assessment of the contribution of in situ combustion of coal to greenhouse gas emission based on a comparison of chinese mining information to previous remote sensing estimates greenhouse gas ghg emission from geological sources such as volcanic activity seabed gas hydrate release hydrocarbon seepage as well as uncontrolled combustion of fossil fuels is erratic and difficult to quantify in this paper we assess the ghg emission due to spontaneous combustion of in situ coal deposits coal fires this problem is aggravated by human intervention mining activities which on the other hand allows better constrained estimations and improved reduction strategies provided modern mining practices are employed in this study we present three case studies on coal fire related ghg emission quantification on provincial scale in china and we propose an integrated quantification aimed at the joint analyses of local knowledge geophysical data and remote sensing data with the objective to retrieve unbiased numbers as a first estimate we conclude that the emissions of ghgs from coal fires in china account for well less than 0 22 of all global fossil fuel related co2 emissions most likely even less than 0 1 this is much less than the previously estimated 2 3

alignment of near surface inclusions and appropriate crack geometries for geothermal hot dry rock experiments ubiquitous splitting of seismic shear waves indicates that most rocks in the upper half of the crust are pervaded by stress aligned fluid filled inclusions called eda cracks these inclusions are expected to be aligned perpendicular to the minimum compressional stress by stress relationships similar to those aligning industrial hydraulic fractures at depths where the overburden stress is sufficiently large typically below a few hundred metres this minimum stress is usually horizontal so that the eda cracks and hydraulic fractures are typically aligned vertically striking parallel or subparallel to the direction of maximum compression this is confirmed by the polarizations of the split shear waves along raypaths at depth in the crust at the free surface however the vertical stress is zero or approximately zero and cracks and hydraulic fractures at shallow depths in intact rock tend to be horizontal thus the directions of minimum stress and the orientations of hydraulic fractures are likely to swing through 90 near the surface of the earth since the behaviour of cracks and stress is often crucial to drilling operations the rotation of the crack and stress geometry near surface has important implications particularly for optimizing hydrocarbon production and geothermal reservoir management consequently evidence gained from experiments for example in hot dry rock geothermal heat extraction in inappropriate crack geometries at shallow depths may not be valid when applied to other crack and stress geometries at depth in hot rock

finite difference modelling of s wave splitting in anisotropic media we have implemented a 3d finite difference scheme to simulate wave propagation in arbitrary anisotropic media the anisotropic media up to orthorhombic symmetry were modelled using a standard staggered grid scheme and beyond monoclinic and triclinic using a rotated staggered grid scheme the rationale of not using rotated staggered grid for all types of anisotropic media is that the rotated staggered grid schemes are more expensive than standard staggered grid schemes for a 1d azimuthally anistropic medium we show a comparison between the seismic data generated by our finite difference code and by the reflectivity algorithm they are in excellent agreement we conducted a study on zero offset shear wave splitting using the finite difference modelling algorithm using the rotated staggered grid scheme our s wave splitting study is mainly focused on fractured media on the scale of seismic wavelenghts small aligned fractures behave as an equivalent anisotropic medium we computed the equivalent elastic properties of the fractures and the background in which the fractures were embedded using low frequency equivalent media theories wave propagation was simulated for both rotationally invariant and corrugated fractures embedded in an isotropic background for one or more than one set of fluid filled and dry fractures s wave splitting was studied for dipping fractures two vertical non orthogonal fractures and corrugated fractures our modelling results confirm that s wave splitting can reveal the fracture infill in the case of dipping fractures s wave splitting has the potential to reveal the angle between the two vertical fractures we also notice that in the case of vertical corrugated fractures s wave splitting is sensitive to the fracture infill

temporal monitoring of coal fires in jharia coalfield india jharia coalfield jcf in india has one of the densest congregations of surface subsurface coal fires in the world evaluation of spatial dynamics and trend of coal fires propagation through aster thermal data has indicated that the magnitude of coal fires in jcf has been fluctuated with time from 2000 to 2009 the area located around western eastern and southeastern parts of the jcf covering shatabdi opencast barora sijua opencast godhar colliery kusunda bokapahari kujama and lodna is under intense coal fire from 2000 to 2004 spatial extent of coal fire has shown a minor decrease of 6 74 and then shows a substantial increase of 11 93 between 2004 and 2009 systematic analysis of the results has revealed that the movement of coal fire in jcf is structurally controlled propagation of the coal fires has been observed in kustor bastacolla and lodna collieries and fire appears to be spreading along the strike of coal seams toward jharia township

the range of effects of azimuthal isotropy and eda anisotropy in sedimentary basins summary combinations of bedding or lithology induced azimuthal isotropy with an axis of symmetry perpendicular to the bedding plane and crack induced extensivedilatancy anisotropy eda with a horizontal axis of symmetry are believed to be common in sedimentary basins and cause the widely observed phenomenon of shear wave splitting combinations of two such transversely isotropic forms of anisotropy with orthogonal axes of cylindrical symmetry lead to orthorhombic symmetry this has two major effects 1 the polarizations of the faster split shear waves may no longer be parallel to the strike of the cracks or fractures even for near vertical propagation and 2 such orthorhombic symmetry systems necessarily have a number of directions called shear wave point singularities where shear waves display disturbed or anomalous behaviour again possibly in near vertical directions unless these effects are correctly identified they could be interpreted mistakenly for the effects of structural irregularities or discontinuities in contrast recognition of the 3 d geometry of this behaviour places comparatively tight constraints on possible combinations of anisotropy in the rockmass in order to give some understanding of the geometry of these phenomena this paper presents 3 d patterns of the behaviour of shear wave splitting that have been computed for a range of combinations of crack and bedding induced anisotropy

crack induced anisotropy models in the geysers geothermal field summary high quality observations of shear wave anisotropy in the shallow crust have allowed us to detect subsurface crack systems in the nw and the se geysers geothermal fields ca shear wave splitting sws parameters fast shear wave polarizations and time delays t are highly sensitive to the anisotropic fabric of the medium through which shear waves propagate and constitute the basic dataset to invert for 3 d crack geometry and crack density in the subsurface variation and asymmetry patterns in and t observations plotted in equal area projections as a function of ray azimuth and incident angle are essential in the determination of crack strike and dip crack aspect ratio and crack density inversion results show that the most common patterns of fracture induced anisotropy in the nw geysers can be simulated by horizontally transversely isotropic hti media or rocks with vertical to steeply dipping systems of parallel cracks which strike subparallel to the n to ne direction of maximum compressive stress on the other hand modelled fracture systems in the se geysers strike both ne and nw percentages of shear wave velocity anisotropy average about 4 per cent in the geysers deviations from horizontal transverse isotropy conditions are modelled with non vertically dipping crack systems or with intersecting crack systems the discussion of specific inversion examples exposes problems related to seismic ray coverage within the shear wave window non uniqueness of crack induced anisotropy models and complementarity between polarization and time delay data

observation of shear wave splitting from natural events south moat of long valley caldera california june 29 to august 12 1982 three component seismograms from local earthquakes recorded by three university of wisconsin stations in the south moat of long valley caldera in 1982 have been analyzed for shear wave splitting the average polarization direction of the first arriving shear waves observed at the two stations south of the resurgent dome is nnw sse time delays between the fast and slow shear waves are proportional to hypocentral distances and the maximum velocity anisotropy inferred is 6 4 the average shear wave polarization observed at the station above the resurgent dome is north south the time delays there are not a function of hypocentral distance and the normalized time delays ms km from events at the southwest boundary of the caldera are larger than those at other azimuths indicating maximum 9 6 velocity anisotropy the east west to ene wsw variation of the tensional stress fields implied by the shear wave polarizations indicates the interaction between the inflating caldera and the sierra nevada range front faults under the generally wnw ese tension expected from the regional tectonics

modelling guided waves in cross hole surveys in uncracked and cracked rock1 the theoretical behaviour of guided waves in cross hole seismic surveys is examined with dispersion curves and synthetic seismograms the modelling suggests and field observations confirm that the dominant energy in many cross hole seismic surveys propagates as guided waves as well as body waves with the energy tied to an interface or combination of interfaces whenever there are interfaces with significant velocity contrasts continuous between the wells the dispersion and waveforms of such modal solutions carry detailed information about the internal structure particularly the stress aligned structure of fluid filled inclusions in the immediate neighbourhood of the waveguide since the information content can be controlled to some extent by the choice of frequency and source type and the level of source and receiver in the two wells such guided waves in cross hole surveys may be important for monitoring changes in nearly horizontal reservoirs during hydrocarbon production processes initially we investigate the basic properties of propagation in isotropic strata and then examine the behaviour of guided waves in the stress aligned fluid filled inclusions

shear wave splitting observations and implications on stress regimes in the los angeles basin california a systematic analysis of three component seismograms recorded at 15 stations from earthquakes occurring at depths of 5 to 18 km beneath the los angeles basin and adjacent areas during the period between 1988 and 1994 shows 20 to 160 ms shear wave splitting shallow events exhibit little splitting while deeper events show progressively greater splitting with depth the preferred polarization direction of the fast shear wave is nearly n s consistent with the direction of the regional maximum horizontal compressive stress but independent of the azimuth between the event and station we interpret that the shear wave splitting is caused by fluid filled crustal microcracks and macrofractures aligned in the n s direction the shear wave splitting observations of 2 8 to 7 8 ms km can be explained in terms of an anisotropic crust containing vertical cracks with the apparent crack density of 0 023 0 08 on a regional basis the crack density may vary from station to station but we find that the apparent crack density in the strike slip region of the newport inglewood fault and the whittier fault is higher than in the reverse thrusting santa monica mountains and palos verdes hills no systematic change of shear wave splitting in the los angeles basin is found in this study

