observations of long period earthquakes accompanying hydraulic fracturing waveforms of most seismic events accompanying hydraulic fracturing have been reported to contain clear p and s waves and have fault plane solutions consistent with shear displacement across a fault this observation is surprising since classical hydraulic fracturing theory predicts the creation of a tensile opening of a cavity in response to fluid pressure very small long period events similar to long period earthquakes observed at volcanoes were found to occur during four hydraulic fracturing experiments carried out at fenton hill new mexico since the long period earthquakes occur in the same region as the shear type events it is concluded that the unusual character of the long period earthquake waveforms is due to a source effect and not a path effect the occurrence of long period earthquakes during hydraulic fracturing could indicate tensile fracturing many waveforms of these events are identical which implies that these events represent repeated activation of a given source a proposed source for these long period events is the sudden opening of a channel that connects two cracks filled with fluid at different pressures the sizes of the two cracks differ which causes two or more peaks to appear in the spectra each peak beingmore associated with one physical dimension of each crack from the frequencies at which spectral peaks occur crack lengths are estimated to be between 3 and 20m less

source parameters and effects of bandwidth and local geology on high frequency ground motions observed for aftershocks of the northeastern ohio earthquake of 31 january 1986 abstract a 10 station array geos yielded recordings of exceptional bandwidth 400 sps and resolution up to 96 db for the aftershocks of the moderate m b 4 9 earthquake that occurred on 31 january 1986 near painesville ohio nine aftershocks were recorded with seismic moments ranging between 9 10 16 and 3 10 19 dyne cm m w 0 6 to 2 3 the two largest aftershocks depth 5 3 5 6 km oblique right slip rake 30 strike n25 e yielded seismic signals above background noise at frequencies as high as 130 hz at epicentral distances up to 17 km the aftershock recordings at a site underlain by 8 m of lakeshore sediments show significant levels of high frequency soil amplification of vertical motion at frequencies near 8 20 and 70 hz viscoelastic models for p and sv waves incident at the base of the sediments yield estimates of vertical p wave response consistent with the observed high frequency site resonances but suggest additional detailed shear wave logs are needed to account for observed s wave response peak acceleration values obtained from the broadband recordings are about two and four times as large as those that would be recorded on strong motion recorders or short period networks with upper bandwidth limits of 30 and 15 hz respectively attenuation corrected acceleration spectra are used to reduce the influence of high frequency up to 100 hz local site effects on corner frequency estimates the moment versus source radius trend inferred for events with moments as small as 9 10 16 dyne cm based on the brune source model extends previous relations inferred for the central united states shows little evidence for a minimum source radius and suggests that stress drops for the smaller events m 0 19 dyne cm decrease with decreasing moment

source parameters of injection induced microseismicity we analyze source parameters of microseismic events m 1 associated with high flow rate water injections in a shale formation at a depth of 220 m two types of events were observed several hundred impulsive events with clear p and s wave arrivals and continuous emissions with peaked spectra detected well into the experiment for a representative collection of impulsive events an 2 model provided satisfactory fits to displacement spectra corrected for attenuation and average quality factors of 34 and 15 were obtained for p and s waves p wave first motion analysis and e s e p ratios indicated the existence of a non double couple component in some events particularly early in the experiment a clear difference was observed for estimates of stress release parameters as non doublecouple events had smaller stress drops and apparent stresses the seismic efficiency of double couple and non double couple events was limited to 0 9 and 0 05 respectively with average values being 0 25 and 0 02 a comparison of our results with those reported for a similar magnitude range in a hard rock formation indicates considerably smaller estimates of stress drop and apparent stress in our case while seismic efficiencies are comparable

source parameters and scaling relations for reservoir induced seismicity in the longtan reservoir area we investigate the relationship between the impoundment and seismicity in the longtan reservoir southwestern china and find evidence that the seismicity was reservoir induced after the reservoir impoundment a pronounced increase in seismicity was observed in five clusters mainly concentrated in the areas where few earthquakes had occurred before the first filling the observed induced seismicity shows a strong correlation with the filling cycles the activity levels in the five clusters are different due to differences in the structures and permeabilities of the faults source parameters for 1 616 earthquakes with m l 0 1 4 2 recorded by 24 fixed and temporary stations deployed around the reservoir were calculated after applying corrections for geometrical spreading frequency dependent q and site effects the static stress drop and apparent stress in this area both appear to increase with increasing seismic moment over the entire magnitude range our results show that reservoir induced earthquakes have ten times lower average stress drop than natural tectonic earthquakes these results may indicate that the reservoir induced seismicity can occur with a lower tectonic stress due to the high pore pressures of the underground medium and that the effect of the water decreases the coefficient of friction

using dual well seismic measurements to infer the mechanical properties of a hot dry rock geothermal system high frequency 8 15 khz seismic waves that travel between two boreholes are used to probe the detailed structure of rock in the los alamos national laboratory fenton hill hot dry rock geothermal energy reservoir we study travel times of direct p waves and spectral characteristics of waveforms in an effort to determine whether or not large fluid filled fractures are present in the rock between the two boreholes data from two experiments separated in time by 1 year are studied we find dramatic differences between the two experiments in p wave velocity and in spectral characteristics of waveforms that traversed the same region we also find variations in p wave velocity and spectra as a function of location during one experiment loss of high frequency content of direct arriving p waves during the second dual well seismic experiment is compared with q found by using the coda wave technique aki and chouet 1975 and also with theoretical results on frequency dependence of transmission through a water filled fracture to determine the minimum number of large fractures in the system two to four fractures with thickness about 4 mm and vertical extent of as much as 200 m are required to explain observations during the second experiment efficient transmission of high frequencies observed during the first experiment is interpreted as meaning that fracture thickness was less than 1 mm at that time increased fracture thickness during the second experiment is probably due to a 75 day period of heat extraction during which in situ temperature dropped by as much as 100 below the virgin temperature since the coefficient of linear thermal expansion of granite is approximately 10 5 c a decrease in temperature of 100 c in a region on the order of 1 m around the fracture is required to explain the increased fracture thickness

