion and other mining cavity collapses and tectonic earthquakes are compared and the separation of the different populations indicates that the seismic moment tensor may be used for source type discrimination

observations of mine seismicity in the eastern wasatch plateau utah u s a a possible case of implosional failure in the summer of 1984 a three dimensional high resolution microearthquake network was operated in the vicinity of two coal mines beneath gentry mountain in the eastern wasatch plateau utah during a six week period approximately 3 000 seismic events were observed of which the majority were impulsive higher frequency 10 hz short duration 2 3 sec events probably associated with the caving of the roof from a longwall operation in contrast 234 of the largest located events appeared to occur predominantly beneath the mines to a depth of 2 to 3 km consistent with previous studies the magnitudes of these events ranged from less than m c 0 to 1 6 in addition to the unusual depths of these latter events an anomalous aspect displayed by the events was an apparent dilatational focal mechanism suggesting a non double couple possibly implosional source implosional events have been observed in other studies of mine seismicity however the generally inadequate instrumental coverage of the focal sphere has cast some doubt on the validity of such mechanisms previously suggested source mechanisms for such implosional events have included tensional failure through strata collapse and a shear implosional displacement mechanism shear failure must be involved in the failure process of the gentry mountain implosional events as evidenced by well defined shear waves in the observed seismograms simultaneous monitoring in the east mountain coal mining area to the south by the university of utah revealed typical shear failure events mixed with implosional events the observed double couple reverse focal mechanisms at east mountain were similar to mechanisms determined in previous studies and a composite focal mechanism determined in this study for a sequence outside the mining areas this suggested that the shear events within the mining areas are being influenced by the regional tectonic stress field thus in addition to the seismic events associated with caving of the roof from the longwall operation there appear to be at least two other types of mining induced seismic events occurring in the eastern wasatch plateau both submine in origin 1 events characterized by apparent non double couple possibly implosional focal mechanisms and well defined shear waves and 2 shear events which are indistinguishable from tectonic earthquakes and may be considered mining triggered earthquakes the small mining induced stress changes that occur beyond a few hundred meters from the mine workings suggest both types of seismic events are occurring on critically stressed pre existing zones of weakness topography overburden method of mining and mine configuration also appear to be significant factors influencing the occurrence of the implosional submine events

volume strain within the geysers geothermal field during the 1970s and 1980s the geysers geothermal region was rapidly developed as a site of geothermal power production the likelihood that this could cause significant strain within the reservoir with corresponding surface displacements led to a series of deformation monitoring surveys in 1973 1975 1977 and 1980 the geysers region was surveyed using first order class i spirit leveling in 1994 1995 and 1996 many of the leveling control monuments were resurveyed using high precision global positioning system receivers the two survey methods are reconciled using the geoid96 geoid model the displacements are inverted to determine volume strain within the reservoir for the period 1980 1994 peak volume strains in excess of 5x10 sup 4 are imaged there is an excellent correlation between the observed changes in reservoir steam pressures and the imaged volume strain if reservoir pressure changes are inducing volume strain then the reservoir quasi static bulk modulus k must be 4 6x10 sup 9 pa however seismic velocities indicate a much stiffer reservoir with k 3 4x10 sup 10 pa this apparent discrepancy is shown to be consistent with predicted frequency dependence in k for fractured and water saturated rock inversion of surface deformation data therefore appears to be a powerful method for imaging pressure change withinmore the body of the reservoir correlation between induced seismicity at the geysers and volume strain is observed however earthquake distribution does not appear to have a simple relationship with volume strain rate c 1999 american geophysical union less

seismicity increase after the construction of the world s tallest building an active blind fault beneath the taipei 101 1 seismic activity began to slightly increase during the construction but rose sharply upon the completion of the tallest building in the world the taipei 101 which stands at 508 m in the taipei basin where local seismicity had historically very low besides an increase in both seismic energy and the number of micro earthquakes two felt earthquakes astonishingly occurred beneath the completed building the focal mechanism of the larger felt earthquake and its aftershocks are indicative of an active blind normal fault just beneath the building estimations of the vertical loading of the taipei 101 show that local stress on its foundation increased at least 4 7 bars making it seem likely that the increased seismicity was a direct product of the loading of the mega structure further investigations in unison with continuous seismic monitoring must be conducted because the safety of the high rise building in the taipei basin be comprehensively assessed

the character and extent of seismic deformation in the focal zone of gazli earthquakes of 1976 and 1984 m 7 0 with objective of investigating the peculiarities of seismic process development and seismotectonic deformation character in the focal area of the gazli earthquakes of 1976 7 0 m 7 3 and 1984 m 7 2 a local seismic network was installed for the field observation period may to june 1991 more than 400 events with magnitudes 0 2 m 4 5 were recorded by at least 6 stations

induced seismicity monitoring of an underground salt cavern prone to collapse within the framework of a large research project launched to assess the feasibility of microseismic monitoring of growing underground caverns this specific work focuses on the analysis of the induced seismicity recorded in a salt mine environment a local seismic network has been installed over an underground salt cavern located in the lorraine basin northeast of france the microseismic network includes four 3 components and three single component geophones deployed at depths between 30 and 125 m in cemented boreholes drilled in the vicinity of the study area the underground cavern under monitoring is located within a salt layer at 180 m depth and it presents a rather irregular shape that can be approximated by a cylindrical volume of 50 m height and 180 m diameter presently the cavern is full of saturated brine inducing a significant pressure on its walls 2 0 mpa to keep the overburden mechanically stable nevertheless some small microseismic events were recorded by the network and analyzed approximately 2 000 events in 2 years of recording in october 2005 and april 2007 two controlled pressure transient experiments were carried out in the cavern in order to analyze the mechanical response of the overburden by tracking the induced microseismicity the recorded events were mainly grouped in clusters of 3 30 s of signal duration with emergent first arrivals and rather low frequency content between 20 and 120 hz some of these events have been spatially located by travel time picking close to the actual cavern and its immediate roof preliminary spectral analysis of isolated microearthquakes suggests sources with non negligible tensile components possibly related to fluid filled cracks rock debris falling into the cavern from delamination of clay marls in the immediate roof is probably another source of seismic excitation this was later confirmed when the most important seismic swarms occurred at the site during may 2007 accompanied by the detachment of more than 8 104 m3 of marly material on top of the cavern roof in any case no clear evidence of classical brittle ruptures in the most competent layers of the overburden has been observed during the analyzed period current work is focused on the discrimination of all these possible mechanisms to better understand the damage processes in the cavern overburden and to assess its final collapse hazard