shear wave velocities anisotropy and splitting in high grade mylonites shear wave velocities have been measured for 10 high grade granulite and upper amphibolite facies mylonites from the snowbird tectonic zone canada at various pressures up to 600 mpa in order to explore the kinematic significance of shear wave splitting in the lower crust at 600 mpa vs anisotropy ranges from 11 in amphibolitic mylonites to 2 in granulite facies pyroxene bearing and tonalitic mylonites hornblende bearing granitic mylonites display intermediate vs anisotropy 6 7 vs anisotropy is lower than vp anisotropy in hornblende rich mylonites whereas vs anisotropy is higher than vp anisotropy in feldspar rich mafic tonalitic granitic and diatexitic mylonites shear wave splitting is pronounced in rocks displaying strong s wave anisotropy in most of the high grade mylonites studied the polarization direction of the first arrival vs1 corresponds to the extension lineation x when propagation is parallel to y perpendicular to lineation and parallel to foliation or to the y direction when propagation is parallel to x for all other propagation directions the polarization direction of vs1 is oblique to the x y and z directions therefore it may be difficult to infer the structural implications of shear wave splitting in lower continental crustal shear zones if one does not know 1 the orientation of the propagation and fast vibration directions with respect to in situ lineation and foliation and 2 the lattice preferred orientation of in situ rock forming minerals

misalignment of the orientation of fractures and the principal axes for p and s waves in rocks containing multiple non orthogonal fracture sets summary azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock a shear wave propagating in an azimuthally anisotropic medium splits into two components with diverent polarizations if the source polarization is not aligned with the principal axes of the medium in the presence of two or more non orthogonal sets of vertical fractures the symmetry of the medium may be approximated as monoclinic with a horizontal plane of mirror symmetry if in the absence of fractures the rock is transversely isotropic ti with the symmetry axis perpendicular to the bedding plane for such a medium the fast and slow polarization directions for vertically propagating shear waves are not parallel or perpendicular to any of the fracture planes but lie in directions given by the principal axes of a second rank fracture compliance tensor this tensor is independent of the normal compliance of the fractures it follows that for vertical propagation of shear waves in a vertically fractured ti medium any number of arbitrarily oriented vertical fracture sets is equivalent to two mutually perpendicular fracture sets provided that the seismic wavelength is large compared to the size of the fractures for ovsets typical of surface seismic acquisition the p wave velocity at fixed ovset varies with azimuth as cos 2 w w 0 where w is the azimuth measured with respect to the fast polarization direction for a vertically polarized shear wave w 0 depends on both the normal and the shear compliance of the fractures and may diver from zero if the ratio of the normal to shear compliance of the fractures varies significantly between fracture sets if this ratio is similar for all fractures w 0 0 and the principal axes of the variation in p wave velocity with azimuth for fixed ovset are determined by the principal axes of the second rank fracture compliance tensor

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

the turkish dilatancy project tdp3 multidisciplinary studies of a potential earthquake source region the section of the north anatolian fault lying near the city of izmit at the east of the marmara sea has been identified as a seismic gap and the possible site of a future major earthquake previously published studies of records from an earthquake swarm within the gap tdpl and tdp2 provided the first evidence that shear wave splitting occurs in earthquake source regions a conclusion since verified by many studies at other locations a third field study tdp3 was mounted in the izmit region during the summer of 1984 observations were made over an eight month period and included geomagnetic and geoelectric measurements in addition to a series of observations utilising dense arrays of three component seismometers earthquake activity in the principal study area was monitored over a period of eight months records showed features similar to those observed in the earlier studies in particular 1 almost all shear waves emerging within the shear wave window displayed shear wave splitting 2 the polarizations of the first arriving faster split shear waves showed sub parallel alignments characteristic of propagation through a distribution of parallel vertical cracks striking perpendicular to the minimum compressional stress these and other observations support the conclusion of earlier studies that the upper crust is pervaded by distributions of micro cracks aligned by stress known as extensive dilatancy anisotropy a search for time dependence in shear wave phenomena has revealed temporal variations in the delays between the split shear waves throughout the course of the tdp3 study but as yet this has not been correlated wi th specific earthquake activity

an integrated study of seismic anisotropy and the natural fracture system at the conoco borehole test facility kay county oklahoma a significant body of published work has developed establishing fracture related seismic anisotropy as an observable effect to further the understanding of seismic birefringence techniques in characterizing natural fracture systems at depth an integrated program of seismic and geologic measurements has been conducted at conoco s borehole test facility in kay county oklahoma birefringence parameters inferred from the seismic data are consistent with a vertical fracture model of density 0 04 striking east northeast that direction is subparallel to a fracture set mapped both on the surface and from subsurface data to the in situ maximum horizontal stress and to the inferred microfabric

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

estimation of fracture parameters from reflection seismic data part iii fractured models with monoclinic symmetry existing geophysical and geological data indicate that orthorhombic media with a horizontal symmetry plane should be rather common for naturally fractured reservoirs here we consider two orthorhombic models one that contains parallel vertical fractures embedded in a transversely isotropic background with a vertical symmetry axis vti medium and the other formed by two orthogonal sets of rotationally invariant vertical fractures in a purely isotropic host rock using the linear slip theory we obtain simple analytic expressions for the anisotropic coefficients of effective orthorhombic media under the assumptions of weak anisotropy of the background medium for the first model and small compliances of the fractures all effective anisotropic parameters reduce to the sum of the background values and the parameters associated with each fracture set for the model with a single fracture system this result allows us to eliminate the influence of the vti background by evaluating the differences between t

effects of the free surface on shear wavetrains summary the behaviour of shear waves is of great importance in identifying and investigating seismic anisotropy in the earth however shear wavetrains recorded at the earth s surface do not always reflect the motion at depth introducing practical problems of interpretation shear wavetrains incident on the surface of an isotropic half space at angles less than critical about 35 are broadly preserved but at greater angles substantial distortions can occur for stations situated close to the source as in local earthquake studies the local sp phase a radially polarized precursor to s may occur the behaviour at the surface of an anisotropic half space is further complicated by the divergence of phase and energy propagation vectors all of these complications suggest that detailed seismogram modelling is essential to any study of shear wave propagation in the earth and in particular to investigations of anisotropy induced shear wave splitting

estimates of upper crustal heterogeneity in the baltic shield from seismic scattering and borehole logs abstract constraints on small scale heterogeneity in the upper crust of the baltic shield were obtained from stochastic modelling of a variety of seismic data from the babel and siljan ring surveys a non linear least squares inversion scheme was applied to wavefield fluctuations in seismic reflections to obtain statistical parameters of the medium through which the wave energy propagated the reflections are from dolerite sills intruded at depths of 6 12 km into the palaeoproterozoic upper crust of the central baltic shield analysis of the babel data set reveals that the upper crust is a weakly scattering stochastic medium with velocities characterised by an exponential autocorrelation function with a correlation length of 150 50 m and rms velocity perturbation of 1 5 0 5 further inversions show that most of the scattering occurs within the upper few kilometres of the crust analysis of the sijan seismic data set reveals heterogeneity with length scales of 210 50 m and a velocity perturbation of about 1 0 the autocovariance and power spectrum of the sonic log and studies of frequency dependent q in vsp data from the gravberg 1 borehole give consistent estimates of basement heterogeneity the upper crust in this region is characterised by a correlation distance of 35 5 m and rms velocity perturbation of 4 6 in the uppermost 1 2 km of crust associated with near surface fracturing and a correlation distance of 140 50 m and rms velocity perturbation of 1 5 0 5 in the uppermost 6 8 km associated with compositional heterogeneity

stress induced seismic azimuthal anisotropy in the upper crust across the north west shelf australia seismic azimuthal anisotropy saa is observed in many areas of the earth and fast polarized directions have been mapped using earthquake surface waves and teleseismic s wave splitting over large regional and tectonic scales higher resolution petroleum exploration data including 3 d seismic surveys and dipole shear logs often exhibit azimuthal anisotropy however we are unaware of any published studies mapping saa from exploration scale data to study large scale regional and tectonic saa trends the physical mechanisms causing saa in earthquake data are a subject of great interest comparing regional trends to the higher resolution exploration trends may help understand these mechanisms we present a saa analysis using seismic exploration data across the north west shelf nws of australia we map the fast polarization directions from 34 dipole shear logs and two 3 d seismic surveys and compare them to in situ maximum horizontal stress orientations h our results show that the and h trends correlate across a geographical area spanning almost 2000 km and are similar to published results in the region from earthquake seismology these results suggest that differential horizontal stress is the likely mechanism causing saa at a wide range of spatial scales on the nws of australia we also show that saa is not observed at exploration scales in some areas of the nws and propose a lithology dependent stress sensitivity model in which saa is strongest in clean unconsolidated quartz dominated sandstones and weaker or nonexistent in well consolidated cemented rocks and shale dominated sediments

the basis for earthquake prediction summary recent advances in understanding the behaviour of shear waves propagating in the crust make the routine prediction of earthquakes seem practicable accumulating evidence suggests that most of the earth s crust is pervaded by distributions of fluid filled cracks and microcracks that are aligned by the contemporary stress field so that the cracked rockmass is effectively anisotropic to seismic waves this causes shear waves to split and shear wave splitting is observed whenever shear waves propagating along suitable raypaths in the crust are recorded by three component instruments these distributions of cracks are known as extensive dilatancy anisotropy or eda many characteristics of the crack and stress geometry can be monitored by analyzing shear waves propagating through the cracked rockmass observations of temporal variations of the behaviour of shear wave splitting in seismic gaps confirm these hypotheses and suggest that stress changes before earthquakes may be monitored by analyzing shear waves in particular monitoring earthquake preparation zones with three component shear wave vertical seismic profiles could lead to techniques for the routine prediction of earthquakes

