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

a detailed seismic anisotropy study during the 2011 2012 unrest period in the santorini volcanic complex abstract the santorini volcanic complex svc is an area in the southern aegean greece which has been characterized by low seismicity rates for the last decades especially in the santorini caldera where they have been very low until 2010 this pattern changed completely in february 2011 when intense microseismic activity was initiated within the caldera during the manual analysis of the events the shear wave splitting phenomenon was observed revealing the existence of an anisotropic upper crust in the svc area a detailed anisotropy study has been conducted using 231 events within the shear wave window that fulfilled the selection criteria the polarization direction of the fast shear wave the time delay between the two split shear waves and the source polarization direction were calculated after visual inspection using both the polarigram and the hodogram representations this procedure applied for eight local stations resulted in the determination of 340 splitting parameters the obtained mean anisotropy directions are not homogeneous revealing a complex regime in the activated area nevertheless these results are explained by the ape model related to the stress sensitive behavior of fluid saturated microcracked rocks a detailed analysis of the temporal evolution of both the time delay and anisotropy direction was carried out the time delays measured in the band 1 window exhibit gradual increase and sudden drop that can be related to imminent bursts of seismicity as well as to the major m w 5 1 and 5 2 events which took place about 40 km sw of santorini on 26 and 27 january 2012 respectively on the other hand no significant temporal variations or 90 flips of the s fast polarization direction were observed

anisotropy and transverse isotropy a number of authors in the exploration literature have written about anisotropy but have restricted their discussions to wave propagation through rock having transverse isotropy with a vertical symmetry axis this note shows that there are fundamental differences between transverse isotropy when the symmetry axis is vertical normal to the free surface and more general anisotropy with an azimuthal variation of properties these differences are important now that effective azimuthal shear wave anisotropy resulting from aligned cracks and pores is becoming recognized as a significant property of crustal rocks

prediction of lateral variability in fracture intensity using multicomponent shear wave surface seismic as a precursor to horizontal drilling in the austin chalk summary a method of multicomponent multisource shear wave interpretation utilizing the output of the alford rotation for separating the fast and slow particle motion stacks in azimuthally anisotropic media yielded a qualitative prediction of lateral fracture intensity variation prior to horizontal drilling the method exploits a fundamental difference between the two particle motion stacks when vertical aligned fractures are the source of the shear wave splitting the fast shear wave s1 section is insensitive to the lateral variation in fracture intensity since its particle motion is parallel to the fractures hence reflectivity on the s1 cdp stacked section should be likewise insensitive to fractures the slow shear wave s2 section however possesses particle motion perpendicular to the fracture planes so variation in lateral fracture intensity will affect velocity and reflectivity examination of the reflectivity on the two sections for a seismic line reveals laterally consistent reflector strength on the s1 section for the austin chalk the horizon of interest but laterally inconsistent reflector strength for the same horizon on the s2 section in the study area reflector dimming on the s2 section was interpreted as indicating fracture intensity maxima horizontal drilling confirmed both fracture azimuth and intensity predictions and resulted in significantly higher initial hydrocarbon production than for surrounding wells

regional gps data confirm high strain accumulation prior to the 2000 june 4 mw 7 8 earthquake at southeast sumatra summary site velocities derived from repeated measurements in a regional gps network in southeast asia help to constrain the motion of tectonic blocks as well as slip rates along major faults in the area using 3 d forward dislocation modelling the influence of seismic elastic loading and unloading on the measured site motions are approximated results suggest that the northwestern sunda arc is fully coupled seismogenically whereas its eastern part along java shows localized deformation higher horizontal velocity gradients than expected from the modelling of a fully coupled plate interface west of manila in the philippines suggest that deformation may be localized there assuming that geodetically derived convergence represents long term rates accumulated geodetic moments are compared to those derived using seismic data from 1977 to 2000 harvard cmt catalogue if areas displaying localized deformation are dominated by creep processes the largest difference between accumulated and seismically released deformation is located where the 2000 june 4 mw 7 8 sumatra earthquake occurred