earthquakes injection wells and the perry nuclear power plant cleveland ohio on january 31 1986 an earthquake of richter magnitude 4 9 occurred in northeastern ohio some 17 0 km south of the perry nuclear power plant pnpp and 12 0 km south of the calhio injection wells accelerometers on site at the pnpp recorded accelerations as high as 0 19 to 0 23 g many instruments tripped due to high amplitude vibrations microearthquake networks have recorded 16 microearthquakes within 5 0 km of the injection wells with focal depths ranging from 1 0 to 3 0 km a hydrological model of an anisotropic reservoir 7 2 km wide and 18 4 km long indicates a pressure buildup of 5 3 mpa at the epicenter and 11 8 mpa at the injection well the assumption of an anisotropic reservoir is consistent with available geophysical and geologic data a pressure increase of 11 8 mpa based on stress ratio estimates in crustal rocks in the region is more than sufficient to induce failure to a depth of 5 0 km furthermore brittle faults and extensive fracture permeability within the basement rocks would allow for the migration of pressure transients to hypocentral distances the indicated pressure buildup of 5 3 mpa at the epicenter may have been sufficient to trigger the january 31 1986 earthquake

induced seismicity in the geysers geothermal area california a simultaneous inversion for hypocenters velocities and station delays was performed from the p arrival times of 55 microearthquakes and four explosions distributed throughout a 90 by 100 km region centered at the geysers geothermal area in northern california the resulting one dimensional layered model shows velocity increasing with depth from 4 4 km s at the surface to 5 9 km s at 8 km and deeper we relocated 7215 earthquakes which occurred between may 1975 and february 1982 using the new velocity model these relocations show a significant spatial clustering near production wells in the steam field that was not apparent in earlier studies we infer from the spatial and temporal pattern of seismicity that the seismicity at the geysers is induced by geothermal production activities with the expansion of power production by approximately 70 in 1979 1980 seismic activity has developed near the new production areas seismicity has also expanded northwestward of the present production area although there are no active steam wells in this area statistical cross correlations between the number of earthquakes per month and both steam withdrawal per month and injection volume per month were calculated for the wells studied we find no consistent pattern of correlation between injection and seismicity likewise we find little or no correlation between steam withdrawal and seismicity for steam wells in production longer than 7 years two mechanisms remain plausible for inducing seismicity at the geysers volumetric contraction due to mass withdrawal may perturb the stress field and cause faulting in reservoir rock already near failure due to the regional stress field alternatively aseismic deformation due to regional tectonic stress may be converted to stick slip seismic deformation due to an increase of coefficient of friction along fault surfaces such as could occur from exsolution of dissolved silica into fractures neither mechanism would be likely to show strong correlation between production and seismicity rates however we expect seismicity to continue in the geysers and for seismicity to appear in areas where new production occurs

moderate to large seismicity induced by hydrocarbon production it is well known that various human activities have the potential to generate seismic activity examples range from subsurface waste injection and reservoir impoundment in the vicinity of large dams to the development of mining geothermal or hydrocarbon resources recently induced seismicity in connection to geologic carbon sequestration projects has emerged as a new field of interest this review discusses seismicity induced by hydrocarbon production in particular i focus on published cases for which earthquakes of moderate to large magnitudes in other words earthquakes that can be felt on the surface have been reported i also discuss current theoretical approaches to model this phenomenon the emphasis on moderate to large magnitudes is intended to complement the other contributions in this issue which focus primarily on microseismicity evidently it is important to understand the conditions under which hydrocarbon production may induce seismic activity in order to ensure that field operations can

moment tensor inversion of induced microseisnmic events evidence of non shear failures in the 4 m 2 moment magnitude range to assess objectively the source mechanism of mining induced seismic events we developed an algorithm to invert for the moment tensor components from this inversion the ratio of isotropic deviatoric component in the source is obtained we are applying this method to microseismic events 4 m 2 recorded at the underground research laboratory url in pinawa manitoba where a tunnel is being excavated at 420 m depth to monitor the seismic activity induced by this opening an array of 16 triaxial accelerometers was installed around the planned tunnel results from the moment tensor inversion on a fist data set provide evidence of non shear failure mechanisms furthermore the location of events displaying a tensile component corresponds to a breakout observed underground in the roof of the tunnel

hydraulic fracturing and the crooked lake sequences insights gleaned from regional seismic networks within central alberta canada a new sequence of earthquakes has been recognized as of 1 december 2013 in a region of previous seismic quiescence near crooked lake 30 km west of the town of fox creek we utilize a cross correlation detection algorithm to detect more than 160 events to the end of 2014 which is temporally distinguished into five subsequences this observation is corroborated by the uniqueness of waveforms clustered by subsequence the crooked lake sequences have come under scrutiny due to its strong temporal correlation 99 99 to the timing of hydraulic fracturing operations in the duvernay formation we assert that individual subsequences are related to fracturing stimulation and despite adverse initial station geometry double difference techniques allow us to spatially relate each cluster back to a unique horizontal well overall we find that seismicity in the crooked lake sequences is consistent with first order observations of hydraulic fracturing induced seismicity

earthquake activity in the aswan region egypt the november 14 1981 aswan earthquake m l 5 7 which was related to the impoundment of lake aswan was followed by an extended sequence of earthquakes and is investigated in this study earthquake data from june 1982 to late 1991 collected from the aswan network are classified into two sets on the basis of focal depth i e shallow or deeper than 10 km it is determined that a shallow seismicity is characterized by swarm activity whereas deep seismicity is characterized by a foreshock main shock aftershock sequence b the b value is equal to 0 77 and 0 99 for the shallow and deep sequences respectively and c observations clearly indicate that the temporal variations of shallow seismic activity were associated with a high rate of water level fluctuation in lake aswan a correlation with the deeper earthquake sequence however is not evident these features as well as the tomographic characteristics of the aswan region awad and mizoue this issue imply that the aswan seismic activity must be regarded as consisting of two distinct earthquake groups