shear wave anisotropy of active tectonic regions via automated s wave polarization analysis abstract using an automated aspect ratio method to evaluate shear wave anisotropy from earthquakes recorded on arrays of three component digital seismographs over the bucaramanga nest of colombia the east rift zone of hawaii and the phlegraean fields uplift area of italy a shear wave velocity anisotropy of 4 7 is observed the anisotropy is attributed to tectonic processes acting either to align anisotropic minerals or to align stress related cracks at most fixed seismographs the polarization of the leading s phase is consistent from event to event independent of earthquake location on a regional basis the polarization may vary from station to station but a regional trend is apparent for events from the bucaramanga nest colombia a small seismogenic volume at 160 km depth in a subducted plate shear wave splitting averages 0 5 0 1 s for more than 100 events the polarization direction of the fast shear wave is 355 n 15 parallel to the strike of the benioff zone at the phlegraean fields in italy which represent an active quaternary caldera the average shear wave splitting observed from 51 events is 0 2 s corresponding to a minimum velocity anisotropy of 7 the strike of the inferred maximum compressive stress is consistent with values obtained from fault plane solutions in hawaii the polarization of the leading shear wave tends to parallel the strike of the east rift zone average velocity anisotropy is about 5 if one assumes that the entire travel path contributes to the observed splitting however station variations in fast shear wave polarization direction imply that some of the splitting is due to near station effects

modeling of three component lg waves in anisotropic crustal models abstract lg wave synthetic seismograms are presented in order to analyze the influence of anisotropy in the continental crust on their three component waveforms the effect of dry and liquid filled open cracks in the upper crust and petrological anisotropy in the lower crust on waves produced by surface explosions and crustal earthquakes is examined the polarization and especially the amplitude of the transverse component proves to be very sensitive to introduction of weak anisotropy in the crust comparisons of the modeling with lg wave data produced by explosions in southern scandinavia show that a realistic crustal anisotropy can explain observations that the transverse components of these waves are as large as their vertical ones a 10 km thick surficial layer with liquid filled open cracks is our perferred model for southern scandinavia

is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of north america on the appalachian plateau of new york ene striking joints parallel the mid continent contemporary stress field and do not correlate with other structures joints within ohio indiana michigan and west virginia also parallel the mid continent maximum horizontal compression as indicated by hydraulic fracture measurements in northern indiana where the contemporary stress field deviates significantly from the orientation of the mid continent stress the attitude of the joints deviates accordingly in addition to joints structures on several scales from subcontinent size fracture systems to microfabrics in shales apparently parallel the mid continent stress field from this i conclude that these joints are mode i cracks whose orientation corresponds to the contemporary tectonic stress field in itself the formation of these structures constitutes a tectonic event that has often been overlooked in favor of tectonic events that produce the more spectacular but less pervasive faults and folds

neotectonics of the marmara sea region of turkey seismological investigations in western anatolia nw turkey have identified linear patterns of earthquake epicentres outlining a wedge shaped block in the area of the marmara sea this block shows different seismic characteristics from the rest of western anatolia and appears to act as a separate tectonic unit earthquake fault plane mechanisms show that the marmara block is being rotated and sheared in order to accommodate the right lateral motion of the north anatolian fault and the extensional tectonics of the southwestern anatolia investigations of the behaviour of seismic shear waves suggest that neotectonic stress aligns water filled microcracks throughout the upper brittle 10 20 km of the crust and that in situ stress directions can be monitored by analysis of shear waves recorded on three component instuments

shear wave polarizations near the north anatolian fault iii observations of temporal changes summary almost all shear waves from local earthquakes recorded on closely spaced three component seismometer networks deployed near the north anatolian fault turkey in two experiments in 1979 and 1980 display shear wave splitting the observations are consistent with the presence of eda extensive dilatancy anisotropy distributions of fluid filled cracks and microcracks aligned by the regional stress field temporal changes in the stress field which may occur before an earthquake may modify the geometry and possibly the orientation of the eda microcracks and lead to corresponding changes in the behaviour of the split shear waves a third experiment was undertaken in 1984 to investigate eda further and to search for possible temporal variations of the polarization of the leading split shear wave and the time delay between split shear waves observations indicate that the polarization alignments which are parallel to the strike of the parallel vertical eda cracks are unaltered between 1979 and 1984 implying that the direction of the regional stress field has not changed significantly temporal changes in the stress field are more likely to cause changes in the crack density and or aspect ratio which would result in a corresponding change in time delay between the split shear waves we examine observations of time delay in relation to their propagation path with respect to the crack geometry since it is then possible to separate the effects of changes in crack density and changes in aspect ratio with this procedure a small temporal variation of time delays is found between 1979 and 1984 consistent with a decrease in crack density and consequently a relaxation of stress in this time period no evidence was found for any observable variation of time delay over a six month observation span in 1984 we suggest that analysis of repeated shear wave vsps offers a technique for monitoring stress changes before earthquakes

suggestions for a consistent terminology for seismic anisotropy seismic anisotropy is an unfamiliar concept to many geophysicists and the use of misleading and ambiguous terminology has made it more difficult to understand i suggest here a consistent terminology in which simple expressions have specific meanings similar to their colloquial meanings it is hoped that use of such language will help to make the increasing number of papers reporting seismic anisotropy more readily comprehensible to the non specialist this not a manual of anisotropy and it is not intended for theoreticians it is a list of terms which may make anisotropy a little easier to understand for those more familiar with wave propagation in isotropic solids

numerical modeling for analyzing thermal surface anomalies induced by underground coal fires abstract coal seams burning underneath the surface are recognized all over the world and have drawn increasing public attention in the past years frequently such fires are analyzed by detecting anomalies like increased exhaust gas concentrations and soil temperatures at the surface a proper analysis presumes the understanding of involved processes which determine the spatial distribution and dynamic behavior of the anomalies in this paper we explain the relevance of mechanical and energy transport processes with respect to the occurrence of temperature anomalies at the surface two approaches are presented aiming to obtain insight into the underground coal fire situation in situ temperature mapping and numerical simulation in 2000 to 2005 annual temperature mapping in the wuda inner mongolia pr china coal fire area showed that most thermal anomalies on the surface are closely related to fractures where hot exhaust gases from the coal fire are released those fractures develop due to rock mechanical failure after volume reduction in the seams the measured signals at the surface are therefore strongly affected by mechanical processes more insight into causes and effects of involved energy transport processes is obtained by numerical simulation of the dynamic behavior of coal fires simulations show the inter relation between release and transport of thermal energy in and around underground coal fires our simulation results show a time delay between the coal fire propagation and the observed appearance of the surface temperature signal additionally the overall energy flux away from the burning coal seam into the surrounding bedrock is about 30 times higher than the flux through the surface this is of particular importance for an estimation of the energy released based on surface temperature measurements finally the simulation results also prove that a fire propagation rate estimated from the interpretation of surface anomalies can differ from the actual rate in the seam

risk assessment of underground coal fire development at regional scale abstract underground coal combustion is a phenomenon known worldwide coal fire monitoring and risk assessment provide important input data for the delineation of coal fire zones and planning of extinguishing activities at present research on coal fire risk focuses mainly on the probability assessment of spontaneous combustion at micro scale based on laboratory investigations of coal molecular structure and composition and their impact on the combustion process coal fire risk assessment at a larger scale such as for mines relies on geological factors and aspects of mining engineering and mine management these scales however are insufficient when considering extinguishing activities in larger areas in order to fill these gaps we studied risk assessment of underground coal fire development ucfd at a regional scale the factors impacting on coal fire development were analyzed under three different aspects coal composition and structure which can influence the direction of underground coal combustion topography and geology which determine the burning environment and climatic conditions and human activities which trigger combustion processes based on this analysis a regional underground coal fire risk assessment ucf ra index system was established it is predicated on the assumption that all indices contribute equally to coal fire risk data layers of 1 km 1 km spatial resolution for each index were calculated and overlaid xinjiang uygur autonomous region was selected as a validation area in view of local conditions and the availability of relevant data the index system was modified the applied method however remained unchanged assessment results are generally satisfying and can be used for monitoring and extinguishing of underground coal fires ucfs in xinjiang

local shear wave observations in joao câmara northeast brazil 1 we present shear wave splitting data from a 10 km aperture eight station digital three component seismograph network which operated from december 1992 to august 1994 near joao c mara 5 33 s 35 51 w to record shear waves from continuing microearthquake activity persisting 6 years after a main shock in november 1986 previous aftershock recording had shown remarkably impulsive simple seismograms and hypocentral errors of typically 200 m in a structure of near uniform wave speed but only one station was three component the pattern of polarization directions shown is consistent with the trend of precambrian foliation observed in the field it is therefore concluded that these shear wave observations are controlled by the seismic anisotropy associated with the precambrian foliation and hence bear the signature of the paleo stress field i e the precambrian deformation regime this conclusion has important implications for the interpretation of shear wave splitting observations made in crustal crystalline rock because other authors have asserted that shear wave splitting normally results from the effect of stress aligned fluid filled cracks extensive dilatancy anisotropy or eda eda is by contrast a signature of the present day stress field in this region there is firm evidence that the present day maximum compressive stress is e w this evidence comes from earthquake focal mechanisms and borehole breakout data and is summarized in the world stress map we therefore conclude that our present data are in maximum conflict with an interpretation in terms of eda our interpretation is supported by the observation that delay times are consistent with those expected from modeling of elastic waves in the crust