induced microearthquake patterns in hydrocarbon and geothermal reservoirs six case studies the injection or production of fluids can induce microseismic events in hydrocarbon and geothermal reservoirs by deploying sensors downhole data sets have been collected that consist of a few hundred to well over 10 000 induced events we find that most induced events cluster into well defined geometrical patterns in many cases we must apply high precision relative location techniques to observe these patterns at three sedimentary sites thin horizontal strands of activity are commonly found within the location patterns we believe this reflects fracture containment between stratigraphic layers of differing mechanical properties or states of stress at a massive carbonate and two crystalline sites combinations of linear and planar features indicate networks of intersecting fractures and allow us to infer positions of aseismic fractures through their influence on the location patterns in addition the fine scale seismicity patterns often evolve systematically with time at sedimentary sites migration of seismicity toward the injection point has been observed and may result from slip induced stress along fractures that initially have little resolved shear in such cases triggering events may be critical to generate high levels of seismic activity at one crystalline site the early occurrence of linear features that traverse planes of activity indicate permeable zones and possible flow paths within fractures we hope the continued development of microseismic techniques and refinement of conceptual models will further increase our understanding of fluid behavior and lead to improved resource management in fractured reservoirs

simultaneous determination of site responses and source parameters of small earthquakes along the atotsugawa fault zone central japan velocity seismograms recorded at nine local stations from 102 small earthquakes on the atotsugawa fault zone central japan with seismic moment from 3 10 17 to 3 10 21 dyne cm are analyzed to obtain the s wave spectra a genetic algorithm is applied to determine the earthquake source parameters and site responses simultaneously the stability of both site responses and source parameters are experimentally tested as well as the effects of different q models the resulting source parameters show that the stress drops appear to vary systematically with the location of earthquakes events located on the mozumi fault or the junction of the mozumi and the atotsugawa faults have higher stress drops than those on the creeping section and southernmost part of the atotsugawa fault the relationship between seismic moment and corner frequency constructed by combining our results and those of iio 1986 for other regions in japan shows that a single scaling relation does not apply in a moment range of 10 10 to 10 25 dyne cm a steplike change in scaling relationship occurs between 5 10 18 and 5 10 20 dyne cm corresponding to corner frequencies between 15 and 25 hz this observation suggests that earthquakes are most likely a multihierarchical system divided by characteristic sizes according to aki9s 1980 model a characteristic source dimension 100 200 meters may represent a critical size of the barriers distributed in the fault zone at which the barrier effects are balanced by the dynamic effect of the source process

precise microearthquake locations and fluid flow in the geothermal reservoir at soultz sous forets france i relocated two microearthquake clusters induced by hydraulic stimulation of a hot dry rock geothermal reservoir in the rhine graben near soultz sous forets france the two clusters were chosen from a collection of clusters identified within a 16 000 event data set obtained in 1993 i determined p and s wave arrival times manually choosing distinctive peaks if waveforms were similar at a given station or first breaks if waveforms were emergent or nodal judging weights accordingly requiring a minimum of five arrival times yielded populous clusters of 226 and 355 events in both cases relocation revealed two distinct planar patches of activity 100 to 200 m across intersecting and truncating one another along a common edge linear segments traverse the planar patches and may mark intersections with seismically inactive fractures the earliest activity occurred along these segments these segments may therefore mark permeable zones that could be important components of the fluid flow network the interpretation of linear segments as flow paths is supported by path dimension constraints derived from fluid flow modeling studies and by the expected confining effects of hydrothermal sealing found in cored fractures orientations of the planar features are verified through consistency with orientations found in core and logging studies in addition focal mechanisms obtained by constraining slip planes parallel to active planes indicate normal to right lateral strike slip consistent with the measured stress field focal mechanisms indicate mass deficits at inside corners implying that the concentrations of activity at fracture intersections may result from slip on one fracture causing extension and a resulting permeability increase in the other the improved location precision results from a ten fold improvement in the precision of arrival time estimation for phases that were similar as estimated from the standard deviation of residuals grouped by station and phase before and after repicking only a two fold improvement was noted for poorly correlated or nodal phases for this data set the manual phase picking yielded more interpretable detail in the location patterns compared to results of an automatic cross correlation technique the manual results could be used to guide improvements to automatic techniques to obtain precise locations of the entire data set

source parameter analysis from strong motion records of the friuli italy earthquake sequence 1976 1977 abstract time and frequency domain analyses are applied to strong motion data recorded in friuli italy during 1976 to 1977 an inversion procedure to estimate spectral parameters low frequency level corner frequency and high frequency decay has been applied to displacement spectra using a simple earthquake source model with a single corner frequency the data were digitized accelerograms from enea enel portable and permanent networks instrument corrected sh waves were selected from a set of 138 three component hand digitized records and 28 automatically digitized records thirty eight events with stations having 8 to 32 km epicentral distance were studied different stress drop estimates were performed showing high values 200 to 300 bars on the average with seismic moments ranging from 2 8 10 22 to 8 0 10 24 dyne cm the observation of systematic higher values of brune stress drop obtained from corner frequencies with respect to other time and frequency domain estimates of stress release and the evidence on time series of multiple rupture episodes suggest that the observed corner frequencies are most probably related to subevent ruptures rather than the overall fault size seven events recorded at more than one station show a good correlation between rms brune and dynamic stress drops and a constant scaling of this parameter as a function of the seismic moment when single station events are also considered a slight moment dependence of these three stress drop estimates is observed differently this may be explained by an inadequacy of the 2 high frequency decay of the source model or by high frequency attenuation due to propagation effects the high frequency cutoff of acceleration spectra indicates the presence of an f max in the range of 5 to 14 hz except for the stations where local site effects produce spectral peaks

the 1982 enola arkansas swarm and scaling of ground motion in the eastern united states abstract the north central arkansas swarm which began in january of 1982 and was still active as of december 1983 has produced over 30 000 events three of these events had duration magnitudes greater than 4 0 fifty three locatable events in the magnitude range 3 8 and below were recorded during the 2 1 2 weeks our 10 digital seismographs were in place the hypocentral locations are confined to a tight cluster with a radius of approximately 1 km depths range between 4 and 7 km focal mechanisms determined for 10 of the events show a combination of strike slip and normal faulting source parameters seismic moment source radius and stress drop were calculated for 48 of the events the seismic moments range from 10 18 to 10 21 dyne cm while the stress drops range from 0 2 to 60 bars acceleration spectra are band limited at the high end at 30 hz the stress drops increase with seismic moment up to 10 20 dyne cm but appear to be constant at greater moment values the arkansas source parameters are similar to other body wave source parameters determined from near source investigations of earthquakes in eastern and central north america e g new brunswick and south carolina and in california e g mammoth lakes oroville and anza but differ substantially from lg wave source parameters estimated for earthquakes in the eastern and central united states

