an algebraic reconstruction technique for weakly anisotropic velocity laboratory measurements and field experiments indicate that many sedimentary rocks display velocity anisotropy robertson and corrigan 1983 white et al 1983 thomsen 1986 winterstein 1986 gaiser et al 1987 accounting for anisotropy is proving important in several areas including the processing and interpretation of shear wave data lynn and thomsen 1986 bush and crampin 1987 and the integration of vertical seismic profile vsp data and surface seismic data chiu and stewart 1987a

rare earth element geochemistry in soils developed in different geological settings of cuba abstract the increased exploitation of rare earth elements rees over the past few decades and the disposal of various products containing these elements have prompted numerous environmental studies and the need to establish regulatory guidelines we determined the concentration of rees in topsoil samples of 33 soil profiles collected from areas of different geological settings in cuba the average ree concentrations revealed the following chemical abundances in decreasing order ce nd la pr sm er yb dy tb lu ho gd eu tm the significant variation in ree levels reflects the geological diversity of cuba the highest ree concentrations were observed in typic rhodudalf and rhodic eutrodox which are derived from hard limestone in contrast the lowest ree concentrations were detected in the typic kandiustalf which originated from mica schist the lower light ree lree contents in cuban soils compared with soils from other countries is related to the low abundance of felsic rocks on the island we also observed a strong positive correlation between rees and some major elements in soils this finding implies that the distribution of rees in soils is partly governed by phosphates and aluminosilicates the low fractionation between lrees and heavy rees is likely related to the predominance of soils developed from carbonates this finding was also supported by the low lan smn and gdn ybn ratios and principal component analysis the spatial distribution of rees normalized to upper continental crust coupled with univariate and multivariate statistical techniques indicated that parent materials control the prevalence of rees in cuban soils the data presented here represent the first national wide survey of rees in cuban soils

fractal structure of spatial distribution of microfracturing in rock summary a constant stress fracture experiment of oshima granite was carried out at the confining pressure of 40mpa hypocentres of 2064 acoustic emissions were located during the experiment using the correlation integral we found that the spatial distribution of hypocentres of acoustic emission is a fractal and that the fractal dimension decreases with the evolution of rock fracturing the spatial distribution of earthquake s hypocentres reveals fractals ranging from regional to worldwide distribution if we extrapolate from laboratory measurements it is possible to predict the occurrence of large earthquakes by the decrease in the fractal dimension

turbulent fountains in a stratified fluid the turbulent fountain that results from the injection of a dense fluid upwards into a large tank of stably stratified fluid of lower density is studied experimentally and theoretically for both axisymmetric and line fountains we have used a combination of dimensional arguments and laboratory experiments to determine the initial height above the source at which the flow first comes to rest depending on the strength of the stratification and the fluxes of momentum and buoyancy at the source the subsequent down flow may either spread along the base of the tank or intrude at an intermediate height in the environment we determine both the height of intermediate intrusion and the critical condition for spreading along the base we also relate numerical solutions of the entrainment equations to our experimental observations and obtain effective entrainment coefficients for both axisymmetric and line fountains finally we discuss the quantitative application of our results to the replenishment of magma chambers and to the heating or cooling of a room

dual role of microcracks toughening and degradation one of the methods of improving the mechanical properties of ceramics is to introduce a defective structure that acts as a restraint for the propagation of cracks in the present study a detailed investigation was carried out by introducing a defective structure in rock to determine if there is any improvement in properties similar to ceramics granite was chosen for the investigation and the microcracks were introduced by a heating and cooling cycle uniaxial compression tests have shown that granite thermally treated to 200 c shows the highest strength and the strength of granite treated to 400 c is comparable to that of unheated granite both ultrasonic images and acoustic emission monitoring show that for thermally treated samples the stress induced microcrack and macrocrack nucleation and their growth are retarded the variations in mechanical properties are explained based on the concept of toughening and degradation uniaxial compression tests on unheated and thermally treated granite samples have clearly established the dual role of microcracks which operate in the toughening and degradation mechanisms

surface deformation of westerly granite during creep holographic interferometry has been used to observe the surface deformation of westerly granite during creep experiments at 50 mpa 500 bars confining pressure interference fringes on the holograms produced by this technique constitute contour maps of the sample surface shape change between exposures several distinctive characteristics of the surface shape change have been observed in general surface deformation during primary and secondary creep while highly inhomogeneous is distributed fairly uniformly no large scale pattern is seen in the surface shape changes at approximately the onset of tertiary creep surface displacements become more locallized eventually approx 85 90 of time to failure a distinct ridge is formed which exactly coincides with the location of the final fracture