pn anisotropic tomography of northeast china and its implications to mantle dynamics abstract in this study we manually pick a total of 9197 high quality pn arrival time data from seismograms of 457 events recorded at 271 provincial seismic stations and 69 necessarray portable seismic stations in northeast ne china using these pn arrivals having much better crosscrissing coverage of ray paths in the uppermost mantle we determine a new high resolution anisotropic tomographic model our new model shows distinct lateral heterogeneities in both velocity and anisotropy which are closely related to surface geology obvious high pn velocities are imaged under the songliao hailar and erlian stable basins prominent low pn velocities are observed under the changbaishan jingpohu longgang wudalianchi and datong volcanic areas as well as the halaha and abaga volcanic groups furthermore some low pn velocities are well connected or they are separated by high pn velocities but connected by the fast directions of pn propagation these results suggest the existence of some correlations among these volcanoes in the present day or the traces preserved in the past tectonic evolution history we propose a possible dynamic model that illustrates the material and energy exchange among the volcanoes which could be caused by a complicated rather than simple e w horizontal flow of mantle convection in the big mantle wedge due to the stagnancy and dehydration of the western pacific slab in the mantle transition zone these results provide new insights into the mechanism of genesis of active volcanoes and complex mantle dynamics of ne china

two dilatancy based models to explain coda wave precursors and p s spectral ratio abstract some coda wave precursors can be predicted by the dilatancy model others need more complicated models the first of the models proposed here takes into account the compression of the medium around the swelling dilatancy zone and explains several observed precursor properties a simple theoretical model is employed to describe anisotropic s wave absorption in a microcracked medium under shear load the second model is used to explain the radiation bursts observed in coda in the preparation period and which overlap background scattered waves this model also explains the unusually high p s energy ratio of seismic waves from shocks in preparation zones the concept of seismic emission is employed to explain these phenomena two possible modes of emission are proposed 1 1 passive radiation generated by an abrupt dilatancy drop produced by stress drop in and around the earthquake source and 2 2 active radiation stimulated by seismic waves in a preparation zone or in any other volume of the lithosphere which is in a state of changing dilatancy

shear wave splitting in cross hole surveys modeling shear wave splitting diagnostic of some form of effective seismic anisotropy is observed along almost all near vertical raypaths through the crust the splitting is caused by propagation through distributions of stress aligned vertical parallel fluid filled cracks microcracks and preferentially oriented pore space that exist in most crustal rocks shear waves have severe interactions with the free surface and may be seriously disturbed by the surface and by near surface layers in principle cross hole surveys chss should be free of much of the near surface interference and could be used for investigating shear waves at higher frequencies and greater resolution along shorter raypaths than is possible with reflection surveys and vsps synthetic seismograms are examined to estimate the effects of vertical cracks on the behavior of shear waves in chs experiments the azimuth of the chs section relative to the strike of the cracks is crucial to the amount of information about seismic anisotropy that can

magnetotellurics in southwestern sweden evidence for electrical anisotropy in the lower crust magnetotelluric data from measurements conducted on the southwestern part of the baltic shield are analyzed with respect to crustal and upper mantle structures the interpretations have been done using ordinary two dimensional and transverse anisotropic layered models in which distortions of the impedance tensors due to near surface electrical structures are included in the interpretation procedures the anisotropic model is able to explain the gross features of the data whereas the two dimensional model only displays features observed at long periods the two main results from the study with anisotropic models are the interpretation of a possible transverse anisotropic lower crust and a depth to the uppermost conductive layer within the mantle of more than 200 km the minimum depth to the conductive mantle layer is found from a most squares analysis

stress directions and shear wave anisotropy observations from local earthquakes in southeastern sicily italy the spatial distribution of 414 earthquakes 1 0 m l 4 6 recorded from 1994 to 2002 in southeastern sicily italy has been analyzed the seismicity generally coincides with mapped plio quaternary faults including the north north west south southeast striking offshore fault system which is the most important tectonic structure of the area for the best located events we computed 70 focal mechanisms by combining p wave polarities with s wave polarizations a predom inance of strike slip and normal faults was observed focal mechanisms were then inverted for stress tensor parameters by using the algorithm of gephart and forsyth the results highlighted a region governed mainly by a north northwest south south east to northwest southeast compressional stress regime moreover anisotropy anal ysis of shear waves showed a polarization of fast s waves coherently aligned with this stress direction a finer scale analysis of the stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis r1 the eastern sector displays a nearly horizontal r1 trending northwest southeast the central sector is affected by a low dip north northwest south southeast r1 whereas in the western sector a north northwest south southeast oriented r1 with a higher dip angle was detected finally the comparison of the spatial distribution of seismicity occurring during 1994 2002 with locations of previous instrumental earthquakes and larger m 5 0 historical events showed that the seismicity patterns are persistent

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

the seismic travel time drop in the charlevoix region from 1979 to 1980 evidence for aligned saturated cracks in the crust a significant drop in seismic travel times of up to 1 0 occurred in the charlevoix region between 1979 and 1980 possibly related to the m 5 0 aug 1979 earthquake in the vicinity a travel time drop of this magnitude could have been produced either by the closing of dry or saturated cracks in the upper crustal material or by the saturation of dry or partly saturated cracks however the anisotropy of travel time changes in this area supports the view that this travel time drop was caused by the closing of water saturated aligned vertical cracks in the crustal material three different crack directions with respect to north were resolved 0 or 90 in the precambrian rocks underneath the st lawrence river 18 or 72 in the shallow rocks 5 km of the charlevoix crater region and 35 or 125 in the paleozoic cover rocks crack closure would require a decrease in the pore volume of the rocks which would be expected to produce subsidence in the charlevoix area since repeated levellings restrict the vertical crustal motion during this time interval to less than 2 cm we conclude that either the effective aspect ratio of cracks is less than 0 0001 or the process of crack closure occurred in a number of unconnected regions more specifically the crack deformation would have to occur in isolated inclusions less than 1 km in diameter and no deeper than about 6 km the process of crack closure may have been triggered by the passage of seismic waves from the m 5 0 earthquake

a remote sensing and gis based investigation of a boreal forest coal fire abstract a coal seam fire in interior alaska was suspected to have started the rex creek forest fire in the summer of 2009 with prevailing winds the forest fire spread rapidly to the north and within eleven days it burned about 410 km 2 of boreal forest coal seam fires can go unnoticed and unreported when present in remote and inaccessible areas however they still pose a serious threat to the surroundings we used summer time thermal infrared images from 1999 through 2009 acquired by the landsat satellite and through the process of image stacking identified a region where the surface persistently showed temperatures 5 c to 14 c higher than the background areas field validation confirmed that this thermal anomaly area corresponds to a previously undocumented shallow coal seam fire superimposing the boundary of the rex creek forest fire revealed that the coal seam fire was at the southern end of the burn area where the forest fire originated plotting the location of all lightning strikes during this period helped to rule out lightning as the cause of the forest fire coal fires and forest fires can have a complex and dynamic relationship one being the possible cause of the other a thorough inventory of all past and present known coal seam fire locations can help to update forest fire hazard maps a detailed map of shallow coal seam areas can help to prioritize fire fighting operations in order to avoid the chance of starting a new coal seam fire

the role of ice lines in the formation of uranus and neptune we aim at investigating whether the chemical composition of the outer region of the protosolar nebula can be consistent with current estimates of the elemental abundances in the ice giants to do s

seismic velocity anisotropy in mica rich rocks an inclusion model summary we calculated seismic wave velocity anisotropy caused by the preferred orientation of mica minerals by using the differential effective medium method dem spheroidal biotite crystals with their c axes coinciding with the symmetry axis of the spheroid are embedded in an isotropic matrix up to a volume ratio of 30 per cent all crystals are aligned with their c axes parallel to the symmetry axis of the effective homogeneous medium which shows transverse isotropy the effect of crystal shape on anisotropy was studied by changing the aspect ratio the ratio between the minor axis and the major axis of the spheroid from 0 01 flat spheroid to 1 sphere the s velocity anisotropy becomes large as the crystal shape becomes flat whereas the p velocity anisotropy shows only small changes with changes of the crystal shape in particular biotite generates large s wave anisotropy and the anisotropy becomes stronger as the aspect ratio of the biotite crystal becomes smaller when the volume ratio of the mica mineral is large the p wave phase velocity surface shows considerable deviation from the ellipse and the sv wave phase velocity surface forms a large bulge and crosses the sh wave phase velocity surface singularity in the plane including the symmetry axis these results show an interesting contrast when compared with the effect of crack or pore shape on seismic velocity anisotropy crack or pore shape affects the p velocity more than the s velocity we also calculated thomsen s anisotropic parameters e and as functions of the crystal aspect ratio and the mica volume ratio