global review of human induced earthquakes the human induced earthquake database hiquake is a comprehensive record of earthquake sequences postulated to be induced by anthropogenic activity it contains over 700 cases spanning the period 1868 2016 activities that have been proposed to induce earthquakes include the impoundment of water reservoirs erecting tall buildings coastal engineering quarrying extraction of groundwater coal minerals gas oil and geothermal fluids excavation of tunnels and adding material to the subsurface by allowing abandoned mines to flood and injecting fluid for waste disposal enhanced oil recovery hydrofracturing gas storage and carbon sequestration nuclear explosions induce earthquakes but evidence for chemical explosions doing so is weak because it is currently impossible to determine with 100 certainty which earthquakes are induced and which not hiquake includes all earthquake sequences proposed on scientific grounds to have been human induced regardless of credibility challenges to constructing hiquake include under reporting which is 30 of m 4 events 60 of m 3 events and 90 of m 2 events the amount of stress released in an induced earthquake is not necessarily the same as the anthropogenic stress added because pre existing tectonic stress may also be released thus earthquakes disproportionately large compared with the associated industrial activity may be induced knowledge of the magnitude of the largest earthquake that might be induced by a project mmax is important for hazard reduction observed mmax correlates positively with the scale of associated industrial projects fluid injection pressure and rate and the yield of nuclear devices it correlates negatively with calculated inducing stress change likely because the latter correlates inversely with project scale the largest earthquake reported to date to be induced by fluid injection is the 2016 m 5 8 pawnee oklahoma earthquake by water reservoir impoundment the 2008 m 8 wenchuan people s republic of china earthquake and by mass removal the 1976 m 7 3 gazli uzbekistan earthquake the minimum amount of anthropogenic stress needed to induce an earthquake is an unsound concept since earthquakes occur in the absence of industrial activity the minimum amount of stress observed to modulate earthquake activity is a few hundredths of a megapascal and possibly as little as a few thousandths equivalent to a few tens of centimeters of water table depth faults near to failure are pervasive in the continental crust and induced earthquakes may thus occur essentially anywhere in intraplate regions neither infrastructure nor populations may be prepared for earthquakes human induced earthquakes that cause nuisance are rare but in some cases may be a significant problem e g in the hydrocarbon producing areas of oklahoma usa as the size of projects and density of populations increase the potential nuisance of induced earthquakes is also increasing and effective management strategies are needed

structure and permeability geologic controls on induced seismicity at nurek reservoir tadjikistan ussr induced seismicity at nurek tadjik s s r is occurring within the upper plate of the ionakhsh thrust which passes at depth directly beneath the reservoir within the immediate vicinity of the reservoir one can contrast two distinct areas one seismic and the aseismic both consist of rocks that can be shown to have a highly anisotropic permeability due to alternating permeable and impermeable strata water flow is favored along bedding while essentially prohibited across bedding however because the two areas differ in structure they also differ in their vertical component of permeability as a direct result of the different attitudes of the bedding in the aseismic area vertical fluid flow is blocked by a syncline that cups the base of the reservoir in the seismic area a plunging anticline that crops out beneath the reservoir allows water to flow to depth in the high permeability direction along bedding because effective stress changes at depth are dependent on permeability characteristics knowledge of subsurface permeability or permeability anisotropy can serve as a means by which to predict the distribution of induced seismic activity thus while regional geologic studies may indicate the presence of lithologies that are conducive to the transmission of pore water pressure access ofmore water to these units depends on the structural arrangement beneath the reservoir on a very local scale less

gas injection may have triggered earthquakes in the cogdell oil field texas between 1957 and 1982 water flooding was conducted to improve petroleum production in the cogdell oil field north of snyder tx and a contemporary analysis concluded this induced earthquakes that occurred between 1975 and 1982 the national earthquake information center detected no further activity between 1983 and 2005 but between 2006 and 2011 reported 18 earthquakes having magnitudes 3 and greater to investigate these earthquakes we analyzed data recorded by six temporary seismograph stations deployed by the usarray program and identified 93 well recorded earthquakes occurring between march 2009 and december 2010 relocation with a double difference method shows that most earthquakes occurred within several northeast southwest trending linear clusters with trends corresponding to nodal planes of regional focal mechanisms possibly indicating the presence of previously unidentified faults we have evaluated data concerning injection and extraction of oil water and gas in the cogdell field water injection cannot explain the 2006 2011 earthquakes especially as net volumes injection minus extraction are significantly less than in the 1957 1982 period however since 2004 significant volumes of gases including supercritical co2 have been injected into the cogdell field the timing of gas injection suggests it may have contributed to triggering the recent seismic activity if so this represents an instance where gas injection has triggered earthquakes having magnitudes 3 and larger further modeling studies may help evaluate recent assertions suggesting significant risks accompany large scale carbon capture and storage as a strategy for managing climate change

deep injection and closely monitored induced seismicity at paradox valley colorado the u s bureau of reclamation s paradox valley unit pvu extracts aquifer brine from nine shallow wells along the dolores river paradox valley in southwestern colorado and after treating high pressure injects the brine 4 3 4 8 km below the surface pvu injects at rates between 800 and 1300 l min since 1991 pvu has emplaced over 4 106 m3 of fluid and induced more than 4000 surface recorded seismic events the events are recorded on the local 15 station paradox valley seismic network the induced seismicity at paradox separates into two distinct source zones a principle zone 95 of the events asymmetrically surrounding the injection well to a maximum radial distance of 3 km and a secondary ellipsoidal zone 2 5 km long and centered 8 km northwest of the injection well the expansion of these zones has stabilized since mid 1999 about three years after the onset of continuous injection within the principal zone hypocenters align in distinct linear patterns showing at depth stratigraphy and the local wray mesa fracture and fault system the primary faults of the wray mesa system are aseismic striking subparallel to the inferred maximum principal stress direction with one or more faults probably acting as fluid conduits to the secondary seismic zone individual seismic events in both zones do not discernibly correlate with short term injection parameters however a 0 5 km2 region immediately northwest of the injection well responds to long term large scale changes in injection rate and the surpassing of a threshold injection pressure focal mechanisms of the induced events are consistent with simple double couple strike slip moments and subhorizontal extension to the northeast in addition the fault planes are consistent with principal stress directions determined from borehole breakouts more than 99 9 of the pvu seismicity is below human detection m 2 5 however approximately 15 events have been felt locally with the largest being a magnitude m 4 3 because of the m 4 3 and two earlier felt m 3 5 events and injection economics pvu changed injection strategies three times since 1996 these changes reduced seismicity from 1100 events year to as low as 60 events year