induced microearthquake patterns and oil producing fracture systems in the austin chalk abstract microearthquakes collected during hydraulic stimulation allowed us to study fracture zones in austin chalk oil reservoirs at two sites in the giddings field texas we deployed three component downhole geophone tools in production wells at depths of 2100 m and greater one near cook s point and two on the matcek lease near caldwell at cook s point we collected 482 microseismic events during a 4000 m 3 25 000 bbl hydraulic stimulation in an offset well we collected 770 events during a similar operation on the matcek lease many seismograms contained reflected phases that constrained location depths to the production zone at the base of the austin chalk by restricting all microearthquake locations to production depths we located 20 of the cook s point events and over 60 of the matcek events at both sites we observed only the fracture wing closest to the observation stations locations formed elongated patterns extending up to 1 km from the stimulation well and trending n60 e parallel to the known regional fracture trend the cook s point seismic zone measured over 100 m in width while long stretches of the matcek seismic zone narrowed to 30 m or less we believe that the width of the seismic zone reflected the density of conductive fractures and thus the volume of the reservoir accessed by the stimulation indeed production rates in the first year following stimulation were much higher at cook s point where we observed the wider of the two seismic zones

understanding the seismic velocity structure of campi flegrei caldera italy from the laboratory to the field scale we report laboratory measurements of p and s wave velocities on samples of tuff from campi flegrei italy and a new tomographic velocity map of the campi flegrei caldera laboratory measurements were made in a hydrostatic pressure vessel during both increasing and decreasing effective pressure cycles selected samples were also thermally stressed at temperatures up to 600 c to induce thermal crack damage acoustic emission output was recorded throughout each thermal stressing experiment and velocities were measured after thermal stressing laboratory p and s wave velocities are initially low for the tuff which has an initial porosity of 45 but both increase by between 25 and 50 over the effective pressure range of 5 to 80 mpa corresponding to a decrease of porosity of 70 marked velocity hysteresis due to inelastic damage processes is also observed in samples subjected to a pressurization depressurization cycle tomographic seismic velocity distributions obtained from field recordings are in general agreement with the laboratory measurements integration of the laboratory ultrasonic and seismic tomography data indicates that the tuffs of the campi flegrei caldera can be water or gas saturated and shows that inelastic pore collapse and cracking produced by mechanical and thermal stress can significantly change the velocity properties of campi flegrei tuffs at depth these changes need to be taken into account in accurately interpreting the crustal structure from tomographic data

can small events m 0 it is becoming evident that under different circumstances such as injection rates and geologic conditions it might be possible to generate larger magnitude m 0 events during hydraulic fracture stimulations by utilizing fully integrated passive seismic monitoring programs we can examine the relationship between the occurrence of small magnitude events m 0 and the fracture and stress conditions that may lead to larger events in our investigations we identify that over all scales of observation the events follow a self similar behavior however the small magnitude events are generally lower in stress release than observed for the larger events these differences can be explained by the observed failure mechanisms where smaller events tend to be driven by shear tensile failures of pre existing discrete fractures joints whereas the larger events appear to be dominated by shear driven failure processes associated with pre existing faults these observations suggest that there can be sufficient

paleovalley morphology and fluvial system evolution of eocene oligocene sediments auriferous gravels northern sierra nevada california implications for climate tectonics and topography in the northern sierra nevada california eocene oligocene fluvial sediments the auriferous gravels rest unconformably in well defined basement incised paleovalleys providing a record of paleolandscape geomorphology and drainage system evolution paleovalley surface mapping shows that local topography and bedrock lithology controlled the location of broad terraced paleovalley segments which formed in the central and eastern metamorphic bedrock belts and subsequently became areas of significant fluvial deposition high gradient high energy deposits are present near the edges of these areas suggesting that tributaries and outlets resided in steeper narrow paleovalleys multiple strath terraces are common in broad paleovalleys indicating that incision events occurred both before deposition and during paleovalley widening followed by periods of aggradation broad paleovalley deposits contain predominantly braided stream facies deposited in an energetically variable system multiple upward fining cycles bounded by erosional surfaces are composed of alternating coarse grained channel belt and fine grained floodplain packages an overall upward fining succession suggests that the locus of fluvial deposition shifted and depositional energy waned over time as the system backfilled successive broad paleovalleys paleochannel slope estimates of 0 004 0 055 were obtained based on average grain sizes of coarse channel fills anomalously thick up to 11 m laterally continuous 1 3 km floodplain units containing kaolinite rich clay and oxisol intervals record high fine grained sediment supply driven by high rates of chemical weathering in the warm eocene climate based on sedimentary characteristics combined with floral and detrital zircon data braided stream deposition within the basin occurred from the late early eocene through late eocene and aggradation primarily occurred in broad lower gradient reaches behind a major bedrock knickpoint created in the late mesozoic early cenozoic possibly due to asymmetric batholith unroofing the diachronous localized nature of paleovalley incision and braided stream aggradation within the northern sierra nevada invalidates previous estimates of the timing and amount of range uplift based on paleochannel reconstructions and gradients both internal and external drivers influenced the evolution of the fluvial system through 1 the pattern of basin filling as the locus of deposition migrated east to west 2 the warm eocene climate which increased chemical weathering and contributed a high sediment supply and 3 changes in base level due to eustasy and or laramide tectonism this record of laramide tectonics in the sierra nevada and basin and range is important for the timing of sierra nevada uplift for tectonic reconstructions of western north america and for studies of landscape evolution in tectonically active orogens

the digital array at anza california processing and initial interpretation of source parameters seismicity along the san jacinto fault suggests that a 40 km long section near the town of anza may constitute a gap in the occurrence of ml 6 7 earthquakes the potential for a gap filling shock and the high rate of seismicity at the southern end of the gap five events of 4 0 ml 5 5 since 1970 provided the impetus for deploying a digital seismic array to collect high quality ground motion recordings of all events 2 ml 4 5 on scale recording for shocks with magnitudes above ml 4 can be obtained from an existing array of eight strong motion accelerographs the anza site also had the advantage of being in the southern california batholith which appears to be relatively homogeneous compared to the franciscan gablian contrast of the central san andreas we expected that the granitic rocks of the batholith would yield relatively accurate earthquake locations and efficiently propagate high frequencies the field instrumentation is specifically designed for broadband recording up to 70 hz and high dynamic range 96 db in the on site digitizer alone since both are necessary for determining the rupture history of earthquakes both local vhf and microwave digital telemetry transmit the data from the anza region to san diego for computer data logging in the first 30 months of the array s operation approximately 292 events have been recorded located and processed for source parameters most events occur in one of five clusters or in a diffuse zone near the buck ridge fault two of these clusters are located at right stepping en echelon offsets coyote creek san jacinto and san jacinto hot springs two others are directly below and about 8 km west of anza respectively the fifth cluster is just to the northeast of the hot springs fault although event depths are generally between 11 and 14 km at the southern end of the hot springs fault depths extend to 18 km these are some of the deepest strike slip earthquakes on the san andreas system we calculate source parameters such as the scalar moment and stress drop for the analysis of high frequency waves scaling relations and earthquake interaction the largest event recorded thus far had a moment of 4 4 1021 dyn cm m 3 8 and a stress drop of 55 bars both arms and brune stress drops increase with moment source radius increases only slowly with moment the maximum values of both the brune and arms stress drops increase with depth down to 10 km remain approximately constant to 14 km and may decrease below 14 km the data suggest that stress drops of a group of earthquakes can be related to the strength of the upper crust calculated from frictional and quasi plastic flow laws although individual events may not be related stress drops on the san jacinto fault are high compared to those on the central creeping section of the san andreas where stress drops are about 10 bars or less this observation is consistent with the relative rupture area of events of equal ml 5 5 and may be related to the lithologie differences between the two fault segments and the amount of normal stress compared to shear stress