radon content of groundwater as an earthquake precursor evaluation of worldwide data and physical basis the properties of a worldwide data set of 91 radon 222 rn anomalies the frequency of occurrence the precursor time interval and the distribution of peak amplitudes are correlated with earthquake data such as the respective magnitude and epicentral distance these anomalies were reported as precursors to earthquakes in the united states ussr china japan and iceland although the data set is incomplete and limited by experimental deficiencies several consistent patterns emerge radon anomalies from different tectonic regions show similar patterns the radon anomalies occur at greater epicentral distances for earthquakes of the larger magnitude anomalies preceding large earthquakes m 6 are frequently observed at a distance of 100 to 500 km these distances are larger than several times the rupture dimensions of the future earthquakes the time from the onset of an anomaly to the time of the earthquake the precursor time increases with magnitude but decreases with distance between epicenter and radon station in addition radon anomalies are observed more frequently prior to large earthquakes than prior to small ones indicating that the preparation zone increases in size as magnitude increases the peak amplitude does not scale with magnitude but forms a consistent pattern with epicentral distance in that the larger the earthquake magnitude the farther away the largest amplitudes tend to occur the preparation zone of the earthquake where the anomalies occur forms an almost continuous annulus that expands with time away from the future rupture zone the outer radius of this annulus scales with the earthquake magnitude model calculations indicate that strain fields of at most 10 6 to 10 8 strain caused the radon anomalies if these strains are divided by the appropriate precursor time minimum strain rates from 10 7 day 1 to 10 10 day 1 are obtained such small strains and strain rates suggest that in most cases neither mechanical crack growth induced by dilatancy nor mechanical coupling between pore pressure and the rock matrix caused the anomalies large changes in the orientation of the local strain field however could occur and affect the local stress intensity factor since changes in the stress intensity factor can result in stress corrosion the occurrence of radon anomalies is attributed to slow crack growth controlled by stress corrosion in a rock matrix saturated by groundwater

observations of shear wave splitting parameters in the western gulf of corinth focusing on the 2014 mw 5 0 earthquake abstract the western gulf of corinth located in central greece is an active neotectonic graben that features intense seismicity events recorded by dense local networks during 2014 including those belonging to an offshore swarm between nafpaktos and psathopyrgos were used to perform a shear wave splitting study with recordings that fulfill strict criteria the splitting parameters determined by visually inspecting polarigrams and hodograms are the polarization direction of the fast shear wave and the time delay between the two split shear waves over 960 observations were obtained for 13 stations in the area temporal variations of normalized time delays were investigated to extract patterns that indicate stress accumulation and release such patterns were identified in 5 stations where linear trends of increase followed by a decrease a couple of weeks before the mw 5 0 7th november 2014 earthquake were identified these observations highlight similarities with behavior of time delays before the occurrence of earthquakes in other areas nevertheless scattering of measurements poses a significant challenge in interpreting these variations the mean polarization direction of the fast shear wave in each station is consistent with the known regional maximum horizontal stress component i e wnw ese except for three stations located in the northern shore of the gulf which present a general ne sw polarization direction the former is interpreted with the anisotropic poro elasticity model where fluids within microcracks aligned with the regional stress regime affect shear wave splitting the latter are located in an area where faults of similar direction have been mapped which could indicate the presence of secondary local stress states

the geysers geothermal field results from shear wave splitting analysis in a fractured reservoir summary clear shear wave splitting sws is observed in 1757 high signal to noise ratio microearthquake seismograms recorded by two high density seismic arrays in the nw and the se geysers geothermal fields in california the geysers reservoir rocks within the study area are largely composed of lithic low grade metamorphism well fractured metagraywackes which commonly lack schistosity warranting the general assumption that shear wave splitting here is induced solely by stress aligned fracturing in an otherwise isotropic medium the high quality of observed shear wave splitting parameters fast shear wave polarization directions and time delays and the generally good data spatial coverage provide an unprecedented opportunity to demonstrate the applicability and limitations of the shear wave splitting approach to successfully detect fracture systems in the shallow crust based on sws field observations from a geothermal reservoir results from borehole stations in the nw geysers indicate that polarization orientations range between n and n60e while in the se geysers ground surface stations show polarization directions that are generally n5e n35e to n60e n75e to n85e and n20w to n55w crack orientations obtained from observed polarization orientations are in good agreement with independent field evidence such as cracks in geological core data tracer tests locally mapped fractures and the regional tectonic setting time delays range typically between 8 and 40 ms km 1 indicating crack densities well within the norm of fractured reservoirs the sizeable collection of high resolution shear wave splitting parameters shows evidence of prevalent vertical to nearly vertical fracture patterns in the geysers field at some locations however strong variations of sws parameters with ray azimuth and incident angle within the shear wave window of seismic stations indicate the presence of more complex fracture patterns in the subsurface

seismic inversion for the parameters of two orthogonal fracture sets in a vti background medium characterization of naturally fractured reservoirs often requires estimating parameters of multiple fracture sets that develop in an anisotropic background here we discuss modeling and inversion of the effective parameters of orthorhombic models formed by two orthogonal vertical fracture sets embedded in a vti transversely isotropic with a vertical symmetry axis background matrix although the number of the microstructural physical medium parameters is equal to the number of effective stiffness elements nine we show that for this model there is an additional relation constraint between the stiffnesses or tsvankin s anisotropic coefficients as a result the same effective orthorhombic medium can be produced by a wide range of equivalent models with vastly different fracture weaknesses and background vti parameters and the inversion of seismic data for the microstructural parameters is nonunique without additional information reflection moveout of pp and ps waves can still be used to find the

stress in the earth s crust the link between crustal fluids and extensive dilatancy anisotropy observations of seismic shear wave splitting birefringence along almost all raypaths in the earth9s crust have led to the recognition that the fluid filled cracks microcracks and pores known to pervade most rocks in the crust have been preferentially aligned by the prevailing stress field this phenomenon is known as extensive dilatancy anisotropy or eda and the fluid filled microstructures as eda cracks because the seismic effects can be modelled by distributions of parallel cracks the effects of these stress aligned eda cracks have been observed in many different areas and they appear ubiquitous in the upper 10 to 20 km of the crust many details of the crack geometry and the current stress field can be readily determined from records of the shear waves which have propagated through these aligned cracks in separate studies geochemists have confirmed the presence of microscopic fluid filled cavities in almost all geological material these are referred to as fluid inclusions and provide unequivocal evidence of crustal fluid circulation often along stress controlled fractures different stress episodes can be recognized in many rock samples and studies of fluid inclusions can be used to interpret the palaeo stress history of a rock where the in situ morphology of these inclusions is related to the current regional stress field this paper discusses the relationship between the fluid inclusions of the geochemical literature and eda cracks and concludes that the former may be regarded as one of the constituent inclusion types of extensive dilatancy anisotropy

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

seismic anisotropy in granite at the underground research laboratory manitoba the shear wave experiment at atomic energy of canada limited s underground research laboratory was probably the first controlled source shear wave survey in a mine environment taking place in conjunction with the excavation of the mine by test tunnel at 420 m depth the shear wave experiment was designed to measure the in situ anisotropy of the rockmass and to use shear waves to observe excavation effects using the greatest variety of raypath directions of any in situ shear wave survey to date inversion of the shear wave polarizations shows that the anisotropy of the in situ rockmass is consistent with hexagonal symmetry with an approximate fabric orientation of strike 023 degree and dip 35 degree the in situ anisotropy is probably due to microcracks with orientations governed by the in situ stress field and to mineral alignment within the weak gneissic layering however there is no unique interpretation as to the cause of the in situ anisotropy as the fabric orientation agrees approximately with both the orientation expected from extensive dilatancy anisotropy and that of the gneissic layering eight raypaths with shear waves propagating wholly or almost wholly through granodiorite rather than granite do not show the expected shear wave splitting and indicate a lower in situ anisotropy which maymore be due to the finer grain size and or the absence of gneissic layering within the granodiorite these results suggest that shear waves may be used to determine crack and mineral orientations and for remote monitoring of a rockmass this has potential applications in mining and waste monitoring less

effects of point singularities on shear wave propagation in sedimentary basins summary in most directions of propagation in anisotropic solids seismic shear waves split in regular and predictable ways that in principle can be directly related to the degree of anisotropy and the anisotropic symmetry of the rockmass in all anisotropic solids however there are directions of propagation known as shear wave singularities where the split shear waves have the same phase velocities for directions of propagation near the commonest type of singularity the point singularity the relationship between the phase and group velocities may undergo rapid variations for small changes in direction this results in shear waves along rays propagating at the grozq velocity behaving anomalously with irregular polarizations and amplitude changes as if they were propagating near cusps although the degree of anisotropy may be too small to cause conventional cusps on the group velocity wave surfaces the effects of propagation near such point singularities have been identified in sedimentary basins where they are features of the well established phenomenon of azimuthal isotropy transverse isotropy with a vertical axis of symmetry caused by horizontal lithology or by fine layering ptl anisotropy combining with the more recently recognized azimuthal anisotropy caused by distributions of near parallel near vertical fluid filled inclusions eda anisotropy this paper demonstrates these irregular effects by calculating synthetic shear waves in directions near a point singularity in a material simulating a possible sedimentary basin such anomalies may be important in exploration seismology as point singularities can occur along nearly vertical ray paths in sedimentary basins if not identified correctly the effects of such point singularities could be mistakenly attributed to structural irregularities and if correctly identified the directions of such singularities can place tight constraints on possible combinations of ptl and eda anisotropy in sedimentary basins