induced seismicity during the stimulation of a geothermal hfr reservoir in the cooper basin australia a long term fluid injection experiment was performed in the cooper basin australia in 2003 to stimulate a geothermal reservoir more than 20 000 m 3 of water were injected into the granitic crust at 4250 m depth during reservoir stimulation about 27 000 induced seismic events were detected by a local eight station seismic monitoring system deployed in nearby boreholes hypocenter locations for 11 068 events were determined by using an averaged velocity model that was calibrated by associating early events with the injection point the spatial hypocenter distribution forms a nearly subhorizontal structure with a lateral extension of 2 km 1 5 km and an apparent thickness of approximately 150 200 m which is in the order of the hypocenter location confidence limits the hypocenter distribution exhibits a high degree of spatiotemporal ordering with the seismic activity systematically migrating away from the injection well with increasing time previously activated regions become seismically quiet indicating relaxation processes high resolution relative hypocenter locations determined for clusters of similar events locally reduce the apparent thickness of the structure to the level of a few tens of meters indicating that the reservoir is dominated by a single fracture zone only consistent with these findings a subsequently drilled well intersects a dominating high permeable fracture within 15 m of the predicted intersection depth based on drilling and logging information the fracture zone is interpreted as a preexisting possibly tectonically formed feature that partly sheared during stimulation triggering of the induced seismicity is found to be predominantly controlled by the increase of fluid pressure implicating a local reduction of the effective normal stress resolved on the fracture plane additionally perturbations of the stress field caused by the largest magnitude events may trigger seismicity aftershocks on a local short ranging scale

investigation of fault mechanisms during geothermal reservoir stimulation experiments in the cooper basin australia abstract subsequent to an initial reservoir stimulation in 2003 a second long term fluid injection experiment was conducted in the cooper basin australia in 2005 to further enhance and enlarge the existent geothermal reservoir approximately 25 000 m 3 of water were injected into the granitic crust at 4250 m depth thereby inducing about 16 000 seismic events which were recorded by a local eight station network event magnitudes calibrated by recordings of the permanent network of geosciences australia are in the range between m l 1 2 and 2 9 with seismic moments between 10 9 and 10 13 n m hypocenter locations could be determined for 8886 events the spatial hypocenter distribution indicates that the seismicity occurred on the same subhorizontal layer structure identified for the initial reservoir stimulation which is further extended in the course of restimulation early seismicity is located near the outer rim of the previous zone of seismic activity and subsequently migrates both toward and away from the injection well the immediate vicinity of the injection well remains seismically quiet during restimulation these effects can be explained by a simple kaiser effect model where the induced seismicity is controlled by the in situ fluid pressure increase compound fault plane solutions based on p phase polarities indicate a similar fault mechanism for nearly all events the dominating fault mechanism is consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution a small number of events with reverse mechanisms might indicate the existence of conjugated fractures locally intersecting the main fracture zone cumulative slip determined by mapping the slip contribution of individual events onto the fracture zone indicates that most fracture patches in the reservoir slipped repeatedly accumulating up to several centimeters of shear displacement

monitoring deformation at the geysers geothermal field california using c band and x band interferometric synthetic aperture radar monitoring deformation at the geysers geothermal field california using c band and x band interferometric synthetic aperture radar d w vasco jonny rutqvist alessandro ferretti alessio rucci fernando bellotti patrick dobson curtis oldenburg julio garcia mark walters and craig hartline d w vasco jonny rutqvist patrick dobson curtis oldenburg earth sciences di vision building 74 berkeley laboratory 1 cyclotron road berkeley ca 94720 e mail dwvasco lbl gov jrutqvist lbl gov pfdobson lbl gov cmoldenburg lbl gov alessandro ferretti alessio rucci and fernando bellotti tele rilevamento europa via vittoria colonna 7 20149 milano italy e mail alessandro ferretti treuropa com alessio rucci treuropa com fernando bellotti treuropa com julio garcia mark walters and craig hartline calpine corporation 10350 socrates mine road middletown ca 95461 e mail julio garcia calpine com mark walters calpine com craig hartline calpine com

induced stresses due to fluid extraction from axisymmetric reservoirs earthquakes can be induced by fluid extraction as well as by fluid injection segall 1989 proposed that poroelastic stresses are responsible for inducing earthquakes associated with fluid extraction here i present methods for computing poroelastic stress changes due to fluid extraction for general axisymmetric reservoir geometries the results ofgeertsma 1973 for a thin disk reservoir with uniform pressure drop are recovered as a special case predicted surface subsidence agrees very well with measured leveling changes over the deep lacq gas field in southwestern france the induced stresses are finite if the reservoir pressure changes are continuous computed stress changes are on the order of several bars suggesting that the preexisting stress states in regions of extraction induced seismicity are very close to frictional instability prior to production

implosive earthquakes at the active accretionary plate boundary in northern iceland non double couple earthquake focal mechanisms imply a mode of seismic failure other than simple shear and movements perpendicular to the fault plane reports of such events are rare and it is controversial whether such failure can occur at depth in the earth following reports of such earthquakes from the hengill and reykjanes volcanic systems on the accretionary plate boundary in iceland1 3 a seismological survey was conducted in the krafla system iceland to explore whether such earthquakes occur elsewhere along the boundary here we report the observation of a mixed suite of focal mechanisms at krafla including shear events explosive tensile crack events such as were observed at hengill and reykjanes and implosive events these last represent cavity collapse and constitute an entirely new class of earthquake the krafla system had undergone a crustal spreading episode4 10 during the decade before the observations were made and our results indicate that immediately after such an episode the accretionary plate boundary is characterized by a heterogeneous stress regime with shear extensional and compressional sources in close juxtaposition