magnitude frequency relation for small earthquakes a clue to the origin of ƒmax of large earthquakes in order to test the hyothesis that the max of large earthquakes is due to the source effect and is causally related to the corner frequency of small earthquakes when it becomes constant below magnitude about 3 we constructed the frequency magnitude relation for earthquakes with magnitude range from 1 3 to 4 recorded at a borehole seismograph station operated by the university of southern california at baldwin hills in the middle of the newport inglewood fault a procedure was developed for an accurate estimation of moment magnitude from the measurement of coda amplitude as a function of lapse time the cumulative number of earthquakes per unit time period and area was estimated by finding the detection limit for ts p for each magnitude we found a clear depature of the observed log frequency magnitude relation for m 3 the observed frequency of earthquakes with m 0 5 is almost 10 times less than that expected from the extrapolation of empirical relation for earthquakes with magnitudes larger than 3 we conclude that the observed frequency magnitude relation departs from self similarity for earthquakes with magnitude smaller than about 3 this departure coincides with the widely observed departure of the moment corner frequency relation from self similarity these observations are consistent with the interpretation of max of large earthquakes in terms of the slip weakening instability model in which the critical slip may be a few tenths of a meter to a few meters and the cohesive zone size may be a few hundred meters to a few kilometers

faulting induced by forced fluid injection and fluid flow forced by faulting an interpretation of hydraulic fracture microseismicity carthage cotton valley gas field texas we analyzed precisely located microearthquake data detected during five hydraulic fracture treatments in the carthage gas field of east texas the treat ments were conducted in two adjacent boreholes within interbedded sands and shales of the upper cotton valley formation the microearthquakes were induced within narrow horizontal bands that correspond to the targeted sandstone layers events throughout all the treatments show strike slip faulting occurring uniformly along vertical fractures trending close to maximum horizontal stress direction these events are consistent with the reservoir s prevalent natural fractures known to be isolated within the sands and trending subparallel to the expected hydraulic fracture orien tation when this uniform fracture system was activated exclusively the detected shear deformation measured as the moment release per unit volume of fluid injected was constant independent of various fluid viscosities and flow rates used within the base of the upper cotton valley formation anomalous event counts and moment release occurred within dense clusters that delineate bends or jogs in the fracture zones the mechanisms are also strike slip but the fault planes are more favorably oriented for failure the dense clusters show location patterns diverging in time suggesting the expulsion of fluids from compressive fault jogs fluid flow forced by the adjacent slip induced loading appears to initially extend the treatments but the loading also tends to lock up and concentrate stress at the jogs as evident by fewer but larger events populating the structures as treatments progress as a result effec tive drainage lengths from the boreholes may be shorter than would be inferred from the seismicity extending past the jogs these high moment asperities are similar to dense patches of seismicity observed along creeping sections of the san andreas fault where they have been attributed to localized zones of strength or stress con centration surrounded by larger regions undergoing stable aseismic slip this sim ilarity plus large moment deficits in terms of volume injected suggests a large component of aseismic slip is induced by the cotton valley treatments

fracture compliance estimation using borehole tube waves abstractwe tested two models one for tube wave generation and the other for tube wave attenuation at a fracture intersecting a borehole that can be used to estimate fracture compliance fracture aperture and lateral extent in the tube wave generation model we consider tube wave excitation in the borehole when a p wave is incident on the fracture the amplitude ratio of the pressure due to the tube wave to that of the incident p wave is a function of fracture compliance aperture and length similarly the attenuation of a tube wave in the borehole as it crosses a fracture intersecting the borehole is also a function of fracture properties numerically solving the dispersion relation in the fracture we study tube wave generation and the attenuation coefficient as a function of frequency we observed that measuring amplitude ratios or attenuation near a transition frequency can help constrain the fracture properties the transition frequency corresponds to the regime in which the viscous skin depth in

earthquake source parameters and the frequency dependence of attenuation at coalinga mammoth lakes and the santa cruz mountains california least squares best fits were calculated to the fourier amplitude spectra of p and s waves of locally recorded earthquakes attenuation amplification and earthquake source parameters were calculated from parameters of the fits approximately 8200 spectra were fit at 39 sites at coalinga mammoth lakes and the santa cruz mountains california typically the spectra were fit from 2 to 45 hz for p waves and 2 to 20 hz for s waves the source spectral falloff of p waves was limited to be no greater than f 2 5 f is frequency by low attenuation sites in the sierra nevada attenuation was measured through t the travel time divided by q attenuation was considered to be frequency dependent according to t to f which is equivalent to q qof negative values of were found to minimize the sum of squared residuals at most of the sites studied attenuation and amplification varied from site to site systematically with site geology source parameters were determined for individual earthquakes by averaging fit parameters from between four and 18 spectra depending on the earthquake seismic moments ranged from roughly 5 1012 to 1015 nm at coalinga and mammoth lakes moments ranged from 2 1011 to 3 1014 nm in the santa cruz mountains when the fits were determined using negative values of a the seismic moments trended approximately as fc 3 fc is the corner frequency for 0 5 stress drops varied from roughly 0 2 to 50 mpa at coalinga and mammoth lakes stress drops were lower in the santa cruz mountains ranging from approximately 0 05 to 8 mpa