seismic energy release of the moon previous estimates of the seismic energy released by lunar events have not properly accounted for instrument bandwidth variations in corner frequency and the effects of intense scattering in this paper equations are developed that include all of these effects and give realistic estimates for source parameters these equations are applied to seismograms and displacement spectra from near surface and deep moonquakes to obtain m0 seismic moment s seismic energy release eyr seismic energy released annually by each lunar event class stress drop and mb body wave magnitude the calculations yield m0 3 1021 dyn cm s 2 1017 ergs 400 bars and mb 5 0 for the largest shallow moonquakes and m0 5 1020 dyn cm s 1 1013 ergs 0 1 bars and mb 3 0 for large deep events the average energy released annually is eyr 2 1017 erg yr and eyr 8 1013 erg yr by shallow and deep events respectively overall lunar energy release is dominated by the shallow events the energy released by the deep events may be accounted for by tidal dissipation and deep event stress drops are comparable to and smaller than the calculated tidal stresses a comparison of the above values with those observed terrestrially eyr 1025 ergs and with the energy available from heat flow and tidal dissipation emphasizes the importance of tectonic style e g plate tectonics in determining the characteristics of planetary seismicity

behaviour of shear waves in rocks with two sets of parallel cracks summary the behaviour of shear wave polarizations and shear wave splitting observed at the surface suggesting propagation through parallel vertical cracks has been the stimulus for many recent investigations both in earthquake and exploration seismology cracks in surface outcrops however frequently display multiple sets of parallel vertical intersecting cracks this paper examines seismic shear wave propagation in media with two sets of parallel vertical cracks biplanar cracks to determine whether the behaviour of shear waves can distinguish between the effects of multiple crack sets and the effects of single sets of parallel cracks monoplanar cracks this study shows that the difference between the overall patterns of polarizations of biplanar and monoplanar systems of vertical cracks within the shear wave window in many circumstances is marginal and unlikely to be easily recognized in the field we conclude that it is frequently not possible from analysis of surface observations of shear wave polarizations alone to distinguish between the effects of biplanar sets of parallel vertical cracks and those of a single parallel set the difference can usually be recognized if an accurate estimate of both polarizations and time delays between the split shear waves is available over a wide range of azimuths and angles of incidence within the shear wave window however in areas with complex fracture and stress systems time delays may be much harder to estimate than the polarization angles of the leading split shear waves and it may not be easy to distinguish from seismic data alone the difference betwekn parallel and multiplanar sets of vertical fractures

equivalent medium representation of fractured rock a similarity may be found between various approaches for determining the effects of parallel fractures or aligned cracks on seismic wave propagation at wavelengths that are long compared with the scale length of the cracks fractures can be modeled using an empirical linear slip condition however natural fracture surfaces can also be simulated directly as planar distributions of small isolated areas of slip cracks model 1 or conversely as planar distributions of imperfect interfacial contacts model 2 an alternative is plane surfaces separated by thin continuous layers of viscous fluid or a soft material model 3 we present analytic expressions for the fracture compliances for these three models and using these analytic results compute the effective compliances and stiffnesses of the fractured material as a result it is possible to relate the measured compliances or stiffnesses directly to the statistics of the microstructural details of a fracture given appropriate a priori information on the fracture surfaces the results for model 1 are equivalent to a volume distribution of cracks as studied by hudson 1980 1981 for small crack density the results for model 2 are basically the same as those given by white 1983 for a packing of spheres and finally the results for model 3 are in agreement with those given by backus 1962 for combinations of two constituent layers these results can be extended to the case of nonaligned fractures and to allow for fluid flow between cracks and into a porous matrix rock finally it is shown that the ratio of the normal to shear fracture compliance is a good indicator of the properties of the fracture infill

deep borehole log evidence for fractal distribution of fractures in crystalline rock summary sonic velocity and electrical resistivity logs run to a depth of 3 5 km in crystalline rock near the san andreas fault at cajon pass in southern california correlate over scale lengths both small sub metre and large tens to hundreds of metres no such correlations are seen with the more lithologically sensitive natural gamma intensity log the correlation between the sonic velocity and electrical resistivity logs suggests that a non lithologic property of the crystalline rock controls fluctuations in situ fracture intensity is a logical candidate for the controlling rock property the fluctuations of the individual sonic velocity and electrical resistivity logs are examined with the hurst rescaled range parameter over borehole log intervals 1 5 m l 1500 m for log fluctuations arising from a scale invariant physical process the hurst rescaled range scales with data interval as lh 0 h 1 a purely random sequence of in situ fractures produces a scaling exponent h 0 50 fluctuations in the cajon pass sonic velocity and electrical resistivity logs yield h 0 70 evidence that in situ fracture sets tend to occur in clusters rather than at purely random intervals the tendency for fracture clustering over log intervals 1 5 m l 1500 m suggests that fracture formation is a fractal process independent of length scale in which larger fracture intervals form from clustering of numerous smaller fracture intervals seismic reflectivity derived from the borehole sonic velocity log is also scale independent over the range of data intervals 1 5m l 1500 m with a hurst exponent h 0 21 if we associate fracture clustering with crustal fault formation the cajon pass borehole sonic velocity and electrical resistivity logs predict that crustal faults scale fractally with fractal dimension d 2 30 the equivalent b value for earthquake size distribution is b d 2 1 15 on this hypothesis observed b values 1 15 indicate a tendency for earthquakes to cluster on existing weak faults a scale independent mechanics for crystalline rock fracture formation in which larger scale fractures form as clusters of smaller scale fractures provides a mechanical link between the small pervasive stress aligned flaws cracks and microfractures which can impart anisotropy to crystalline rock and the larger scale fractures associated with finite strain and faulting thus in crustal regions of active but low strain fracture anisotropy is observed to be aligned with the inferred maximum principal stress while in actively faulted crust fracture anisotropy is observed to be more nearly fault parallel as if the fractures are aligned by the finite strain faulting process

coal fires in indonesia abstract indonesia s fire and haze problem is increasingly being ascribed to large scale forest conversion and land clearing activities making way for pulpwood rubber and oil palm plantations fire is the cheapest tool available to small holders and plantation owners to reduce vegetation cover and prepare and fertilize extremely poor soils fires that escaped from agricultural burns have ravaged east kalimantan forests on the island of borneo during extreme drought periods in 1982 1983 1987 1991 1994 and 1997 1998 estimates based on satellite data and ground observations are that more than five million hectares were burned in east kalimantan during the 1997 1998 dry season not only were the economic losses and ecological damage from these surface fires enormous they ignited coal seams exposed at the ground surface along their outcrops coal fires now threaten indonesia s shrinking ecological resources in kutai national park and sungai wain nature reserve sungai wain has one of the last areas of unburned primary rainforest in the balikpapan samarinda area with an extremely rich biodiversity although fires in 1997 1998 damaged nearly 50 of this reserve and ignited 76 coal fires it remains the most valuable water catchment area in the region and it has been used as a reintroduction site for the endangered orangutan the office of surface mining provided indonesia with the capability to take quick action on coal fires that presented threats to public health and safety infrastructure or the environment the us department of state s southeast asia environmental protection initiative through the us agency for international development funded the project technical assistance and training transferred skills in coal fire management through the ministry of energy and mineral resource s training agency to the regional offices giving the regions the long term capability to manage coal fires funding was also included to extinguish coal fires as demonstrations to the indonesian government successful demonstrations obtained visible benefits for a large number of local interests and received overwhelmingly favorable public response this built the public and political support needed to continue this work these successes encouraged the ministry to officially assume the responsibility for coal fire management by decree 1539 20 mpe 1999 the ministry reallocated internal funds to support portions of the immediate suppression projects and committed a portion of the coal royalty fund to provide long term support for coal fire suppression activities coal fires continue to present a serious risk to indonesia s ecosystems population and forest resources the ministry and local governments in east kalimantan are still working on the coal fire inventory it presently contains 164 coal fires but it is far from complete unless these coal fires are managed or extinguished they will add to the already catastrophic cycle of anthropogenic forest fires that further reduce indonesia s forest resources and endangered species while contributing unnecessarily to global carbon emissions

paris basin vsps case history establishing combinations of fine layer or lithologic anisotropy and crack anisotropy from modelling shear wavefields near point singularities summary this paper examines shear wave splitting in multi offset vsps at a borehole site in the paris basin and demonstrates that the sedimentary structure must have at least orthorhombic anisotropic symmetry it is suggested that the most likely cause for such an orthorhombic symmetry is the combination of fine layer or lithologic anisotropy and crack anisotropy transition zones close to shear wave singularities where there are rapid changes of shear wave polarizations and delays can account for the anomalous observed waveforms observations of large variations in shear wave polarizations in propagation directions close to point singularities are probably the first positive identification of the effects of singularities in sedimentary basins

neptune and uranus ice or rock giants existing observations of uranus and neptune s fundamental physical properties can be fitted with a wide range of interior models a key parameter in these models is the bulk rock ice ratio and models broadly fall into ice dominated ice giant and rock dominated rock giant categories here we consider how observations of neptune s atmospheric temperature and composition h2 he d h co ch4 h2o and cs can provide further constraints the tropospheric co profile in particular is highly diagnostic of interior ice content but is also controversial with deep values ranging from zero to 0 5 parts per million most existing co profiles imply extreme o h enrichments of 250 times solar composition thus favouring an ice giant however such high o h enrichment is not consistent with d h observations for a fully mixed and equilibrated neptune co and d h measurements can be reconciled if there is incomplete interior mixing ice giant or if tropospheric co has a solely external source and only exists in the upper troposphere rock giant an interior with more rock than ice is also more compatible with likely outer solar system ice sources we primarily consider neptune but similar arguments apply to uranus which has comparable c h and d h enrichment but no observed tropospheric co while both ice and rock dominated models are viable we suggest a rock giant provides a more consistent match to available atmospheric observations this article is part of a discussion meeting issue future exploration of ice giant systems