earthquakes induced by hydraulic fracturing in poland township ohio felt seismicity induced by hydraulic fracturing is very rare with only a handful of reported cases worldwide using an optimized multistation cross correlation template matching routine 77 earthquakes were identified in poland township mahoning county ohio that were closely related spatially and temporally to active hydraulic fracturing operations we identified earthquakes as small as local magnitudes m l 1 up to 3 potentially one of the largest earthquakes induced by hydraulic fracturing in the united states these events all occurred from 4 to 12 march 2014 and the rate decayed once the ohio department of natural resources issued a shutdown of hydraulic fracturing at a nearby well on 10 march using a locally derived velocity model and double difference relocation the earthquakes occurred during six stimulation stages along two horizontal well legs that were located 0 8 km away nearly 100 stimulation stages in nearby wells at greater distances from the earthquake source region did not coincide with detected seismicity during the sequence hypocenters migrated 600 m along an azimuth of 083 defining a vertically oriented plane of seismicity close to the top of the precambrian basement the focal mechanism determined for the m l 3 event had a vertically oriented left lateral fault plane consistent with the earthquake distribution and the regional stress field the focal mechanism orientation and depth of hypocenters were similar to those of the 2011 youngstown earthquake sequence that occurred 18 km to the northwest and was correlated with wastewater injection instead of hydraulic fracturing considering the relatively large magnitude of the poland township events and the b value of 0 89 it appears the hydraulic fracturing induced slip along a pre existing fault fracture zone optimally oriented in the regional stress field

geomechanical modeling of the nucleation process of australia s 1989 m5 6 newcastle earthquake abstract inherent to black coal mining in new south wales australia since 1801 the discharge of ground water may have triggered the m5 6 newcastle earthquake in 1989 4 dimensional geomechanical model simulations reveal that widespread water removal and coal as deep as a 500 m depth resulted in an unload of the earth s crust this unload caused a destabilization process of the pre existing newcastle fault in the interior of the crust beneath the newcastle coal field in tandem an increase in shear stress and a decrease in normal stress may have reactivated this reverse fault over the course of the last fifty years elevated levels of lithostatic stress alterations have accelerated in 1991 based on the modeling of the crust s elastostatic response to the unload there has been the minimal critical shear stress changes of 0 01 mega pascal 0 1 bar that reached the newcastle fault at a depth where the 1989 mainshock nucleated hence it can be anticipated that other faults might also be critically stressed in that region for a couple of reasons first the size of the area volume that is affected by the induced stress changes is larger than the ruptured area of the newcastle fault second the seismic moment magnitude of the 1989 m5 6 newcastle earthquake is associated with only a fraction of mass removal 1 of 55 following mcgarr s mass moment relationship lastly these findings confirm ongoing seismicity in the newcastle region since the beginning of the 19th century after a dormant period of 10 000 years of no seismicity

south african seismicity april 1997 to april 1999 and regional variations in the crust and uppermost mantle of the kaapvaal craton abstract events induced by deep gold mining activity on the edge of the witwatersrand basin dominate the seismicity of south africa the deployment of 54 broad band seismic stations at 84 separate locations across southern africa between april 1997 and april 1999 kaapvaal network enabled the seismicity of south africa to be better defined over a 2 year period seismic events located by the south african national network and by localized seismic networks deployed in mines or across gold mining areas were used to evaluate earthquake location procedures and to show that the kaapvaal network locates mining induced tremors with an average error of 1 56 0 10 km compared with 9 50 0 36 km for the south african network travel times of seismic events from the mines recorded at the kaapvaal network indicate regional variations in the thickness of the crust but no clearly resolved variations in seismic wavespeeds in the uppermost mantle greater average crustal thicknesses 48 50 km compared with 41 43 km are observed in the northern parts of the kaapvaal craton that were affected by the bushveld magmatism at 2 05 ga estimates of average crustal thickness for the southern part of the kaapvaal craton from receiver functions 38 km agree well with those from refracted arrivals from mining induced earthquakes if the crustal thicknesses below the sources are assumed to be 40 43 km in contrast the average crustal thickness inferred from refracted arrivals for the northern part of the kaapvaal craton is larger by about 7 km 51 km than that inferred from receiver functions 44 km suggesting a thick mafic lower crust of variable seismic properties due to variations in composition and metamorphic grade pn wavespeeds are high 8 3 8 4 km s indicating the presence of highly depleted magnesium rich peridotite throughout the uppermost mantle of the craton seismic pg and sg phases indicate that the upper crust around the witwatersrand basin is comparatively uniform in composition when averaged over several kilometres

nanoseismicity and picoseismicity rate changes from static stress triggering caused by a mw 2 2 earthquake in mponeng gold mine south africa static stress changes following large earthquakes are known to affect the rate and distribution of aftershocks yet this process has not been thoroughly investigated for nanoseismicity and picoseismicity at centimeter length scales here we utilize a unique data set of m 3 4 earthquakes following a mw 2 2 earthquake in mponeng gold mine south africa that was recorded during a quiet interval in the mine to investigate if rate and state based modeling is valid for shallow mining induced seismicity we use dieterich s 1994 rate and state dependent formulation for earthquake productivity which requires estimation of four parameters 1 coulomb stress changes due to the main shock 2 the reference seismicity rate 3 frictional resistance parameter and 4 the duration of aftershock relaxation time comparisons of the modeled spatiotemporal patterns of seismicity based on two different source models with the observed distribution show that while the spatial patterns match well the rate of modeled aftershocks is lower than the observed rate to test our model we used three metrics of the goodness of fit evaluation the null hypothesis of no significant difference between modeled and observed seismicity rates was only rejected in the depth interval containing the main shock results show that mining induced earthquakes may be followed by a stress relaxation expressed through aftershocks located on the rupture plane and in regions of positive coulomb stress change furthermore we demonstrate that the main features of the temporal and spatial distributions of very small mining induced earthquakes can be successfully determined using rate and state based stress modeling