the cause of frequency dependent seismic absorption in crustal rock summary the absorption of 5 to 200 hz seismic wave energy propagating at depth in fracture bearing crystalline basement in southern california is observed to increase as a power law in wave frequency as f0 43 0 1 so that energy absorption per unit frequency q 1 decreases with frequency as f 0 57 and the seismic quality factor q increases with frequency as f0 57 the value of q at 10 hz is 1000 crustal fracture density which elastic scattering and borehole logs show to vary with fracture dimension l as a power law l0 4 cannot easily explain the observed frequency dependence of absorption a likely hypothesis identifies the f0 43 0 1 frequency dependence of seismic absorption with the f1 12 frequency dependence of a thermal diffusion process seismic absorption computed for thermal gradients induced by localized strain concentrations depends on a single free parameter the thermal absorption domain characteristic size d if d 0 2 mm the computed magnitude of absorption agrees with the observed q 1000 at 10 hz thin sections from deep borehole rock samples show that 0 2 mm is the characteristic dimension of microfracture shear fabric and crystalline grains where seismic elastic scattering occurs over a wide range of fracture sizes seismic absorption occurs predominantly at the dimension of microfractures and granularity systematic wide band borehole seismic observation of seismic absorption magnitude and particularly frequency dependence combined with thin section analysis of the rock microfabric can test the hypothesis for a range of crustal rocks the evidence of thermal activity at the scale of microfractures suggests that static stress concentrations possess a free energy that can act to align microfractures thus explaining the shear wave birefringence widely observed in the crust

source parameters and rupture velocity of small m 2 1 reservoir induced earthquakes summary we calculate stress drop and rupture speed for ml 2 1 shallow reservoir induced earthquakes and find them to be similar to those of large natural earthquakes previous studies have suggested that hydrofractures mining and reservoir induced earthquakes have lower average stress drop than natural tectonic earthquakes this difference might result from the different tectonic setting or the shallower hypocentral depths of induced earthquakes alternatively difficulties in correcting for attenuation and site effects in earlier studies may lead to underestimation of stress drop in addition most studies assume the rupture velocity of small reservoir induced earthquakes to be the same as for the large earthquakes we analyse a set of 101 ml 2 1 earthquakes induced by changing water level in the acu reservoir ne brazil the earthquakes are shallow depth 5 km and the region has negligible natural seismicity we use three different approaches to calculate the source parameters of the six largest 1 9 ml 2 1 earthquakes we model the individual spectra to find corner frequency frequency independent q and long period amplitude we use collocated small earthquakes as empirical green s functions to calculate the spectral ratios and determine the relative source time functions estimates of the source duration and corner frequency imply stress drops in the range of 26 179 mpa these are similar to or higher than tectonic earthquakes suggesting that the shallow hypocentral depth and the presence of water do not affect stress drop we observe clear directivity for one of the earthquakes and use the azimuthal variation in pulse width to estimate a rupture velocity of 0 6

a model for attenuation and scattering in the earth s crust the mechanisms contributing to the attenuation of earthquake ground motion in the distance range of 10 to 200 km are studied with the aid of laboratory data coda waves rg attenuation strong motion attenuation measurements in the northeast united states and canada and theoretical models the frequency range 1 10 hz has been studied the relative contributions to attenuation of anelasticity of crustal rocks constant q fluid flow and scattering are evaluated scattering is found to be strong with an albedo b 0 0 8 0 9 and a scattering extinction length of 17 32 km the albedo is defined as the ratio of the total extinction length to the scattering extinction length the rg results indicate that q increases with depth in the upper kilometer or two of the crust at least in new england coda q appears to be equivalent to intrinsic anelastic q and indicates that this q increases with frequency as q q 0 f n where n is in the range of 0 2 0 9 the intrinsic attenuation in the crust can be explained by a high constant q 500 q 0 2000 and a frequency dependent mechanism most likely due to fluid effects in rocks and cracks a fluid flow attenuation model gives a frequency dependence q q 0 f0 5 similar to those determined from the analysis of coda waves of regional seismograms q is low near the surface and high in the body of the crust

source parameters of the 1980 mammoth lakes california earthquake sequence from the more than 1500 mammoth lakes earthquakes recorded on three component digital seismographs spudich et al 1981 150 were used in an analysis of the locations mechanism and source parameters a composite fault plane solution of nine earthquakes 3 9 m 5 1 defines a right lateral strike slip mechanism on a steeply dipping nearly east west plane striking s75 e or left lateral strike slip on a nearly north south plane striking n10 e vertical cross sections of well located aftershocks indicate possibly three east west planes that coincide with the locations of the four largest earthquakes with ml 6 0 using the spectral analysis of s waves brune 1970 source parameters for 67 earthquakes were determined forty eight had magnitudes greater than or equal to 3 0 seismic moments ranged from 9 20 1018 dyn cm to 2 33 1024 dyn cm earthquakes with seismic moment greater than about 1 0 1021 dyn cm had nearly constant stress drops 50 bars earthquakes with seismic moment less than about 1 0 1021 dyn cm had stress drops that apparently decrease as seismic moment decreases

interpretation of seismic data from hydraulic fracturing experiments at the fenton hill new mexico hot dry rock geothermal site an attempt was made to synthesize the results of active seismic experiments carried out by the los alamos national laboratory s hot dry rock project staff for determining the geometrical and physical properties of the fracture system produced by hydraulic fracturing in a hot low permeability rock interpretation of data from several reflection transmission and attenuation experiments using seismic probes in the frequency range from a few hundred to more than 10 sup 4 hz led us to postulate the existence of a highly complex fracture system consisting of major discrete vertical cracks intersected by several inclined joints which are surrounded by a large volume of rock containing small scale cracks we found an interesting coincidence between the mean square fluctuation of p arrival time and q sup 1 of coda waves both are increased by nearly the same amount 35approx 40 when fluid pressure in the reservoir is increased another coincidence is the scale length of the inhomogeneity of roughly 3 m obtained from the frequency dependence of attenuation and also from the spatial variation of p arrival time these results suggest that the seismic attenuation in the fractured region is caused primarily by scattering

effects of viscoelasticity on seismic wave propagation in fault zones near surface sediments and inclusions by defining anelastic effects through the superposition of relaxation mechanisms and incorporating them into the wave equation in terms of memory variables the time domain computation of responses of viscoelastic media with arbitrary spatial variations is feasible this approach is applied to simulation of the effects of realistic distribution of compressional and shear wave quality factors qp and qs in a sequence of viscoelastic models of increasing complexity from a homogeneous medium to a fault zone a near surface attenuating zone and inclusions with anomalous qp and qs values comparisons of elastic and viscoelastic responses for both surface and borehole recordings show pronounced q dependent attenuation analysis of data from viscoelastic media using elastic rather than a viscoelastic model may result in misleading or erroneous interpretations