temporal changes in shear wave splitting during an earthquake swarm in arkansas shear wave splitting has been observed in seismograms of two aftershock and one foreshock sequence recorded over 12 days by a local network of nine three component digital seismometers during the 1982 enola arkansas earthquake swarm polarizations of the faster split shear wave show alignments within the shear wave window which correlate with the regional stress field the time delay between split shear waves appears to increase before and to decrease at or after the time of each main shock this behavior is similar to observations before and after a m 6 0 event near the anza seismic gap southern california and may correspond to variations in crack geometry caused by changes in the stress field before and after the earthquakes there are indications that the decrease in delay may occur a few hours before the main shock and could be a possible precursor for earthquake prediction

correlation of paleotectonic fracture and microfracture orientations in cores with seismic anisotropy at cajon pass drill hole southern california fractures and microfractures in oriented cores from the cajon pass drill hole southern california have strong preferred orientations there are two major sets of extensional fractures and microfractures that have northwest and northeast strikes and vertical dips relative and absolute densities of both sets vary greatly within a single core the regional tectonic history implies that these fractures and microfractures probably formed before late miocene pliocene vertical seismic profiling experiments in the drill hole show shear wave splitting with polarization directions parallel to the fracture and microfracture orientations the measured seismic anisotropy is due to these inherited deformation fabrics and does not appear to be caused by the current local stress field

latitudinal variation in the abundance of methane ch4 above the clouds in neptune s atmosphere from vlt muse narrow field mode observations abstract observations of neptune made in 2018 using the new narrow field adaptive optics mode of the multi unit spectroscopic explorer muse instrument at the very large telescope vlt from 0 48 to 0 93 m are analysed here to determine the latitudinal and vertical distribution of cloud opacity and methane abundance in neptune s observable troposphere 0 1 3bar previous observations at these wavelengths in 2003 by hst stis karkoschka and tomasko 2011 icarus 205 674 694 found that the mole fraction of methane above the cloud tops at 2 bar varied from 4 at equatorial latitudes to 2 at southern polar latitudes by comparing the observed reflectivity at wavelengths near 825 nm controlled primarily by either methane absorption or h2 h2 h2 he collision induced absorption we find a similar variation in cloud top methane abundance in 2018 which suggests that this depletion of methane towards neptune s pole is potentially a long lived feature indicative of long term upwelling at mid equatorial latitudes and subsidence near the poles by analysing these muse observations along the central meridian with a retrieval model we demonstrate that a broad boundary between the nominal and depleted methane abundances occurs at between 20 and 40 s we also find a small depletion of methane near the equator perhaps indicating subsidence there and a local enhancement near 60 70 s which we suggest may be associated with south polar features spfs seen in neptune s atmosphere at these latitudes finally by the use of both a reflectivity analysis and a principal component analysis we demonstrate that this depletion of methane towards the pole is apparent at all locations on neptune s disc and not just along its central meridian

calibration of shear wave splitting in the subcontinental upper mantle beneath active orogenic belts using ultramafic xenoliths from the canadian cordillera and alaska abstract despite the abundance of measurements of shear wave splitting around the world some fundamental questions about its geological interpretation have not yet been answered in order to constrain 1 i the orientation and magnitude of s wave anisotropy 2 ii the thickness of the anisotropic layer and 3 iii the possible variation of the anisotropy with depth in the subcontinental upper mantle beneath active orogenic belts we have carried out a systematic investigation on shear wave properties of mantle xenoliths from recent alkaline basalts of the canadian cordillera and alaska it is found that the polarization direction of the fast s wave as well as the time delay t between the two arrivals are strongly dependent on the propagation direction with respect to the structural frame at a single station the variations of measured and t values for sks phases from different events can result from this dependence because the average grain size of olivine in upper mantle samples is commonly larger than 0 5 mm dislocation creep prevails over a thickness of at least 250 km and results in olivine lpo and hence anisotropy and shear wave splitting a single thick anisotropic layer in mountain parallel subvertical shear zones may cause high values of tgd while multiple subhorizontal layers will produce multiple splitting making the measurement and interpretation of s wave splitting difficult our results also suggest that the s wave splitting observed in cold old stable cratons such as the canadian shield is likely dominated by fossil anisotropy whereas that observed in hot active orogenic regions such as the northwest american cordillera and the tibetan plateau may be dominated by modern tectonic deformation in the lithosphere and flow in the asthenosphere

evidence for shallow and pervasive seismic anisotropy in the wellington region new zealand the shear waves of local earthquakes were recorded during a 5 month deployment of seven three component digital seismographs on the wellington peninsula new zealand the seismographs were spaced an average of less than 5 km apart and over 300 local earthquakes were recorded with phase arrivals within the shear wave window a significant number 37 of the earthquakes recorded showed clear evidence of shear wave splitting identifiable fast and slow shear wave arrivals with similar pulse shapes consistent polarization directions at particular stations were also observed even when poor signal to noise ratio or scattering meant that no slow shear wave arrival could be identified however there were large station to station differences in the polarization directions correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double couple focal mechanism the cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy large station to station differences in the polarization alignments ranging from 61 24 to 137 18 suggest that most anisotropy is confined to the top 2 3 km of the crust however there is evidence from one station for a small amount of pervasive anisotropy if such a trend existed on the other stations it could not be identified because of the large scatter in the data points the measured delay times between split shear waves vary from 0 02 to 0 22 s with a mean value of 0 1 0 06 s this translates to a near surface shear wave velocity anisotropy of about 10 with up to 2 pervasive anisotropy possible throughout the crust this data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important there is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment

seismic anisotropy of the shallow crust at the juan de fuca ridge microearthquake data recorded on four ocean bottom seismometers are used to study shear wave splitting on the endeavour segment of the juan de fuca ridge the covariance matrix decomposition method is used to determine the sensor orientation from explosive shot data and to estimate the anisotropy parameters for 238 earthquake records at three of the four sites the results show a remarkably consistent fast direction parallel to the ridge axis the time delays between the fast and the slow waves range from 40 to 200 ms with an average of 90 ms they are not clearly related to earthquake range focal depth or source receiver azimuth the splitting of the shear waves is interpreted as an effect of structural anisotropy due to the presence of ridge parallel cracks in the shallow crust if we assume that anisotropy is concentrated in the upper 1 2 km the splitting times require a high crack density of 0 1

geomorphology of coal seam fires abstract coal fires occur in underground natural coal seams in exposed surface seams and in coal storage or waste piles the fires ignite through spontaneous combustion or natural or anthropogenic causes they are reported from china india usa south africa australia and russia as well as many other countries coal fires lead to loss of a valuable resource coal the emission of greenhouse relevant and toxic gases and vegetation deterioration a dangerous aspect of the fires is the threat to local mines industries and settlements through the volume loss underground surface collapse in coal fire areas is common thus coal fires are significantly affecting the evolution of the landscape based on more than a decade of experience with in situ mapping of coal fire areas worldwide a general classification system for coal fires is presented furthermore coal seam fire geomorphology is explained in detail the major landforms associated with and induced by these fires are presented the landforms include manifestations resulting from bedrock surface fracturing such as fissures cracks funnels vents and sponges further manifestations resulting from surface bedrock subsidence include sinkholes trenches depressions partial surface subsidence large surface subsidence and slides additional geomorphologic coal fire manifestations include exposed ash layers pyrometamorphic rocks and fumarolic minerals the origin evolution and possible future development of these features are explained and examples from in situ surveys as well as from high resolution satellite data analyses are presented the geomorphology of coal fires has not been presented in a systematic manner knowledge of coal fire geomorphology enables the detection of underground coal fires based on distinct surface manifestations furthermore it allows judgments about the safety of coal fire affected terrain additionally geomorphologic features are indicators of the burning stage of fires finally coal fire geomorphology helps to explain landscape features whose occurrence would otherwise not be understood although coal fire induced thermal anomalies and gas release are also indications of coal fire activity as addressed by many investigators no assessment is complete without sound geomorphologic mapping of the fire induced geomorphologic features