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

estimating crustal seismic anisotropy with a joint analysis of radial and transverse receiver function data summary we developed an integrated technique for estimating crustal anisotropy with a horizontal axis using radial r and transverse t receiver functions the technique includes computing three individual and one joint objective function jof and a reliability analysis of the estimated anisotropy the individual objective functions iofs are designed to 1 maximize the peak energy of the stacked r receiver function after a cosine moveout correction in the ps arrival time 2 to maximize the correlation of the radial receiver functions after a full correction of anisotropy or 3 to minimize the total energy of transverse receiver functions stacked after a removal of crustal anisotropy the jof was computed by a weighted average of the three iofs and the reliability analysis uses the principle that stacking coherent signals can lead to an increase of signal to noise ratio we applied the technique to synthetic receiver functions generated with 30 60 per cent white noise from a variety of anisotropic and heterogeneous models the synthetic tests indicate that the proposed technique has good capability to recover the input models despite the presence of random and other coherent noises such as those caused by inhomogeneous structures in the data the technique can always provide accurate estimates of crustal anisotropy we applied the technique to two permanent seismic stations in western china and found significant crustal anisotropy beneath one station located at the northern edge of the tibetan plateau the observed fast polarization direction at this station follows the direction of the maximum horizontal tensile stress suggesting that the observed seismic anisotropy is likely caused by mineral alignment in the lower crust the station situated in the sichuan basin on the other hand shows little to no seismic anisotropy which may suggest that the crust beneath the basin is nearly rigid with very little deformation the developed technique can be applied to any broadband seismic stations that have a good backazimuthal coverage of teleseismic events

modelling the anisotropic seismic properties of partially molten rocks found at mid ocean ridges abstract the problem of modelling the seismic properties of mid ocean ridge rocks of the axial magma chamber amc and the low velocity triangle lvt immediately below it has been addressed using two samples from oman ophiolite as examples the specimens are a layered gabbro from the lower oceanic crustal sequence and a harzburgite from the upper most mantle section at the palaeo ridge axis the seismic properties have been simulated at a temperature of 1200 c and pressure of 200 mpa so that the basalt melt is above its solidus various effective medium methods are discussed in the perspective of modelling rocks that have a strong background elastic anisotropy due to crystal preferred orientation cpo and an introduced anisotropy due to oriented melt filled inclusions calculations using various methods show a wide range of predicted melt fractions for a given seismic velocity the self consistent scheme scs and differential effective medium dem are compared in some detail a tensorial model was developed using a poro elastic method of gassman at low seismic frequency and a standard dem with isolated basalt inclusions at high frequency at low frequency the basalt pore fluid is considered to be everywhere connected and the pressure uniform whereas at high frequency basalt inclusions are isolated assuming the medium to be a standard linear solid the low and high frequency velocities were used to calculate the anisotropy of attenuation calculations with spherical basalt inclusions show that the seismic velocities decrease and attenuation increase with increasing melt fraction the symmetry of the background anisotropy due to cpo is preserved but gradually reduced with increasing melt fraction for the seismic velocities observed for the axial magma chamber amc with v p of 3 5 to 4 7 km s the model predicts 50 70 melt for the low velocity triangle lvt below the amc with v p of 4 8 5 6 km s the model predicts 35 50 melt predictions from the observed attenuations q 1 at the amc of 0 05 0 02 and in the lvt of 0 02 0 01 are around 60 and 45 melt respectively seismic anisotropy has been modelled with ellipsoidal basalt pancake shaped inclusions with their circular sections in the xy foliation plane only a 2 3 of ellipsoidal basalt inclusions are required to over print the anisotropy of the background medium there is a rapid decrease in v p and increase of q 1 for propagation normal to the foliation z with increasing axial ratio of the inclusions with increasing axial ratio the model predicts decreasing amounts of melt for given velocity v p in the z direction for the lvt only 15 25 melt is predicted for a 10 1 inclusion instead of the 35 50 of the spherical inclusion