characterizing in situ stress domains at the aecl underground research laboratory the underground research laboratory access shaft was excavated from the surface to about the 185 m depth in jointed pink granite below this depth to the 443 m depth the shaft was excavated in massive grey granite the grey granite is essentially unjointed except for a major low dipping thrust fault and associated minor splays overcoring hydraulic fracturing convergence measurements microseismic monitoring and observations of shaft wall failure and core discing indicate that unusually high in situ stresses can be associated with large volumes of massive unjointed granite at fairly shallow depth the database of in situ stress measurements collected at the underground research laboratory indicates that major geological features such as thrust faults can act as boundaries for in situ stress domains and that both the magnitude and direction of the in situ stress state can change when these geological features are traversed key words in situ stress anisotropy stress domains thrust faults overco

frequency characteristics of acoustic emission in rocks under uniaxial compression and its relation to the fracturing process to failure acoustic emission generated during the uniaxial compression of brittle rocks were counted after band pass filtering throughout the whole process from application of load to failure the count rate monitored through a low pass window was compared statistically with the monitored through a high passs window the emission rate monitored through a low pas window increases more rapidly than that monitored through a high pass window as rock approaches failure two possible explanations for this effect are 1 generation of larger cracks or coalescence of cracks into larger sizes and 2 the relative attenuation of higher frequency wave components this finding is consistent with past observations i e the relative number of large amplitude events increases i e a decrease in b value as rock approaches failure we suggest that larger cracks tend to generate events with larger amplitudes and containing lower frequency components the frequency characteristics of acoustic emissions are used to characterize the microfracturing processes leading to failure the beginning of a sequence of acoustic emission activity is discussed in relation to the stress strain curve and the onset of dilatancy the present observations may be utilized in understanding structural instability of a region where rockbursts or earthquakes can potentially occur if a sizable microseismicmore population is observed through sensors placed near the fault then a potential exists for predicting major earthquakes less

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

spatial and temporal analysis of a seismic series using a new version of the three point method application to the 1989 antequera spain earthquakes abstract the three point method tpm has been successfully applied to several seismic series and has provided information about the spatial characteristics azimuth and dip of the fault planes activated in the rupture process a new development of the tpm to determine temporal characteristics is presented to obtain the evolution of the fracturing process of an active fault system for the analysis of the 158 microearthquakes and earthquakes that took place in the seismic series of antequera in june 1989 the choice of a threshold magnitude m u 2 5 has permitted the events related to the most relevant fractures to be distinguished secondly only the events between two concentric spheres here named spatial crown with respect to a given earthquake have been used in order to avoid taking into account earthquakes that are too close to each other together with the very distant events that have little relation to the event analysed the spatial crown has permitted some clear results in the antequera series where we have found that the fracturing process began fundamentally with n 80 e planes and evolved to n 65 w planes finally an error analysis enables an estimation of the uncertainty in the results to be obtained from the errors in the data

rare earth elements absorption patterns in grapevine vitis vinifera l cultivated in carbonate terrains south eastern sicily italy vitis vinifera l vineyards grown on carbonate soil hyblean plateau se sicily have been characterized in terms of rare earth elements rees distribution results highlighted that the absorption of rees by plants depends on the composition of the underlying soil which in this case derives from limestone parent rock allowing us to recognize the area of origin indeed even slight differences in rees content in soils may affect the absorption pattern of each grapevine cultivar importantly the various parts of the plants showed differences in rees absorption such rees fractionation is particularly evident in the leaf and juice samples in general the uptake and concentrations of rees in plant tissues may be related to many factors such as geographical climatic and lithological features this is also pointed out by the statistical investigation which took into account either the grapevine variety or each part of the plant by taking into consideration both the grape variety and the type of soil the present multi elemental approach aims to provide a useful geochemical tool for assessing the geographical origin of the production area of wine

time dependent brittle creep in darley dale sandstone 1 the characterization of time dependent brittle rock deformation is fundamental to understanding the long term evolution and dynamics of the earth s crust the chemical influence of pore water promotes time dependent deformation through stress corrosion cracking that allows rocks to deform at stresses far below their short term failure strength here we report results from a study of time dependent brittle creep in water saturated samples of darley dale sandstone initial porosity 13 under triaxial stress conditions results from conventional creep experiments show that axial strain rate is heavily dependent on the applied differential stress a reduction of only 10 in differential stress results in a decrease in strain rate of more than two orders of magnitude however natural sample variability means that multiple experiments must be performed to yield consistent results hence we also demonstrate that the use of stress stepping creep experiments can successfully overcome this issue we have used the stress stepping technique to investigate the influence of confining pressure at effective confining pressures of 10 30 and 50 mpa while maintaining a constant 20 mpa pore fluid pressure our results demonstrate that the stress corrosion process appears to be significantly inhibited at higher effective pressures with the creep strain rate reduced by multiple orders of magnitude the influence of doubling the pore fluid pressure however while maintaining a constant effective confining pressure is shown to influence the rate of stress corrosion within the range expected from sample variability we discuss these results in the context of microstructural analysis acoustic emission hypocenter locations and fits to proposed macroscopic creep laws