twelve years of geophysical research in the charlevoix seismic zone abstract various and extensive geophysical monitoring began in 1974 in the charlevoix seismic zone csz the historical record and a major earthquake in 1925 characterize this zone as the likely site of a future damaging earthquake i e m 5 5 or larger and the aim of the monitoring was to develop the capability of predicting such earthquakes the parameters studied included microseismicity seismic travel times electrical impedance vertical movement horizontal movement tilt gravity change and strain through water well level changes although no clear precursor was detected in the months before the largest seismic event that occurred during the observation period m 5 0 the multiparameter experiment provided new insights into the structure and the mechanics of this active region observations of several hundred well located microearthquakes have permitted the delineation of ancient rift faults that are now being activated by high horizontal compression the geographical extent of the zone is sharply defined by the microseismicity which occurs in an area of 35 km 80 km straddling the st lawrence river observation of p waves from accurately timed explosions and s waves from earthquakes support the hypothesis of extensive dilatancy anisotropy eda involving saturated parallel cracks in the crust the data have permitted an estimate of both crack density and the direction for the principal compressive stress in the region n 30 e changes in electrical parameters with periods of months to years were detected at five of the six magnetotelluric stations installed in the region changes in the polarization angle of the earth s electrical telluric field correlated well for seven years with calculated groundwater accumulation levels at one site electric impedance changes showed no clear correlation from station to station but changes at one station appear to correlate with a seismic p wave travel time change anomaly and with the disappearance of a negative gravity anomaly at the time of an unusual number of earthquakes in the range m 2 to 3 from mid 1976 to mid 1977 it is suggested that these correlated changes could be related to aseismic stress redistribution and fluid flow along one of the ne striking rift faults about two years before the m 5 0 earthquake in 1979 continuous monitoring of tilt surface and borehole and well water levels was carried out at a single site charlevoix geophysical observatory although there were unexplained near surface tilts of the order of the earth tide amplitude within two or three days before some earthquakes they were either not spatially coherent or were related to small unexplained temperature changes in the tiltmeter vault no intermediate term one week to one year precursors were seen against a noise level of 1 3 rad rms for near surface tilt observations and a noise level of 0 2 0 3 rad rms for borehole tilt measurements at a depth of 50 100 m the threshold of detectable tidal tilt admittance changes for the borehole tiltmeters was lowered to 2 but no significant changes were seen four coseismic changes in the levels of water in boreholes were observed all were drops in water level which were much larger than expected on the basis of elastic dislocation theory it is suggested that these could be explained either by the collapse of a dilatant zone around the respective hypocentres or by triggered aseisnuc movement on a nearby active fault borehole water level tides observed from 1980 to 1986 exhibit significant variations in amplitude and phase which are thought to be the result of time varying elastic properties in a nearby structure some constraints can now be placed on the long term deformation of the csz as a whole from tiltmeter levelling and horizontal surveys data from a levelling array observed from 1976 to 1986 puts a limit of 0 54 rad yr on the secular tilt rate at the observatory a further limit of 0 1 rad yr is provided by borehole tilt measurements over the period 1983 1986 a limit of 2 cm is placed on vertical movements northwest of the st lawrence river by levelling surveys in 1965 1978 1980 and 1982 no significant 90 confidence deviatoric strain has been detected across the csz from horizontal control surveys carried out in 1912 1919 1965 1978 and 1983 the maximum allowable long term strain rate across the csz from 1919 to 1983 is 0 16 strain yr

3d modeling of fracture induced shear wave splitting in the southern california basin splitting of shear waves generated by local earthquakes in the southern california basin and their polarization changes during propagation through layers with different orientations of anisotropy are investigated from multicomponent seismograms and their hodograms synthetic data for earthquakes in the southern california basin illustrate the effects on polarization caused by dry vertical microcracks with a crack density of 0 04 at the source and receiver locations hodograms for observed and synthetic data show that the polarization at the receiver depends only on the orientation of the anisotropy within two wavelengths of the receiver which qualitatively explains the frequency dependence of anisotropy in previous studies seismograms are simulated for a model with a range of station azimuths and distances if the anisotropy producing crack orientations do not change between the source and a receiver and the source motion is parallel to the cracks there is no shear wave splitting splitting occurs when the anisotropy orientation at the receiver or between the source and receiver is different from the source orientation

investigating dynamic underground coal fires by means of numerical simulation summary uncontrolled burning or smoldering of coal seams otherwise known as coal fires represents a worldwide natural hazard efficient application of fire fighting strategies and prevention of mining hazards require that the temporal evolution of fire propagation can be sufficiently precise predicted a promising approach for the investigation of the temporal evolution is the numerical simulation of involved physical and chemical processes in the context of the sino german research initiative innovative technologies for detection extinction and prevention of coal fires in north china a numerical model has been developed for simulating underground coal fires at large scales the objective of such modelling is to investigate observables like the fire propagation rate with respect to the thermal and hydraulic parameters of adjacent rock in the model hydraulic thermal and chemical processes are accounted for with the last process complemented by laboratory experiments numerically one key challenge in modelling coal fires is to circumvent the small time steps resulting from the resolution of fast reaction kinetics at high temperatures in our model this problem is solved by means of an operator splitting approach in which transport and reactive processes of oxygen are independently calculated at high temperatures operatorsplitting has the decisive advantage of allowing the global time step to be chosen according to oxygen transport so that time consuming simulation through the calculation of fast reaction kinetics is avoided also in this model because oxygen distribution within a coal fire has been shown to remain constant over long periods an additional extrapolation algorithm for the coal concentration has been applied in this paper we demonstrate that the operator splitting approach is particularly suitable for investigating the influence of hydraulic parameters of adjacent rocks on coal fire propagation a study shows that dynamic propagation strongly depends on permeability variations for the assumed model no fire exists for permeabilities k 10 10 m 2 whereas the fire propagation velocity ranges between 340 m a 1 for k 10 8 m 2 and drops to lower than 3 m a 1 for k 5 10 10 m 2 additionally strong temperature variations are observed for the permeability range 5 10 10 m 2 k 10 8 m 2

shear wave splitting above small earthquakes in the kinki district of japan abstract we have identified shear wave splitting diagnostic of the effective anisotropy induced by aligned microcracks in the wavetrains of micro earthquakes at four stations of the abuyama network in the kinki district of japan we find that the directions of polarization of the faster split shear waves are nearly parallel for all azimuths of arrival and for all angles of incidence less than the critical angle at three of the four stations these directions of polarization are consistent with the axis of maximum compression obtained from earthquake fault plane mechanisms and also agree with the directions of the general trends of geological structures which represent the orientations of the cleavage or lamination these results suggest that crack induced anisotropy is present in the brittle upper crust beneath japan as has been found elsewhere but we could not distinguish whether this reflects the distributions of cracks induced by the present stress field or results from the general trends of surface geology although the delay times between faster and slower shear waves are difficult to estimate reliably because of their high sensitivity to internal interfaces the delay times can be interpreted as the result of a distribution of parallel vertical cracks with a crack density of about 0 04 the consistency or lack of consistency of the directions of the shear wave polarizations at the four stations demonstrates the effects of surface topography and near surface layering on the shear wave polarizations

developing stress monitoring sites using cross hole seismology to stress forecast the times and magnitudes of future earthquakes abstract a new understanding of rockmass deformation suggests that changing stress in the crust modifies the geometry of the distributions of fluid saturated grain boundary cracks and pores pervading almost all rocks in the crust these stress aligned micro cracks cause the widely observed splitting of seismic shear waves which are sensitive to the details of the microcrack geometry this means that analysing shear wave splitting along appropriate ray paths above small earthquakes can monitor the build up of stress before large earthquakes this allowed the time and magnitude of an m 5 earthquake in iceland to be successfully stress forecast such forecasting using small earthquakes as the source of shear waves is possible only on those rare occasions when the comparatively severe restrictions on the geometry of source receiver and large earthquake allow shear wave splitting to be analysed along appropriate ray paths this paper suggests that in the absence of such pronounced local seismicity and optimum recording geometry the time and magnitude of future earthquakes can be estimated by analysing shear wave splitting in controlled source cross well seismology between three boreholes we suggest that stress forecasting at such stress monitoring sites smss is the best deterministic option for reliable forewarning of large earthquakes near any earthquake vulnerable township or vibration sensitive installation a preliminary stress monitoring site is currently being set up in northern iceland

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

shear wave polarizations near the north anatolian fault ii interpretation in terms of crack induced anisotropy summary the polarizations of shear waves recorded by networks of digital thxee component seismometers immediately above small earthquakes near the north anatolian fault in turkey display shear wave splitting on almost all shear wave seismograms recorded within the shear wave window this splitting is incompatible with source radiation patterns propagating through simple isotropic structures but is compatible with effective anisotropy of the internal structure of the rock along the ray paths this paper interprets the phenomena in terms of widespread crack induced anisotropy distributions of stress induced cracks model many features of the observations and synthetic polarization diagrams calculated for propagation through simulated cracked rock are similar to the observed patterns this evidence for widespread crack induced anisotropy lends strong support to the hypothesis of extensive dilatancy anisotropy eda suggested by laboratory experiments in subcritical crack growth the crucial evidence confirming some form of eda would be observations of temporal changes in shear wave splitting as the stress field alters the crack density and crack geometry there is some weak evidence for such temporal changes at one site but further analysis of suitable digital three component seismometer networks in seismic areas is required to confirm eda

theory of time dependent rupture in the earth at a velocity x which is a well defined function of k this propagation is stable and quasi static and is referred to as subcritical crack growth we shall use the terms stable quasistatic and subcritical to mean propagation at velocities much less than the sonic velocities of the medium this behavior results from stress induced corrosion at the crack tip the principal corrodent for the present application being h20 this behavior has been firmly established in the laboratory for mode i tensile cracks in a wide variety of materials ineluding silicates and silicate glasses see cholz 1968a 1972a martin 1972 wiederhorn and bolz 1970 atkinson 1979 lawn and wilshaw 1975 knott 1973 the form of the relationship between k and x does not vary significantly with the material only the parameters in the law vary as an example we show in figure 1 data on subcritical crack growth in quartz after atkinson 1979 the empirical relationship is found to take the form

the variation of delays in stress induced anisotropic polarization anomalies summary polarization anomalies in seismic shear wavetrains diagnostic of propagation through anisotropic media have now been observed in dilatancy zones in seismic regions stress induced dilatancy will open cracks with preferred orientations which will be effectively anisotropic to short period seismic waves the polarization anomalies are due to the shear waves splitting in propagation through anisotropic media into components with different polarizations and different velocities this writes characteristic signatures into the shear wavetrains the paper examines ways in which the differential shear wave anisotropy the delay between the split shear waves varies with direction by plotting stereograms of the relative delays and their polarizations for possible dilatancy symmetry systems it seems likely that if sufficient observations of these anomalies can be obtained at each stage of the dilatancy episode it will be possible to estimate the symmetry directions of the dilatancy and the geometry of the stress induced crack system as well as monitoring the progress of the dilatancy episode