locations and spectral properties of earthquakes accompanying an eruption of mount st helens a joint hypocenter determination routine has been developed to accurately determine the locations of earthquakes occurring under mount st helens the method has a special application to mount st helens where most of the earthquakes are suspected to occur at shallow depth in a rather limited volume under the volcano differences in elevation among stations are allowed for in this routine the location procedure determines a best half space velocity and one station correction for each station simultaneously with earthquake hypocenters arrival time data from an array of portable digital seismic event recorders are supplemented with data from stations of the permanent university of washington seismic network operating at the volcano earthquakes are found to occur along a line trending south east of the dome inside the crater the shallowest events occur near the dome with hypocentral depth increasing with increasing distance from the dome station corrections can be interpreted to imply the existence of a high velocity body beneath the crater which extends at least between depths of 5 and 2 1 km beneath the crater floor spectra are calculated from earthquake waveforms recorded at three stations the spectra show very little evidence for local site or path effects on frequency content of waveforms stress drops of earthquakes are calculated from measurements of corner frequency and low frequency spectral amplitude stress drops of earthquakes vary between 5 and 5 bars there is no evidence for a limiting corner frequency for small earthquakes as has been found for earthquakes occurring in mammoth lakes california data fit well a linear trend between log seismic moment and log source radius fit to data for earthquakes ranging in size from 1014 to 1027 dyn cm suggesting that there is no limiting source radius observed for earthquakes in this range of size

locating microearthquakes induced by hydraulic fracturing in crystalline rock microearthquakes induced by hydraulic fracturing in crystalline rock at a depth of 3 5 km were located with a precision of better than 30 m to obtain information about the geometry and dimensions of the fracture system produced the induced microseismicity was monitored by a network of five vorehole seismic stations a total of about 800 induced events were reliably located from arrival times event locations show a tabular distribution that strikes 350 sup 0 and dips 65 sup 0 east subparallel to the injection well the injection was intended to produce a fracture system that would hydraulically connect two subparallel wells a lack of fluid communication between them is consistent with a lack of induced microearthquakes near the target wellbore the 150 m thickness of the zone of seismicity far exceeds the relative locational uncertainties the injected fluid appears to have stimulated a zone of rock rather than simply a single fracture the distribution of microseismic events presumably envelopes the zone of fluid paths created by the fracture experiment

detailed joint structure in a geothermal reservoir from studies of induced microearthquake clusters microearthquake clusters form distinct planar patterns within five study regions of a geothermal reservoir undergoing hydraulic fracturing at fenton hill new mexico the patterns define individual slipping joint surfaces of dimension 40 120 m containing 80 150 events each sharp straight edges truncate the clusters we interpret these as marking intersections with aseismic joints each edge orientation is consistent with an intersection between the active joint and a plane oriented parallel to one of the other clusters we identify therefore it appears that cluster shapes constrain the geometry of seismic and aseismic joints both could be important components of the fluid flow network the distribution of inferred slip plane orientations is consistent with but fails to provide sufficient constraint to differentiate conclusively between two very different stress field estimates one measured using pressurization and wellbore breakouts the other using focal mechanisms of the largest microearthquakes an impermeable joint model requiring pore pressure in excess of the normal stress on a joint before slip can occur was inconsistent with many of the inferred slip plane orientations the high quality locations were possible because events from the same cluster generated nearly similar waveforms permitting the precise determination of relative arrival times standard deviations of arrival time residuals fall between 0 1 and 1 1 ms for these clusters major axes and aspect ratios of the 90 confidence ellipsoids range from 6 to 28 m and 1 5 to 8 respectively small events dominate the seismic energy release and thoroughly populate the identified active joints allowing the hypocenters to reflect details of the joint structure to further investigate the reservoir structure we applied a source array slant stack technique to waveforms from the well located clusters yielding directions that scattered energy left each cluster by studying paths of scattered waves we expected to pinpoint impedance contrasts that might have indicated concentrations of fluid filled joints however results show that scattered energy in the s wave coda left the source region in the same direction as the direct s wave direct waves may have excited borehole tube waves that became trapped in the vicinity of the geophone tool overwhelming any energy scattered from the reservoir

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

seismic monitoring of the growth of a hydraulic fracture zone at fenton hill new mexico the hydraulic fracturing technique is an important method for enhancing hydrocarbon recovery geothermal energy extraction and solid waste disposal determination of the geometry and growth process of a hydraulic fracture zone is important for monitoring and assessing subsurface fractures a relative source location approach based on a waveform correlation and a grid search method has been developed to estimate relative hypocenter locations for a cluster of 157 microearthquakes induced by hydraulic fracturing at the los alamos hot dry rock hdr geothermal site among the 157 events 147 microearthquakes occurred in a tight cluster with a dimension of 40 m roughly defining a vertical hydraulic fracture zone with an orientation of n40 w the length height and width of the hydraulic fracture zone are estimated to be 40 35 and 5 m respectively analysis of the spatial temporal pattern of the induced microearthquakes reveals that the fracture zone grew significantly averaging 0 2 m minute in a two h

analysis of the cleburne texas earthquake sequence from june 2009 to june 2010 on 9 june 2009 an graphic 1 2 8 earthquake shook cleburne texas a community not known to have previously experienced earthquakes over 50 small earthquakes followed by the end of december 2009 a temporary network of four and then five iris passcal broadband systems was deployed from june 2009 to june 2010 recording data that were used to locate 38 events with the most confident p and s arrival picks event locations were distributed along a 2 km long north northeast trend the location centroid was at 32 298 n 97 372 w and at 3 6 km depth this location is approximately 1 3 km from a saltwater disposal well that began injection in october 2007 and 3 2 km away from a second injection well that was active from september 2005 to late july 2009 focal mechanisms estimated for the best recorded events suggest a north northeast south southwest trending normal fault with a dip of 50 and a component of oblique motion rake of 80 this average solution is generally consistent with the north northeast trending extensional faults that are prevalent across parts of texas oklahoma louisiana and arkansas stress drops calculated from p and s spectra for seven different events ranged from 3 9 to 90 bars with most estimates between 40 and 50 bars typical values for intraplate earthquakes because there were no known previous earthquakes and the located events were close to the two injection wells and near the injection depth the possibility exists that earthquakes may be related to fluid injection online material tables of absolute and relative earthquake locations and a figure summarizing focal mechanism solutions 1 embed inline graphic 1 gif