ultrasonic imaging and acoustic emission monitoring of thermally induced microcracks in lac du bonnet granite concurrent ultrasonic tomography and acoustic emission monitoring were employed to study thermally induced microfracturing in an unconfined 15 cm cube of lac du bonnet granite from atomic energy of canada limited s underground research laboratory an electrical resistance cartridge heater placed in a central vertical borehole was used to cycle the sample to progressively higher peak temperatures between 75 c and 175 c tomography data were collected at room temperature before and after each thermal cycle acoustic emission monitoring proceeded during both heating and cooling phases of each thermal cycle microfractures opened above 80 c and eventually coalesced into a macroscopic fracture plane the macroscopic fracture originated at the outer edges of the sample and then extended inward parallel to the fast velocity direction and eventually intersected the borehole both acoustic emission locations and slowness difference tomography clearly delineated the fracture plane we attribute the development of the macroscopic fracture to a thermal gradient cracking mechanism acting upon a brittle anisotropic medium

failure zone development in andesite as observed from acoustic emission locations and velocity changes three dimensional 3 d locations of acoustic emissions ae and p wave velocity anisotropy were investigated to analyse the failure zone development in two yugawara andesites the results were obtained during the deformation of rock under incremental loading and short duration creep regimes at a confining pressure of 40 mpa creep tests were carried out at a constant differential stress of about 90 95 of short term fracture strength of andesites for the single and two stage creep experiments in both the experiments the ae activity during loading was mostly confined to two region where ae hypocentres concentrated near the cylindrical surface of the rock prior to fault nucleation the localized zone of microcracks attained a spheroidal shape in one sample and planar shape in the other p wave velocity anisotropy increased to 44 as final failure approached in the rock the loading rate and the magnitude of creep stress were found to have a profound influence on the details of failure zone development and times of failure of the rocks studied

fracturing of volcanic systems experimental insights into pre eruptive conditions conditions for fracturing are a primary control on the behaviour of volcanic systems especially during the approach to eruption we here present the results of deformation experiments under simulated volcanic conditions on a porhyritic andesite from ancestral mount shasta andesite was chosen as a representative material because it is common at subduction zone volcanoes among both erupted products and country rock we deformed the lava in tension and triaxial compression tests at strain rates of 10 5 s 1 confining pressures from 0 to 50 mpa and temperatures up to 900 degrees c we also concurrently recorded acoustic emissions ae in order to monitor cracking activity the results show that deformation behaviour changes significantly in the temperature range 600 750 degrees c thus as temperatures increased across this interval the tensile fracture toughness increased from 2 5 0 5 mpa m 1 2 to 3 5 1 mpa m 1 2 the compressive strength decreased from 110 30 mpa to 55 35 mpa at 900 degrees c and the corresponding young s modulus decreased from 20 4 gpa to 6 4 gpa the changes occur when the deformation of the sample changes from elastic brittle to brittle ductile behaviour which we attribute to the blunting of crack tips due to melting of the glass phase and enhanced crystal plasticity at high temperature ae activity was observed in all experiments indicating that earthquakes can be generated not only in country rock but also in hot magma such as may be found in lava domes and at the margins of magma conduits in addition the trends in accelerating ae event rates before sample failure were comparable to those seen in earthquakes before some volcanic eruptions and a minimum in the seismic b value coincided with sample failure applied to volcanic systems the results suggest that 1 andesite strength and elasticity will not be affected by temperature or pressure beyond similar to 10 100 m from active magma 2 before eruptions fractures propagate preferentially through weaker horizons in a mechanically heterogeneous volcano and 3 volcanic rocks have characteristic seismic b values that are perturbed during the approach to bulk failure each of these conditions provides quantitative constraints on models for seismic precursors to eruption or intrusion c 2009 elsevier b v all rights reserved

ultrasonic tomography and acoustic emission in hydraulically fractured lac du bonnet grey granite ultrasonic tomography and acoustic emission ae data were obtained during laboratory hydraulic fracturing tests on two large unconfined cylinders of lac du bonnet grey granite the cylinders were internally pressurized over four to five cycles prior to final failure compressional velocities were measured before and after each pressurization cycle with an array of 16 evenly spaced transducers around the central cross sectional plane of each sample sixteen channels of whole waveform ae data were recorded during most pressurization cycles and for a period of about 1 hour after final failure in one sample compressional velocities were found to be strongly anisotropic with the in situ vertical direction being the most rapid direction in both samples the velocity anisotropy is related to the rock s preexisting microcrack fabric owing to radial penetration of fluid into the rock compressional velocities rose over the course of the experiment a regression analysis showed that the velocity changes can be explained by variations in crack density inferred from initial velocities and radial distance from the borehole saturation levels consistent with the observed velocity changes were calculated on the basis of the o connell and budiansky theory acoustic emissions reoccurred in a few distinct zones over several pressurization cycles the ae locations allowed two distinct fracture planes to be sharply delineated in one sample the fracture plane orientations were controlled by the preexisting microcrack fabric in both samples ae occurred too rapidly during peak pressure failure to permit us to isolate distinct events source mechanism analysis of the ae which occurred prior to peak pressure failure in both samples and during postfailure monitoring in one sample showed a predominance of double couple sources compressive sources thought to be related to crushing of asperities during crack closure and tensile sources related to mode i crack growth were also recorded as well as more complex sources that could not be modeled by the simple source types listed above