geological structure of the paradox valley region colorado and relationship to seismicity induced by deep well injection the bureau of reclamation reclamation operates a deep injection well at paradox valley in western colorado seismicity has been recorded since 1985 with more than 5900 likely induced earthquakes detected since injection operations began in 1991 making this project a unique and valuable case study in the long term development of injection induced seismicity likely induced earthquakes occur in a complex pattern with clusters separated by aseismic gaps of 2 km or more and have been observed up to 16 km from the injection well to evaluate how the subsurface geologic structure may control the occurrence of induced seismicity we compare the earthquake locations to a set of geologic models that reclamation commissioned prior to drilling the injection well our analysis indicates that many aspects of the observed seismicity pattern can be attributed to the complex geologic structure the earthquake depths and locations are generally consistent with the structural trends of the primary injection target formation within the uncertainty of the models and some of the mapped basement faults appear to serve as boundaries in the earthquake locations suggesting that they are barriers to flow additionally the majority of events follow the predicted fluid flow pattern which trends northwest southeast and wraps around paradox valley some features of the seismicity pattern such as the early onset of seismicity to the northwest of the injection well and the distinct clustering are not explained by the current geologic models and may be related to unmapped geologic features or local variations in the stress field

the 1994 cacoosing valley earthquakes near reading pennsylvania a shallow rupture triggered by quarry unloading a mb lg 4 6 mainshock on january 16 1994 in the cacoosing valley 10 km west of reading southeastern pennsylvania caused modified mercalli vi vii intensities and the highest earthquake damage 2 million in the eastern united states since 1944 aftershock hypocenters from a temporary local seismic network are confined to the upper 2 5 km of the crust they are clustered around the periphery of a tabular 3 3 km zone that is interpreted to outline the mainshock rupture this zone matches the nodal plane with reverse and left lateral slip strike 135 dip 54 southwest and rake 55 of a focal mechanism obtained from aftershock first motions and from mainshock waveforms this rupture does not correlate with any of the faults mapped in the epicentral area but it is parallel to the most prominent fracture set including joints and small faults maximum possible strike slip accumulated on the causative fault is no more than several tenths of meters a large carbonate rock quarry is centered above the rupture we calculate a small but significant 0 13 mpa coulomb stress increase caused by the quarry on the shallow portion of the rupture most of this increase was caused by pore pressure rise after the quarry was abandoned in december 1992 and flooded seismicity started may 1993 we conclude that the 1993 1997 cacoosing valley sequence is probably triggered by the quarry about 200 km northeast of cacoosing another quarry in early paleozoic carbonate rocks triggered the 1974 mb lg 3 0 wappingers falls earthquake

effect of the rock properties on mining induced seismicity around the ventersdorp contact reef witwatersrand basin south africa the ventersdorp contact reef is characterised by a variety of different rock types that are found above and below the reef in different mining areas these variations in the host rock type have been classified into six main geotechnical areas knowledge of the difference in seismicity related to these geotechnical areas could influence the design of deep level stopes and optimise the support systems

post rifting stress relaxation at the divergent plate boundary in northeast iceland interaction of the elastic lithosphere with the underlying anelastic asthenosphere causes strain to propagate along the earth s surface in a diffusion like manner following tectonism at plate boundaries this process transfers stress between adjacent tectonic segments and influences the temporal tectonic pattern along a plate boundary observations of such strain transients have been rare and have hitherto been confined to strike slip and underthrusting plate boundaries1 here we report the observation of a strain transient at the divergent spreading plate boundary in iceland a global positioning system survey undertaken a decade after an episode of dyke intrusion accompanying several metres of crustal spreading reveals a spatially varying strain field with the expected diffusion pulse shape and an amplitude three times greater than the 5 7 cm that would be expected from the average spreading rate2 a simple one dimensional model with a thin elastic layer overlying a viscous layer fits the data well and yields a stress diffusivity of 1 1 a 0 3 m2s 1 combined with struc tural information from magnetotelluric measurements this implies a viscosity of 0 3 2 1019 pa s a value comparable to that derived for iceland from post glacial rebound23 but low compared with estimates for mantle viscosity obtained elsewhere3

felt seismicity associated with shale gas hydraulic fracturing the first documented example in europe we describe the origin of felt seismicity during the hydraulic fracturing of the carboniferous bowland shale at the preese hall 1 exploration well near blackpool in the uk during 2011 the seismicity resulted from the interaction of hydraulic fracturing and a fault the location of which was unknown at the time but has subsequently been located and does not intersect the well borehole waveform cross correlation is used to detect 50 events in the sequence a representative hypocenter and strike slip focal mechanism is calculated using the best recorded seismic event the hypocenter is calculated to lie 300 400 m east and 330 360 m below the injection point and shown to lie on a fault imaged using 3 d seismic at a depth of about 2930 m the 3 d survey shows that not only the event hypocenter but also the focal mechanism correlates strongly with a subsequently identifiable transpressional fault formed during the late carboniferous variscan basin inversion

assessment of focal mechanisms of microseismic events computed from two three component receivers application to the arkema vauvert field france we have developed a method that enables computing double couple focal mechanisms with only a few sensors this method is based on a non linear inversion of the p sv and sh amplitudes of microseismic events recorded on a set of sensors the information brought by the focal mechanism enables determining the geometry of the rupture on the associated geological structure it also provides a better estimate of the conventional source parameters full analysis has been performed on a data set of 15 microseismic events recorded in the brine production field of vauvert the microseismic monitoring network consisted of two permanent tools and one temporary borehole string the majority of the focal mechanisms computed from both permanent tools are similar to those computed from the whole network this result indicates that the double couple focal mechanism determination is reliable for both permanent 3c receivers in this field

a monte carlo method for probabilistic hazard assessment of induced seismicity due to conventional natural gas production abstract a monte carlo approach to probabilistic seismic hazard analysis is developed for a case of induced seismicity associated with a compacting gas reservoir the geomechanical foundation for the method is the work of kostrov 1974 and mcgarr 1976 linking total strain to summed seismic moment in an earthquake catalog our monte carlo method simulates future seismic hazard consistent with historical seismic and compaction datasets by sampling probability distributions for total seismic moment event locations and magnitudes and resulting ground motions ground motions are aggregated over an ensemble of simulated catalogs to give a probabilistic representation of the ground motion hazard this approach is particularly well suited to the specific nature of the time dependent induced seismicity considered we demonstrate the method by applying it to seismicity induced by reservoir compaction following gas production from the groningen gas field a new ground motion prediction equation gmpe tailored to the groningen field has been derived by calibrating an existing gmpe with local strong motion data for 2013 2023 we find a 2 chance of exceeding a peak ground acceleration of 0 57 g and a 2 chance of exceeding a peak ground velocity of 22 cm s above the area of maximum compaction disaggregation shows that earthquakes of m w 4 5 at the shortest hypocentral distances of 3 km and ground motions two standard deviations above the median make the largest contributions to this hazard uncertainty in the hazard is primarily due to uncertainty about the future fraction of induced strains that will be seismogenic and how ground motion and its variability will scale to larger magnitudes