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

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

calculable fluid rock interactions this paper introduces a model of rock deformation anisotropic poro elasticity or ape where the response of fluid saturated rock to changing conditions prior to fracturing can be calculated the driving mechanism for deformation is fluid migration along pressure gradients between neighbouring intergranular microcracks and pores at different orientations to the stress field the parameters that control changes to microcrack geometry also control the splitting birefringence of seismic shear waves so that changes in deformation can be directly monitored by analysing the shear wave splitting which is observed in almost all rocks analysis of shear wave splitting in the earth9s crust and ape modelling show that distributions of mostly intergranular cracks in the crust are always geometrically close to fracturing with the implication that shear wave splitting is sensitive to comparatively minor changes of stress and minor changes of in situ conditions this has important implications for the state of criticality of the rockmass and as a consequence changes in shear wave splitting have been observed before larger earthquakes on those few occasions when suitable source receiver geometry coincides with appropriate seismic activity ape also has implications for monitoring changing conditions in reservoirs during hydrocarbon recovery

seismic detection of a hydraulic fracture from shear wave vsp data at lost hills field california a shear wave s wave vsp experiment was performed at lost hills field california in an attempt to detect hydraulic fractures induced in a nearby well the hydrofrac well was located between an impulsive s wave source on the surface and a receiver well containing a clamped three component geophone both direct and scattered waves were detected immediately after shut in when the hydraulic pumps were shut off and recording started the scattered energy disappeared within about an hour which is consistent with other measurements that indicate some degree of fracture closure and leak off within that period although s wave splitting was evident no change was detected in the fast wave polarized parallel to the fracture however the slow wave polarized perpendicular to the fracture did change over a period of about an hour after which the prehydrofrac wavelet shape was recovered the fact that only the wave polarized perpendicular to the fracture was affected is a dramatic confirmation of both theoretical predictions and laboratory observations of s wave behavior in a fractured medium subtracting the prehydrofrac wavelet from the wavelets recorded within the first hour after shut in revealed scattered wavelets that were diminished and phase rotated versions of the incident prehydrofrac wavelet arrival times of the direct and scattered waves were matched by ray tracing we accounted for the scattered wave amplitudes by using numerical solutions of s wave diffractions off of ribbon shaped fractures amplitudes derived from full wavefield born scattering however did not match recorded amplitudes the phase of the scattered wavelets was matched very well by born scattering when the incident wavelet was input but only for fracture lengths no larger than half those predicted from fracture simulator models these results show that a carefully controlled experiment combined with accurate modeling can provide important information about the geometry of induced fractures

depth variations of azimuthal anisotropy beneath the tian shan mt range nw china from ambient noise tomography abstract seismic anisotropy beneath the tian shan mt range has been well studied with shear wave splitting the origin of asthenospheric anisotropy is mainly attributed to the north south shortening however the crustal anisotropy and its depth variation are poorly constrained in this study we focus on inversion of rayleigh wave phase velocity azimuthal anisotropy in the 5 35 s period bands beneath the tian shan mt range through analysis of continuous seismic ambient noise we use approximately 7 months of available continuous seismic data recorded by 31 broadband seismic stations deployed in the tian shan mt range the tomographic results reveal substantial variations in the azimuthal anisotropy with an increase in the depth in the upper crustal depths the orogen parallel or orogen subparallel azimuthal anisotropy observed in the tian shan mt range likely originates from the lattice preferred orientation of crustal minerals in response to compression whereas the orogen perpendicular or orogen sub perpendicular azimuthal anisotropy in the southern and northern piedmont of the tian shan mt range is related to the stress alignment of cracks fractures due to ongoing north south shortening the pattern of azimuthal anisotropy in the middle to lower crustal and upper most mantle is different from that at the upper crustal depths an orogen perpendicular or orogen sub perpendicular fast direction associated with prominent lower velocities has been observed north and south of lake issyk kul the spatial correlation between the azimuthal anisotropy and the lower velocity anomalies at the middle to lower crustal and upper most mantle depths may suggest a warm melt rich upper mantle below the tian shan mt range which could be attributed to the upwelling asthenospheric mantle flow in that region