the effects of site response on source parameters deduced for the 1980 long valley california earthquake sequence data taken from the university of nevada unr array of surface located wideband digital displacement seismographs placed over the 1980 long valley earthquake sequence show very strong site effects which control spectral parameters and hence inferred source parameters up to ml 5 0 observations from 4 000 seismograms show that differences in site effects can lead to inferred seismic moments at least a factor of two lower than previously reported observed s wave spectral corner frequencies show upper bound cutoffs which are widely disparate at different sites e g 15 hz at mg1 and 6 hz at wt1 thus fmax is quite variable by site this high frequency cutoff clearly a site effect at wt1 could be misinterpreted as an apparent departure from source similarity for seismic moments below 1021 dyn cm estimates of seismic moment and source duration from the unr rock sites may be more accurate than those previously published both because the unr instrumentation has greater resolution at longer periods 0 1 to 1 0 hz and because of less near site signal distortion at high frequencies

source parameters of seismic events at heinrich robert mine ruhr basin federal republic of germany evidence for nondouble couple events abstract a three component digital seismic station is in operation at heinrich robert coal mine in the ruhr basin at a depth of 890 m a cluster of seismic events occurred at the beginning of 1987 at a distance of about 5 km from the station and continued in 1988 source parameters of 46 tremors selected from the cluster with local magnitude from 1 2 to 2 3 are studied the ratio of p wave to s wave corner frequency is variable implying its dependence on the angle of observation and in turn the variability of focal mechanism with time the scaling relations indicate that characteristic tremors with a fault length of about 200 m are possibly generated in the area but the evidence is not conclusive the seismic energy for a given seismic moment can vary by a factor of 20 and the ratio of s wave to p wave energy ranges from 1 5 to 30 stress release estimates show low values not exceeding a few bars typical for tremors induced by mining a comparison of the apparent stress with the static stress drop divides the selected tremors into high and low apparent stress events the latter events are characterized by high p wave energy and almost constant and large ratio of p wave to s wave corner frequency which differs from the lower ratios of other events implying similar focal mechanism these high p wave energy tremors are the most likely candidates for nondouble couple events

microearthquake imaging of the parkfield asperity microearthquake data from a downhole seismometer network on the san andreas fault appear to outline two aseismic asperities that may correspond to the locations of the foreshocks and main shocks of the parkfield characteristic earthquakes the source parameters of the microearthquakes show that a few of the earthquakes have significantly higher stress drops than most furthermore the magnitude frequency statistics suggest that at local magnitude 0 6 the cumulative number of small events begins to fall off the usual gutenberg richter b 1 relation in which the number of events increases exponentially with decreasing magnitude the downhole seismometer data establish a baseline from which the evolution of the earthquake process at parkfield can be monitored and suggest that different mechanical conditions than those that lead to the typical gutenberg richter relation may be operating for the smallest of parkfield microearthquakes

waveform inversion of seismic refraction data and applications to young pacific crust summary we present a method for constraining the velocity depth structure in the earth using seismic refraction waveform data we test the method with synthetic data from known models and apply it to a set of data collected in 1982 june from the east pacific rise at 13 n from the magma expedition in this iterative process wkbj seismograms are computed for a starting model the difference between these and the observed seismograms is used to update the model subject to physical constraints an important first step in the inverse scheme is the linearization of the wkbj seismogram equation allowing us to compute differential seismograms partial derivatives of the synthetic seismogram with respect to specific model parameters this linearization provides the means for estimating required model perturbations based on the misfit in the seismograms the choice of a suitable numerical strategy for computing an updated model is a crucial second step in formulating a working algorithm because the data contain noise synthetic seismograms can only fit the data to this noise level in this case infinitely many models fit the data to this tolerance and some of these estimates are non physical involving negative layer thicknesses a successful strategy must choose from among these possibilities a well defined physically reasonable new model in a commonly used approach to solving non linear problems the perturbation to the starting model is minimized while improving the fit to the data after several iterations the final model which possesses no special properties still tends to resemble the starting model when used with the magma data this technique essentially does not perturb the model at all a method we find much more satisfactory involves solving for the new model directly while applying physically important constraints as constraints we require the velocity gradient remain below a fixed value and penalize the roughness of the new model we thus solve for the smoothest model fitting the data to the specified misfit

reliable and economical high temperature deep borehole seismic recording abstract recording earthquakes with borehole seismometers has become increasingly popular in recent years as the advantages in noise reduction and also the distorting effects of near surface rocks especially sediments have become well known borehole recording can be extremely complex involving active sensors special cables and downhole electronics such installations however are often not very reliable at the high temperatures reached in active tectonic areas at depths of 1 km and greater here we describe a simple and reliable system for 3 component recording of local earthquakes at single and multiple depths greater than 1 5 km and temperatures up to 120 c our system was designed and developed for experiments in the cajon pass scientific drillhole of southern california the borehole packages are made of titanium with spring loaded clamping allowing easy retrieval standard seven conductor oil well logging cables are used together with a specially designed cablehead the data recorded have been used for investigating earthquake source scaling attenuation in the mid crust and also near surface site effects

simultaneous inversion for q and source parameters of microearthquakes accompanying hydraulic fracturing in granitic rock an inversion that fits spectra of earthquake waveforms and gives robust estimates of corner frequency and low frequency spectral amplitude has been used to determine source parameters of 223 microearthquakes induced by hydraulic fracturing in granodiorite assuming a 2 source model the inversion fits the p wave spectra of microearthquake waveforms to determine individual values of corner frequency and low frequency spectral amplitude for each event and one average frequency independent q for all source receiver paths we also implemented a constraint that stress drops of all microearthquakes be similar but not equal and found that this constraint did not significantly degrade the quality of the fits to the spectra the waveforms analyzed were recorded by a borehole seismometer the p wave q was found to be 1070 for q values as low as 600 and as high as 3000 the misfit between model and spectra increased by less than 5 per cent and the average corner frequency changed by less than 15 per cent from those obtained with a q of 1070 average stress drop was 3 7 bars seismic moments obtained from spectra ranged from 1013 to 1018 dyne cm the low stress drops are interpreted to result from underestimation of the actual stress drops because of a nonuniform distribution of stress drop and slip along the fault planes spatially varying stress drops and slips result from the strong rock heterogeneity due to the injection of fluid into the rock stress drops were found to be larger near the edges of the seismic zone in regions that had not been seismically active during previous injections the seismic moments determined from spectra were used to obtain a coda length to moment relation then moments were estimated for 1149 events from measurements of coda lengths from events whose moments could not be measured from spectra because of saturation or a low signal to noise ratio the constant of proportionality between cumulative number of events and seismic moment is higher than that found for tectonic regions the slope is so high that the seismic energy release is dominated by the large number of small events in the absence of information about the number of events smaller than we studied we cannot estimate the total seismic energy released by the hydraulic injection