positive feedback fracture process induced by nonuniform high pressure water flow in dilatant granite during forced water infiltration into a dry granite rock in the dilatant state migration of a water front and a clear positive feedback between acoustic activity and water flow into microcracks were revealed by means of ae hypocenter locations and velocity tomography water was injected into a cylindrical granite sample subjected to 40 mpa confining pressure and constant axial stress equal to about 70 of short term dry strength the injection water pressure was held constant at 17 mpa until failure occurred soon after water injection into the bottom end of the sample ae hypocenters began near the bottom end subsequently they moved up with a strong preference for the region where a weak concentration of ae events had been observed during a steady state creep stage before the water injection the p wave velocity structure was determined in the section parallel to the loading axis and crossing the cluster of ae hypocenters the velocity structure reconstructed by seismic tomography showed a nonsymmetric distribution of the velocity with regard to the loading axis during the injection the strong concentration of ae hypocenters suggests that water flow channels are preferentially formed in a very limited region in the dilatant rock the concentration of ae events and formation of these water channels marks a positive feedback process in which the local strength of the rock is reduced by the decrease of the effective confining pressure caused by increasing pore pressure which in turn triggers more microfracturing

sequential velocity imaging and microseismic monitoring of mining induced stress change sequential imaging of the temporal changes inp wave velocity offers a practical tool to monitor a rock mass using established correlations between the location of seismic events and velocity structure the temporal seismic potential characteristics of the rock may be monitored furthermore the temporal velocity differences isolate the time dependent factors effecting velocity such as stress while cancelling static factors such as lithology various sequential imaging techniques were compared with respect to accuracy differences between successive velocity images were found to have relatively high associated error estimates however images of velocity differences calculated from measured travel time delays between successive velocity surveys were found to have lower error estimates in particular travel time delays measured using cross correlation techniques resulted in the most accurate sequential image two 3d sequential imaging studies were carried out at strathcona mine in sudbury ontario canada results of the average static images indicated an association between the location of induced microseismicity and a zone of both high velocity and high gradient additional examples are described from the global seismology literature which also show a similar correlation between seismicity and velocity structure we attribute this association to an interrelated stress and strength effect the strathcona mine sequential images show zones of significantly decreased velocity in regions of concentrated microseismic activity which are postulated to be indications of localized destressing and relaxation of the clamping forces resulting in the microseismicity the zones of decreased velocity corresponded to an increase in the velocity gradient one of the case studies also shows an increase in velocity in a zone of high static velocity which is later the site of amn 2 5 mining induced seismic tremor the increase in velocity is believed to correspond to a region of stress concentration resulting in the subsequent seismic tremor

hypocenter distribution and focal mechanism of ae events during two stress stage creep in yugawara andesite more than a thousand acoustic emission ae hypocenters were determined in a cylindrical andesite specimen under two stage uniaxial creep at stresses of 204 and 214 mpa strains were monitored for 6 peripheral points at the middle part of the cylindrical specimen s wall the strain data indicate gradual increase of nonuniform deformation during steady creep and strong intensification of the nonuniformity during acceleration creep and therefore biased stress distribution within the specimen the correlation between dilatant strain and ae hypocenters was investigated for whether or not tensile cracks emit ae the region with high ae activity shows only a small dilatant strain this negative correlation between ae and the dilatant region may eliminate tensile cracks as possible ae sources a composite focal mechanism solution of local ae events covering a wide solid angle of the focal hemisphere indicates that shear fractures emit ae waves the direction of the compressional axis in this solution shows a significant deviation from that inferred from the applied external force suggesting that the local stress field is governed by preexisting weak zones that are presumably produced by tensile cracks within the specimen ae hypocenters tended to form clusters during steady creep under the constant compressional stress during acceleration creep caused by a small step increase of the external stress the preceding clusters disappeared while a new cluster appeared in an incipient fault plane this suggests that changes in seismicity pattern such as migrations or quiescences of swarm important clues for earthquake predictions may be caused by an instantaneous change in the tectonic stress levels

crack location in granitic samples submitted to heating low confining pressure and axial loading summary until now observations of mechanically and thermally induced microcracks in rocks could only be carried out by indirect measurements or destructive observations on samples brought back to atmospheric pressure conditions a special triaxial test cell was designed in order to perform direct observations during loading the use of a cell in tomography apparatus involves new devices 1 a movable horizontal load frame around a scanner and 2 a test cell transparent to x rays able to withstand up to 28 mpa maximum confining pressure and temperatures of up to 180 c volumetric strains are compared with radiological density measurements the first processed x ray images locating microcracks during propagation are also presented mineralogical evects on the crack location can be demonstrated strain inferred from ct density measurement is clearly correlated with the strain usually measured by a strain gauge diverent phases of mechanical behaviour are described contracted phase and failure by macrocrack formation the principal results obtained with this tool are the description of the porosity formation and macrocracking results show two principal factors localizing the porosity first the divused porosity volume is controlled by mineralogical parameters quartz and plagioclase grains and boundaries of biotite grains during the thermal and mechanical loading second macrocracking begins at the perimeter of the central section of core and grows towards the sample piston interface it seems that the first macrocracking is not located in the high porosity zone formed during the loading phase but in a relatively low porosity zone