modelling the compliance of crustal rock ii response to temporal changes before earthquakes summary there have been several claims that seismic shear waves respond to changes in stress before earthquakes the companion paper develops a stress sensitive model ape for the behaviour of low porosity low permeability crystalline rocks containing pervasive distributions of fluid filled intergranular microcracks and this paper uses ape to model the behaviour before earthquakes modelling with ape shows that the microgeometry and statistics of distributions of such fluid filled microcracks respond almost immediately to changes in stress and that the behaviour can be monitored by analysing seismic shear wave splitting the physical reasons for the coupling between shear wave splitting and differential stress are discussed in this paper we extend the model by using percolation theory to show that large crack densities are limited at the grain scale level by the percolation threshold at which interacting crack clusters lead to pronounced increases in rock matrix permeability in the simplest formulation the modelling is dimensionless and almost entirely constrained without free parameters nevertheless ape modelling of the evolution of fluid saturated rocks under stress reproduces the observed fracture criticality and the narrow range of shear wave azimuthal anisotropy in crustal rocks it also reproduces the behaviour of temporal variations in shear wave splitting observed before and after the 1986 m 6 north palm springs earthquake southern california and several other smaller earthquakes the agreement of ape modelling with a wide range of observations confirms that fluid saturated crystalline rocks are stress sensitive and respond to changes in stress by critical fluid rock interactions at the microscale level this means that the effects of changes in stress and other parameters can be numerically modelled and monitored by appropriate observations of seismic shear waves

an alternative scenario for earthquake prediction experiments b current earthquake prediction experiments investigate behaviour before potentially large earthquakes there is some evidence that comparatively small typical events in isolated swarms of earthquakes may have precursory behaviour similar to that before much larger earthquakes in more complicated areas of seismicity such typical events in isolated swarms frequently recur with similar magnitudes in similar locations with repetition times sometimes as little as a few days it is argued that monitoring such typical events in isolated swarms could be an effective way to gain experience of precursory activity and might well be a good guide for investigations of precursors to large earthquakes

indication of high pore fluid pressures in a seismically active fault zone summary polarisations of seismic shear wave splitting observed above small earthquakes in iceland are typically approximately ne to sw parallel to the direction of maximum horizontal stress in contrast the polarisations of shear waves at three new stations sited over the husav ik flatey fault a major seismically active transform fault in northern iceland are approximately nw to se orthogonal to the stress aligned polarisations elsewhere modelling suggests that these 90 flips in polarisations are caused by propagation through cracks containing fluids at high pore fluid pressures within one or two mpa of the critical stress these observations suggest that high pore fluid pressures which play a key role in earthquake source mechanisms can be monitored by analysing shear wave splitting above seismically active fault planes

are earthquakes predictable summary the answer to the above question depends on the definition of earthquake prediction we discuss several definitions and possible classifications of earthquake prediction methods we also consider various measures of prediction efficiency review several recent examples of earthquake prediction and describe the methods that can be used to verify prediction schemes we conclude that an empirical search for earthquake precursors that forecast the size of an impending earthquake has been fruitless despite considerable effort in several countries no statistically rigorous validation of proposed precursory phenomena is available therefore reported cases of precursors can be explained by random noise or by chance coincidence we present evidence that earthquakes are non linear chaotic scale invariant phenomena the most probable consequence of earthquake self similarity is a lack of earthquake predictability as popularly defined that is a forecast of a specific individual earthquake many small earthquakes occur throughout any seismic zone demonstrating that the critical conditions for earthquake nucleation are satisfied almost everywhere apparently any small shock can grow into a large event thus it is likely that an earthquake has no preparatory stage this sceptical view of current earthquake prediction efforts should not be interpreted as a statement that any further attempts to mitigate the destructive effects of earthquakes are futile the seismic moment conservation principle when combined with geodetic deformation data offers a new way to evaluate the seismic hazard not only for tectonic plate boundaries but also for areas of low seismicity that is the interiors of continents earthquake clustering with a power law temporal decay omori s law can be used to estimate the rate of future earthquake ocurrence realtime seismology can facilitate relief efforts after large earthquakes and eventually provide an immediate warning of severe shaking a few seconds or tens of seconds before the shaking starts