source parameters of earthquakes recorded near the itoiz dam northern spain we calculate the source parameters and attenuation from earthquakes recorded near the itoiz dam from 2004 to 2009 with magnitudes ranging between 1 2 and 5 2 we use a genetic algorithm in order to fit the three component p wave spectra with the spectral level corner frequency and attenuation factor as searching parameters the obtained moments range from 1 72 1011 to 2 65 1015 nm the radii span from 0 09 to 1 00 km and the stress drops vary from 0 006 to 29 462 mpa the maximum value for the q attenuation factor is 794 and the minimum value is 53 we find a good agreement between empirical and theoretical relationships between moment and magnitude there seems to be a breakdown of self similarity but it could be due to the method used we group the data by means of a self organizing map and the clusters found are related by their magnitude and not by other considerations

seismic imaging using microearthquakes induced by hydraulic fracturing abstractseismic imaging using microearthquakes induced by hydraulic fracturing produces a three dimensional 3 d s wave velocity model of the fractured zone improves the calculated locations of the microearthquakes and may lead to better estimates of fractureplane orientations fracture density and water flow paths such information is important for predicting the amount of heat energy that may be extracted from geothermal reservoir a fractured zone was created at the los alamos hot dry rock reservoir in north central new mexico within otherwise impermeable basement rock by injecting 21000 m3 of water into a borehole under high pressure at a depth of 3 5 km induced microearthquakes were observed using four borehole seismometers the p wave and s wave arrival times have been inverted to find the 3 d velocity structures and the microearthquake locations and origin times the inversion was implemented using the separation of parameters technique and constraints were incorporated to require smooth vel

parameters and a magnitude moment relationship from small earthquakes observed during hydraulic fracturing experiments in crystalline rocks using source parameters estimated from seismic spectra and magnitudes estimated from coda lengths we demonstrate that the log linear relationship between moment and magnitude holds for events with magnitudes as low as 6 using as a data set events induced by hydraulic fracturing experiments at the fenton hill new mexico hot dry rock hdr geothermal site we find that the relationship between magnitude m and seismic moment mo is log mo 17 27 0 77m moreover the linear relationship between seismic moment and source radius r holds for the fenton hill microearthquakes analyses of the fenton hill data yield the following relationship log r 2 28 0 19log mo

petroleum reservoir characterization using downhole microseismic monitoring imaging of microseismic data is the process by which we use information about the source locations timing and mechanisms of the induced seismic events to make inferences about the structure of a petroleum reservoir or the changes that accompany injections into or production from the reservoir a few key projects were instrumental in the development of downhole microseismic imaging most recent microseismic projects involve imaging hydraulic fracture stimulations which has grown into a widespread fracture diagnostic technology this growth in the application of the technology is attributed to the success of imaging the fracture complexity of the barnett shale in the fort worth basin texas and the commercial value of the information obtained to improvecompletions and ultimately production in the field the use of commercial imaging in the barnett is traced back to earlier investigations to prove the technology with the cotton valley imaging project and earlier experiments at the m site in the piceance

breakdown of the gutenberg richter relation for microearthquakes induced by hydraulic fracturing influence of stratabound fractures hydraulic fracturing a powerful completion technique used to enhance oil or gas production from impermeable strata may trigger unintended earthquake activity the primary basis for assessment of triggered and natural seismic hazard is the classic gutenberg richter g r relation which expresses scale independent behaviour of earthquake magnitudes here we use a stochastic approach to simulate and test magnitude distance trends expressed by microseismic catalogues derived from three hydraulic fracture monitoring programmes in north america we show that a widely observed rapid fall off in large magnitude events almost universally quantified using the g r b value may in our case be an artefact of the strongly laminated character of the stimulated oil and gas reservoirs we also show that for the three reservoirs considered mechanical bed thickness can be approximated by a lognormal distribution for a stratabound fracture network this leads asymptotically to a gaussian decay for induced magnitudes we show that the stratabound model provides a more significant correspondence with our observations if applicable in general this result has important implications for determining the energy balance of hydraulic fracture systems i e radiated seismic energy versus injected energy as well as hazard assessments based on the probability of occurrence of anomalous seismic events key words monitoring passive method reservoir geophysics

spatio temporal evolution of frequency magnitude distribution and seismogenic index during initiation of induced seismicity at guy greenbrier arkansas abstract in this study we carry out detailed analyses of the spatio temporal variations of seismic b value during the onset of the potentially induced earthquake sequence between guy and greenbrier arkansas to investigate correlations with pore pressure change the range of b values suggests that the seismicity in the guy greenbrier area is mostly a result of the activation of pre existing faults the spatial distribution of b value correlates with modeled pore pressure changes in the northern segment of the fault b value increases with depth due to large pore pressure changes and opening of new fractures in the deeper part and stress relaxation in the shallower parts whereas in the southern segment the shallower part shows higher b values due to higher pore pressure fluctuations but the deeper part in the crystalline basement has low b value due to higher confining stress the correlation between the temporal variation of b value and hypocentral depth explains a previously observed temporal drop of b value this suggests that temporal variations should be interpreted along with the spatial variations estimation of the seismogenic indices for the guy greenbrier fault is provided our analysis suggests that monitoring changes in b value and seismogenic indices during an injection period might be used to avoid the occurrence of significant events if injection volume is reduced at critical times

rock anelasticity measurements at high pressure low strain amplitude and seismic frequency a new apparatus has been constructed which will ultimately facilitate the study of rock anelasticity under conditions which closely approach those of teleseismic wave propagation simultaneous high pressure and temperature low frequency and strain amplitude and controlled pore pressure of volatiles its performance has been tested in a series of preliminary experiments conducted at room temperature measurements on a steel standard demonstrate the sensitivity of the apparatus to very small departures from ideal elasticity q 1000 experimental data for a fine grained granitic rock show that both the shear modulus g and quality factor q increase sharply with increasing pressure below 100 mpa beyond which pressure both parameters become markedly less pressure sensitive these observations are in accord with those of previous studies at higher frequencies and larger strains and are consistent with the view that the anelasticity of rocks at ambient pressure is dominated by mechanisms operative at open cracks and grain boundaries