distinct element modeling of hydraulically fractured lac du bonnet granite 1 the aim of this study is to better understand the mechanics of fracture development and propagation during hydraulic fracturing this paper presents some development and applications of discrete particle modeling of this problem a discontinuum modeling approach idealizes the material as separate particles bonded together at their contact points and utilizes the breakage of individual structural units or bonds to represent damage the numerical models are correlated with existing hydrofracture laboratory experiments which are presented in other publications a simulation of a laboratory scale hydrofracture experiment and the acoustic emission ae data from the experiment is used to validate the synthetic aes produced in the hydrofracture model this technique has been used to examine the mechanics of fracture initiation and time and spatial distributions of ae the modeling results demonstrate that the mechanism of hydraulically induced fracture in the lac du bonnet ldb granite core sample is predominantly tensile failure and that the shear cracks recorded in the hydrofracture experiment were due to slip on preexisting fractures numerical modeling of hydrofracture on homogeneous and heterogeneous synthetic samples seems to capture much of the behavior observed in the laboratory hydrofracture experiments

surface expression of a wall fountain application to subglacial discharge plumes abstractwe use laboratory experiments and theoretical modeling to investigate the surface expression of a subglacial discharge plume as occurs at many fjords around greenland the experiments cons

seismic and strain studies on large laboratory rock samples being stressed to failure abstract large granite 700 700 700 mm and basalt 580 580 915mm samples were cyclicly loaded to failure strain and ultrasonic velocity measurements were made and acoustic emissions recorded during deformation all measurements were correlated in space and time several zones of acoustic emission sources were identified the acoustic patterns of emission activity within these zones varied greatly with increasing stress 1 a initially active then quiet 2 b initially active then quiet and active again before failure and 3 c activity increasing continuously and at accelerating rate toward failure at constant applied stress the seismic character of the acoustic emission signals was quite different in terms of frequency content and amplitude for the different zones the signals emanating from within a particular zone were very reproducible as recorded on a particular transducer but quite different from transducer to transducer the kaiser effect seems to hold up to about one half of the stress of failure i e very few acoustic emissions were detected when the sample was cycled below previously attained stress as long as was less than about one half of the eventual stress at failure as failure was approached the individual acoustic emission events became more energetic and relatively more power was recorded at lower frequencies strain and ultrasonic velocity measurements indicate large variations in dilatancy in the sample similar measurements on much smaller samples are consistent with these results

fracturing process of granite inferred from measurements of spatial and temporal variations in velocity during triaxial deformations fracturing in fine grained homogeneous granite during triaxial deformation was investigated by measuring the spatial and temporal variations of elastic wave velocities velocities along 20 ray paths were measured with 12 piezoelectric transducers attached to a sample under confining pressures up to 400 mpa systematic velocity differences observed among the 20 paths apparently indicate the formation of a dilatant region near the surface of a sample the damaged region formed near the surface due to asymmetric development of dilatancy seemed to play an important role in controlling ultimate failure no evidence of the formation of a crack concentration zone adjacent to the ultimate fracture plane could be found before failure insofar as could be estimated from the velocity variations of elastic waves propagating along its plane however a highly concentrated region of cracks in which the fracture nucleated was generated in the damaged region at stresses of more than 97 99 of the ultimate strength in all the specimens based on tomographic images computed using an algebraic reconstruction technique the elastic waves were strongly affected by the damaged region even under the high confining pressures at which cracks are considered to close

acoustic emission study of microfracturing during the cyclic loading of westerly granite a rectangular prism of westerly granite was deformed in a uniaxial cycling experiment acoustic emission ae activity was recorded using eight transducers in real time hypocenters of 116 events were determined using a standard least squares approach event clustering was definitely evident and could easily be assessed using a stereographic projection of hypocenters this clustering was observed at volumetric strains well below 0 03 the distribution of hypocenters changed from cycle to cycle suggestive of a spatial migration the cumulative distribution of hypocenters documents the strongly anisotropic development of dilatancy in westerly granite once the sample underwent the initial deformation and a zone of weakness was established subsequent events clustered in and about the initial zone volumetric strain differential stress acoustic emission event rate and cumulative numbers were continously measured during the experiment the cumulative number of events per cycle generally decreased with the number of cycles the majority of events occurred upon loading those that occured during unloading were few in number and not as systematically related to stress as were those for loading in general waveform quality deteriorated with increased cycling and in later cycles waveforms typically displayed low frequency components in initial onsets followed by a more typical higher frequency arrival