use of liquefaction induced features for paleoseismic analysis an overview of how seismic liquefaction features can be distinguished from other features and how their regional distribution and properties of source sediment can be used to infer the location and strength of holocene paleo earthquakes abstract liquefaction features can be used in many field settings to estimate the recurrence interval and magnitude of strong earthquakes through much of the holocene these features include dikes craters vented sand sills and laterally spreading landslides the relatively high seismic shaking level required for their formation makes them particularly valuable as records of strong paleo earthquakes this state of the art summary for using liquefaction induced features for paleoseismic interpretation and analysis takes into account both geological and geotechnical engineering perspectives the driving mechanism for formation of the features is primarily the increased pore water pressure associated with liquefaction of sand rich sediment the role of this mechanism is often supplemented greatly by the direct action of seismic shaking at the ground surface which strains and breaks the clay rich cap that lies immediately above the sediment that liquefied discussed in the text are the processes involved in formation of the features as well as their morphology and characteristics in field settings whether liquefaction occurs is controlled mainly by sediment grain size sediment packing depth to the water table and strength and duration of seismic shaking formation of recognizable features in the field generally requires a low permeability cap above the sediment that liquefied field manifestations are controlled largely by the severity of liquefaction and the thickness and properties of the low permeability cap criteria are presented for determining whether observed sediment deformation in the field originated by seismically induced liquefaction these criteria have been developed mainly by observing historic effects of liquefaction in varied field settings the most important criterion is that a seismic liquefaction origin requires widespread regional development of features around a core area where the effects are most severe in addition the features must have a morphology that is consistent with a very sudden application of a large hydraulic force this article discusses case studies in widely separated and different geological settings coastal south carolina the new madrid seismic zone the wabash valley seismic zone and coastal washington state these studies encompass most of the range of settings and the types of liquefaction induced features likely to be encountered anywhere the case studies describe the observed features and the logic for assigning a seismic liquefaction origin to them also discussed are some types of sediment deformations that can be misinterpreted as having a seismic origin two independent methods for estimating prehistoric magnitude are discussed briefly one method is based on determination of the maximum distance from the epicenter over which liquefaction induced effects have formed the other method is based on use of geotechnical engineering techniques at sites of marginal liquefaction in order to bracket the peak accelerations as a function of epicentral distance these accelerations can then be compared with predictions from seismological models

investigation of regional seismicity before and after hydraulic fracturing in the horn river basin northeast british columbia we systematically re analyzed historical seismograms to verify the existence of background seismicity in the horn river basin of northeast british columbia before the start of regional shale gas development we also carefully relocated local earthquakes that occurred between december 2006 and december 2011 to delineate their spatiotemporal relationship with hydraulic fracturing hf operations in the region scattered seismic events were detected in the horn river basin throughout the study periods the located seismicity within 100 km of the fort nelson seismic station had a clearly increasing trend specifically in the etsho area where most local hf operations were performed the number of events was increased from 24 in 2002 2003 prior to hf operations to 131 in 2011 peak period of hf operations in addition maximum magnitude of the events was shifted from ml 2 9 to ml 3 6 as the scale of hf operation expanded from 2006 2007 to 2011 based on our relocated earthquake catalog the overall b value i

evidence of tectonic release from underground nuclear explosions in long period s waves the sh waves from 21 underground nuclear explosions at pahute mesa nts were used to investigate tectonic release the equivalent double couple representation of the tectonic release which was constrained by waveform modeling and the polarity of sp is very similar for all the explosions the average orientation is a right lateral strike slip fault trending n15 w seismic moments were determined on the basis of comparisons with two western united states strike slip earthquakes benham has the largest tectonic release moment 5 6 10 24 dyne cm and stilton 0 1 10 24 dyne cm the smallest of the events studied in general the seismic moments increase with the size of the explosion but the location of the explosion relative to previous explosions can strongly affect the tectonic release the pahute mesa events can be separated into two populations 1 events which are well separated 4 km from previous explosions and 2 those events which are close 4 km to previous explosions those events which are close to previous explosions show a marked decrease in tectonic release a least squares fit of seismic moment to event size as determined from the world wide average ab amplitude shows that the two populations are approximately parallel but offset the fact that the trends remain separated even at small yields e g pipkin and scotch suggests that for pahute mesa there is not a threshold for tectonic release since spatial position gives the best separation of high and low tectonic release events a volume model is favored for the source mechanism the preferred model is motion on a system of faults and joints

earthquake hypocenters and focal mechanisms in central oklahoma reveal a complex system of reactivated subsurface strike slip faulting the sharp increase in seismicity over a broad region of central oklahoma has raised concern regarding the source of the activity and its potential hazard to local communities and energy industry infrastructure since early 2010 numerous organizations have deployed temporary portable seismic stations in central oklahoma in order to record the evolving seismicity in this study we apply a multiple event relocation method to produce a catalog of 3639 central oklahoma earthquakes from late 2009 through 2014 regional moment tensor rmt source parameters were determined for 195 of the largest and best recorded earthquakes combining rmt results with relocated seismicity enabled us to determine the length depth and style of faulting occurring on reactivated subsurface fault systems results show that the majority of earthquakes occur on near vertical optimally oriented ne sw and nw se strike slip faults in the shallow crystalline basement these are necessary first order observations required to assess the potential hazards of individual faults in oklahoma