further testing of the bedding plane slip model for hydraulic fracture opening using moment tensor inversions abstractmoment tensors are calculated by using the p wave first motion peak amplitudes of 59 microseismic events with high signal to noise ratio these events are from a surface microseismic data set gathered during hydraulic fracture stimulation of the marcellus gas shale in washington county pennsylvania usa the majority of these 59 events have a horizontal nodal plane a few degrees characteristic of a dip slip horizontal slip moment tensor if the horizontal nodal plane is an auxiliary the vertical nodal plane has a pure dip slip motion which is inconsistent with the opening motion for vertical hydraulic fractures that enables proppant loading this points to slip on horizontal nodal planes with the auxiliary vertical nodal planes aligned with the local maximum horizontal stress orientation as indicated by drilling induced fractures in nearby vertical wells these 59 microseismic events are caused by slip on horizontal mechanical discontinuities such as bedding planes during the opening of ver

etnean and hyblean volcanism shifted away from the malta escarpment by crustal stresses abstract a fraction of the volcanic activity occurs intraplate challenging our models of melting and magma transfer to the earth s surface a prominent example is mt etna eastern sicily offset from the asthenospheric tear below the malta escarpment proposed as its melt source the nearby hyblean volcanism to the south and the overall northward migration of the eastern sicilian volcanism are also unexplained here we simulate crustal magma pathways beneath eastern sicily accounting for regional stresses and decompression due to the increase in the depth of the malta escarpment we find non vertical magma pathways with the competition of tectonic and loading stresses controlling the trajectories curvature and its change in time causing the observed migration of volcanism this suggests that the hyblean and etnean volcanism have been fed laterally from a melt pooling region below the malta escarpment the case of eastern sicily shows how the reconstruction of the evolution of magmatic provinces may require not only an assessment of the paleostresses but also of the contribution of surface loads and their variations at times the latter may even prevail accounting for these competing stresses may help shed light on the distribution and wandering of intraplate volcanism

acoustic emission and ultrasonic velocity methods used to characterise the excavation disturbance associated with deep tunnels in hard rock abstract acoustic emission ae and ultrasonic velocity monitoring studies have been undertaken at both the atomic energy of canada limited aecl underground research laboratory url and at the swedish nuclear fuel waste management company skb hard rock laboratory hrl at both locations the excavations were tunnels in granitic material at approximately 420 m depth however the stress regime was more severe at the url mine by tunnel site than the hrl zedex tunnel different parts of the zedex tunnel were created using different excavation techniques using ae and ultrasonic techniques to study these tunnels we have been able to examine the nature of the excavation disturbed zone around the tunnel as well as examining the effects of different stress regimes and excavation techniques studies were undertaken both during and after the mine by tunnel excavation and during excavation in the zedex tunnel ae monitoring in the wall of the mine by tunnel during excavation showed that some activity occurred in the sidewall regions but the spatial density of ae hypocentres increased toward the regions in the floor and roof of the tunnel where breakout notches formed this sidewall activity was clustered primarily within 0 5 m of the tunnel wall ae monitoring in the floor of the tunnel showed that small numbers of ae continued to occur in the notch region in the floor of the tunnel over 2 years after excavation was completed this activity became more acute as the rock was heated imposing thermally induced stresses on the volume ultrasonic velocity studies both in the floor and the wall of the tunnel showed that the velocity is strongly anisotropic with the direction of slowest velocity orthogonal to the tunnel surface the velocity increased with distance into the rock from the tunnel surface in the floor this effect was seen up to 2 m from the tunnel surface most of the change occurred within the first 0 5 m from the tunnel perimeter at the lower stress hrl most of the ae again occur very close to the tunnel surface the occurrence of ae under relatively low stress conditions suggests that the regions experiencing ae activity were damaged during the excavation process thereby reducing their strength the section of tunnel excavated by a tunnel boring machine had fewer events clustered much closer to the tunnel surface than the sections excavated using drill and blast excavation techniques p wave velocity changes of only about 0 1 were experienced due to the tunnel excavation for ray paths within zero to 2 m from the tunnel surface indicating that crack damage was relatively low

rare earth elements in pore waters of marine sediments abstract the rare earth elements rees were measured in pore waters of the upper 25 cm of sediment from one site off peru and three sites on the california margin the pore water ree concentrations are higher than sea water and show systematic down core variations in both concentration and normalized pattern from these analyses and from comparison to other chemical species measured dissolved fe mn ba oxygen nitrate phosphate it is suggested that pore water rees can be grouped into three categories those that are from an fe source those that are from a poc source and cerium oxide rees from the fe source appear where anoxia is reached they have a distinctive middle ree mree enriched pattern the concentrations in this source are so elevated that they dominate ree trends in the fe oxide reduction zone the net result of flux from the poc source is relative enrichment of heavy rees hrees over light rees lrees reflecting remineralizing poc and complexation with doc a common linear ree pattern seen in both oxic and anoxic sediments is associated with this poc source as well as a hree enriched pattern that is seen in surficial sediments at the peru site overall the pore water results indicate that mn oxides are not an important carrier of rees in the oceans a ree biogeochemical model is presented which attempts to reconcile ree behavior in the water and sediment columns of the oceans the model proposes that poc fe oxide and ce oxide sources can explain the ree concentration profiles and relative abundance patterns in environments ranging from oxic sea water to anoxic pore water the model is also consistent with our observation that the ce anomaly of pore water does not exceed unity under any redox condition

the relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks several hundred microearthquakes with local magnitudes ranging between 4 and 2 were recorded during hydraulic stimulation experiments at the hot dry rock demonstration site fenton hill new mexico these events are probably caused by shear failure induced by high pore fluid pressures since the event locations seem to cluster in a narrow band near the hydraulic fracture we were able to use micro seismic techniques to locate the hydraulic fracture and monitor its growth we calculate the minimum pore pressure increase necessary to induce rock failure using a simple model of slippage on preexisting fractures by comparing a pore pressure distribution calculated using a one dimensional diffusion model with the distribution of event foci we demonstrate that high pore pressures are a probable cause for these microearthquakes stress drops for these microearthquakes calculated from spectral parameters increased by a factor of 3 during the course of the experiment the stress drop increase correlates with a migration of seismic activity away from the injection well bore which suggests relatively low in situ shear stresses near the well bore caused by strain release in this region during previous pressurizations

determination of seismogenic structures in southeastern sicily italy by high precision relative relocation of microearthquakes abstract in december 1990 southeastern sicily was struck by a moderate earthquake m l 5 4 the event and the subsequent aftershocks represent renewed activity for the malta escarpment fault system mefs three centuries after the most recent of several large destructive historical earthquakes an 11 yr period of quiescence was interrupted with a minor swarm in 2002 m max 3 6 these sequences and regional seismicity were relocated through 3d tomography the resulting velocity model indicates a velocity contrast in the vicinity of mefs high precision relative relocations were derived by the cross spectral method for 18 events from the 1990 sequence and 10 earthquakes from the 2002 swarm these swarms were aligned along fault segments striking east northeast west southwest conjugate to the north northwest south southeast orientation of the mefs we conclude that the mefs is fragmented in segments with different seismic behaviors and that great stress accumulation along the main segment of the mefs is less likely

a study on the time dependent microfracturing and strength of oshima granite abstract uniaxial compressive loading experiments where axial strain rate or dilatant volumetric strain rate was kept constant were carried out using a servo controlled hydraulic testing machine the rock studied was oshima granite in order to investigate the effect of the strain rate on the strength a theory based on the assumption of redistribution of the pre existing microcracks due to subcritical crack growth was examined results expected from the theory were well borne out by the observations the strength was expressed by power functions of both axial strain rate and dilatant strain rate the stress corrosion index of oshima granite appeared to be 31 3 which agreed well with the values reported previously when the axial strain rate was held constant the acoustic emission rate increased exponentially and the b value the exponent in the frequency magnitude relation of the acoustic emissions decreased with increasing applied stress when the dilatant volumetric strain rate was kept constant the acoustic emission rate and the b value remained constant and independent of the stress increase furthermore when the dilatant strain rate was held constant at a higher value the b value was constant at a lower level as the decrease in the b value indicates the higher dilatant strain rate the b value is one of the most useful key parameters in the fracture predictions

a self exciting process of acoustic emission occurrence in steady creep of granite under uniaxial stress clustering of acoustic emissions ae in space and time was found in a laboratory experiment to investigate the stochastic process of ae occurrence event times of ae were measured during steady creep phases under different stress levels oshima granite was continuously subjected to constant uniaxial stress of different intensity the occurrence time of ae event was analyzed as a combination of a poisson process and a self exciting part in the latter events are excited by preceding events the self exciting character was more enhanced under higher crack density in accordance with a stronger concentration in hypocenter distribution at higher stress

the ventilated filling box containing a vertically distributed source of buoyancy this paper describes the fluid mechanics within a box containing a vertical plane distributed source of buoyancy a theoretical analysis is presented that models the development of plumes from such sources in an unconfined ambient of uniform density two extensions are considered the first concerns a sealed box and the second involves the more general situation where the box is ventilated by openings at top and bottom in the sealed box the stratification develops in much the same way as for a filling box containing a single point source of buoyancy on the floor an initial front descends from the ceiling of the box and an asymptotic stratification eventually develops which is continuous in the vertical direction in the case of the ventilated box it is found that a complex stratification develops where one or more horizontal intrusions are formed by detachment of the plume boundary layer from the vertically distributed source where the buoyancy of the plume is less than or equal to that of the stratified ambient at a given height experimental results are presented to demonstrate the validity of the theory the findings are relevant to both forced and naturally ventilated buildings containing non adiabatic vertical surfaces

paleogeography and structure of the central mediterranean sicily and its offshore area abstract the geology of the mainland and offshore of sicily is illustrated by a few geologic sections and seismic profiles across the late cenozoic orogenic belt of central and western sicily and across the sardinia channel and sicily straits this belt is the result of several tectonic events deformation involved mainly the sedimentary cover of the old african continental margin characterized by a broad basinal domain flanked along its external southern margin by a shallow water carbonate platform attached to africa in the triassic compressional deformation started in the more internal basinal rock assemblages overlying a thinned crust the most important structural characteristic of the early phase of thrusting is the duplex pile forming the bulk of the chain in central western sicily the structure consists of a basal allochthon made up of permian to middle triassic layers an intermediate duplex wedge composed of competent mesozoic carbonates and a roof complex including upper mesozoic lower tertiary less competent rocks large scale clockwise rotation of the thrusts accompanied transpressional movements in the hinterland during the pliocene right oblique reverse faults modified the previous tectonic contacts between the allochthons in the hinterland zones contemporaneous south directed imbrications affected the southern external areas progressively incorporating foreland and piggyback basins development of the gela thrust system appears to be linked to the transpressional event its accretion is also related to contemporaneous underthrusting at deeper levels of mesozoic carbonate substratum the older buried thrust sheets were pushed up to the surface breaching the deformed tertiary cover of the gela ts northwards in the belt post messinian normal growth faults opened half graben whose sedimentary fill underwent structural inversion alternation of extension and compression tectonics characterizes the sicilian continental margin in the last million years

fracture nucleation process in intact rocks abstract yukutake h 1992 fracture nucleation process in intact rocks in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 247 257 we produced tomographic images for cross sections of granite samples by measuring the elastic wave velocity along 20 ray paths at high sampling rates during triaxial compression tests the dynamic fracturing process just before ultimate fracture was investigated from the rapid changes in the associated tomographic images it was found that microcracks concentrated locally in several regions as axial load was increased and the velocity then recovered in some of the intensively microcracked regions 6 60 s before fracture subsequently the amplitude of the elastic waves through the most intensively microcracked region also recovered in a few cases the frequency of the elastic wave through the most intensively microcracked region became extremely high we consider that fracture nucleated in the intensively microcracked regions and infer that several macrocracks occur due to an abrupt coalescence of microcracks in the fracture nucleated region a crack length in the process zone for brittle fracture was evaluated from the stress drop observed just before fracture and the published value of the shear fracture energy for westerly granite we expect from the results of both tomography and stress drops that subsequent instability occurred when one of larger macrocracks formed by the coalescence of the macrocracks reached a critical length in the most intensively nucleated region

acoustic emission microstructure and damage model of dry and wet sandstone stressed to failure twenty three uniaxial compression tests were performed on dry and wet flechtingen sandstone from germany compressive strength of wet core is 60 of the strength of dry core before fracture the transverse p wave speed drops by 13 and the pulse amplitude by 22 for wet and 37 for dry cores accumulated strain energy doubles for dry core acoustic emissions ae are detected with 10 sensors for 19 cores ae activity starts at 84 of the fracture strength of wet cores 55 mpa and at 91 of the strength of dry cores 87 mpa the ratio of located to recorded ae is 0 37 for dry and 0 13 for fully wet cores ae hypocenter patterns document the development of two opposite fracture cones the negative slope of cumulative ae amplitude frequency distribution drops by 50 before failure in dry cores the slope of the wet core drops and recovers energy discrimination of ae detected by a broadband sensor resolves different stages of damage and captures the onset of the dilatant throughgoing macrofracture using the analogy to visible light microfracturing events are separated into high energy short pulses blue ae and low energy pulses with long duration times red ae blue ae are explained by intragranular grain breakage red ae by multiple stick slip on crack planes or grain boundaries deformed cores show highly fractured calcite cement and mostly intact quartz grains the stochastic damage model for brittle composites developed highlights that microfracturing of the sandstone is controlled by the amount and distribution of the weak mineral calcite

evolution of shear bands and cracks in multi stage triaxial tests with water saturated sandstone a study of micro tectonics with a fractal perspective abstract a series of multi stage triaxial compression tests revealed quite chaotic reactions of water saturated permotriassic sandstones on changing pore water pressures not before focussing the various test stages with respect to occurring critical phenomena which strike the fabric structures due to their fractal nature during any triaxial testing eventually an approach became tractable pore systems with cracks and shear bands can be widened in a quasi static way but only up to a stability limit thereafter the fabric collapses with a seismogenic contraction of the pore system causing a rise of the pore water pressure changes of structural patterns occur as formation either of fissures up to collapse with acoustic emission or of a rather fractal pattern of crossing shear bands generated without audible noise but with fine grit due to comminution dilation and contraction of the pore system are visible from overall volume changes both modes of pattern formation can be understood as critical phenomena of the mineral solid scale independent features of these changes of fabric patterns can be transferred to the meso and macro scales in the field and in the lithosphere respectively however questions of scale dependent features related to cohesion should be further investigated