three dimensional seismic structure and moment tensors of non double couple earthquakes at the hengill grensdalur volcanic complex iceland summary the volcanic and geothermal areas of iceland are rich sources of non double couple non dc earthquakes a state of the art digital seismometer network deployed at the hengill grensdalur volcanic complex in 1991 recorded 4000 small earthquakes we used the best recorded of these to determine 3 d v p and v p v s structure tomographically and accurate earthquake moment tensors the v p field is dominated by high seismic wave speed bodies interpreted as solidified intrusions a widespread negative 4 per cent v p v s anomaly in the upper 4 km correlates with the geothermal field but is too strong to be caused solely by the evect of temperature upon liquid water or the presence of vapour and requires in addition mineralogical or lithological diverences between the geothermal reservoir and its surroundings these may be caused by geothermal alteration well constrained moment tensors were obtained for 70 of the best recorded events by applying linear programming methods to p and s wave polarities and amplitude ratios about 25 per cent of the mechanisms are within observational error consistent with dc mechanisms consistent with shear faulting the other 75 per cent have significantly non dc mechanisms many have substantial explosive components one has a substantial implosive component and the deviatoric component of many is strongly non dc many of the non dc mechanisms are consistent within observational error with simultaneous tensile and shear faulting however the mechanisms occupy a continuum in source type parameter space and probably at least one additional source process is occurring this may be fluid flow into newly formed cracks causing partial compensation of the volumetric component studying non shear earthquakes such as these has great potential for improving our understanding of geothermal processes and earthquake source processes in general

source mechanisms of mine related seismicity savuka mine south africa we report full moment tensor solutions for 76 mine tremors with moment magnitudes mw between 0 5 and 2 6 recorded by a network of 20 high frequency geophones in a deep gold mine in south africa source mechanisms convey important information on how in mine stresses are relaxed and understanding the nature of such mechanisms is essential for improving our assessment of rock mass response to mining our approach has consisted of minimizing the l2 norm of the difference between observed and predicted p sv and sh spectral amplitudes with visually assigned polarities to constrain all six independent components of the seismic moment tensor our results reveal the largest principal stresses in the mine are com pressive oriented near vertically and relaxed through a mix of volumetric closure and normal faulting consistent with a gravity driven closure of the mined out areas pre vious moment tensor studies in deep mines had suggested that the distribution of seis mic sources in terms of the volumetric shear mix was bimodal a bimodal distribution is compatible with our moment tensor solutions only for moment magnitudes above 2 2 events in the 0 5 mw 2 2 moment magnitude range display a continuous distribution of their volumetric shear mix online material focal parameters for mine tremors at savuka

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

induced seismic deformation in the cogdell oil field of west texas a sequence of induced earthquakes occurred at the cogdell oil field in west texas from about 1974 to 1982 the earthquakes occurred within an array of injecting wells which surrounded the perimeter of the field and which decreased in radius during the life of the injection project we examine this sequence of earthquakes in terms of two failure models in the first model seismic failure occurs when and where fluid pressures exceed some critical value in the second model earthquakes occur at the edges of low fluid pressure areas where stresses have been transferred as a result of aseismic deformation in adjacent high fluid pressure regions we calculate the history of fluid pressures in the cogdell field using the theis equation for fluid flow in an areally infinite aquifer the calculations show that the earthquakes occur in a region of relatively low pressure surrounded by regions of higher fluid pressure the cogdell earthquakes may occur on pre existing faults however fluid pressures are sufficiently high in surrounding portions of the field to also produce failure a long time delay between the start of injection and the onset of seismicity may correspond to the time required for fluid pressures on a pre existing fault to reach critical values or may be the result of an evolving injection pattern whereby the inward migration of injecting wells produces aseismic failure which loads stress onto stronger regions within the center of the field injection of fluids for secondary recovery occurs at over three thousand sites in the state of texas many of these sites are geographically near cogdell and many have higher injection pressures and volumes of injected fluid than cogdell however they do not exhibit any seismicity a failure model which incorporates stress loading suggests that the perimeter injection pattern is responsible for the onset of seismicity a consequence of this interpretation is that many other injection fields may be undergoing large amounts of aseismic deformation

hengill triple junction sw iceland 1 tectonic structure and the spatial and temporal distribution of local earthquakes the hengill area is an unstable ridge ridge transform triple junction in sw iceland it contains the active central volcano hengill associated with the present accretionary zone and the extinct grensdalur central volcano associated with an accretionary zone that became inactive at about 0 7 m y when the locus of spreading migrated about 5 km w to the hengill zone the dominant tectonic trend of the area is n25 e parallel to the accretionary zones but transverse tectonic topographic features have also developed in the neighborhood of the two central volcanoes these result from local modification of the regional stress field associated with the spreading by radially symmetric stress fields associated with the volcanoes and the migration of volcanism a double high temperature geothermal system mirrors the double volcanic system the transform branch of the triple junction is represented by an ew striking zone of historic destructive earthquakes in the s of the area a seismological study was conducted with the aims of studying the tectonic structure and evaluating the passive seismic method as a geothermal prospecting tool the seismicity may be broadly divided into two groups first infrequent intense episodes of crustal movement occur that are associated with the release of tectonic stress along the plate boundary second continuous small magnitude activity occurs on a day to day basis this activity is mostly associated with the extinct grensdalur central volcano not the present plate boundary statistically significant temporal variations in b accompanied a swarm in the transform zone the continuous small magnitude activity may be geothermal in origin releasing thermal stress and not associated with plate boundary tectonics

the coso geothermal field a nascent metamorphic core complex investigation of the coso range using seismicity gravity and geochemistry of rocks and fluids supports the interpretation that the structure hosting the geothermal resource is a nascent metamorphic core complex the structural setting is a releasing bend in a dextral strike slip system that extends from the indian wells valley northward into the owens valley this tectonic setting results in nw directed transtension which is accommodated by normal and strike slip faulting of the brittle upper 4 6 km of the crust and shearing and ductile stretching below this depth accompanied by shallow igneous intrusions focal mechanisms of some small earthquakes that have occurred from 1996 to the present beneath the coso range exhibit depth dependent rotation of seismic p and t axes indicating that the local orientations of the principal stresses likely favor resolved shear stress on low angle faults these small earthquakes occur near the base of seismicity which we interpret as coincident with the brittle ductile transition geochemical results show a significant asthenospheric influence in the isotopic composition of rocks and fluids indicating that the crust is thinned within the coso structure thinned upper crust is underlain by a more dense mafic body at depths of 10 km or less this is consistent with observed gravity anomalies and models the mafic body may represent cumulates left over from the fractional crystallization of rhyolite which occurs as endogenous domes at coso or it could be a sheeted dike complex in the upper mid crustal area transtension began at 2 3 ma and continues today using a long term crustal deformation rate of 2 mm yr we infer that the basal detachment fault commonly observed in fully exhumed metamorphic core complexes will reach the surface in two to four million years