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

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

fluid overpressure and stress drop in fault zones using the average aspect length maximum thickness ratios of 379 mineral filled extension mode i veins from an active fault zone the fluid overpressure during their development with reference to the minimum compressive principal stress 3 is estimated at 20 mpa emplacement of such veins increases 3 and can generate a temporary stress barrier to the propagation of subsequent hydrofractures on meeting a subhorizontal stress barrier vertically propagating hydrofractures may change into water sills where the fluid pressure is at or above lithostatic in this model stress barriers and thus water sills can form at any depth in and in any type of fault zones for such a high fluid pressure the product of the coefficient of sliding friction and the normal stress in the modified griffith criterion becomes essentially zero and the driving stress associated with faulting equal to twice the in situ tensile strength of the host rock for typical in situ tensile strengths of 2 3 mpa the driving stresses for slip on overpressured fault planes is 4 6 mpa these results are in good agreement with the commonly measured average static stress drops of 3 6 mpa during earthquakes

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 north pacific oscillation observations compared with simulations in a general circulation model the study of the sea level pressure and the surface air temperatures in the north pacific region in a 23 year integration of the oregon state university coupled ocean atmosphere general circulation model has shown that there is a signature of the north pacific oscillation in the model comparable to that recorded in observations in the model the npo index was defined in terms of an opposition in sign of mean temperature anomaly between two regions in the north pacific region close to those found in observations viz dutch harbor alaska and edmonton canada fluctuations of this index prompted the grouping of some of the 23 model years as those when the temperature at the model equivalent of dutch harbor was above normal and that at the model equivalent of edmonton below normal analogous to aleutian above in observations while other years displayed the opposite scenario aleutian below composites of sea level pressure surface air temperatures precipitation and sea surface temperatures in the northern hemisphere were found to be at times strikingly similar to those documented in observations by rogers

comment on quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction by richard c aster peter m shearer and jon berger

earthquake prediction a critical review summary earthquake prediction research has been conducted for over 100 years with no obvious successes claims of breakthroughs have failed to withstand scrutiny extensive searches have failed to find reliable precursors theoretical work suggests that faulting is a non linear process which is highly sensitive to unmeasurably fine details of the state of the earth in a large volume not just in the immediate vicinity of the hypocentre any small earthquake thus has some probability of cascading into a large event reliable issuing of alarms of imminent large earthquakes appears to be effectively impossible key word earthquake prediction

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

observations of stress relaxation before earthquakes summary theory and observations suggest that seismic shear wave splitting caused by fluid saturated stress aligned microcracks directly monitors low level deformation before fracturing faulting and earthquakes occur in the past it had been assumed that the accumulation of stress before earthquakes continued until stress was released by faulting at the time of the earthquake however new data and reappraisal of existing data sets now suggests that the stress begins to relax and cracks close from tens of minutes to months before the earthquake actually occurs with the logarithm of the duration of the relaxation proportional to the magnitude of the impending earthquake the duration of the relaxation appears to be directly correlated with earthquake magnitude and may have implications for the earthquake source process and the ability to predict earthquakes

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

the scatter of time delays in shear wave splitting above small earthquakes summary measurements of time delays in seismic shear wave splitting above small earthquakes typically display a scatter of often as much as 80 per cent about the mean changes in the average timedelay appear to be related to changes of stress but applications of this potentially powerful tool have been handicapped by the previously inexplicable scatter in time delays above earthquakes in contrast measurements of shear wave time delays in controlled source exploration seismics are typically well controlled and display little scatter previous estimates of possible causes of scatter cannot produce sufficient variation specifically above earthquakes here we show that 90 flips in shear wave polarizations due to fluctuating high pore fluid pressures on seismicallyactive fault planes are the most likely cause of the scatter

variations in near global sea level pressure abstract analysis of a near global sea level pressure slp data set for the period 1951 80 was carried out in terms of complex empirical orthogonal function a large scale propagating slp signal was discovered that seems to include variability previously associated with the southern oscillation monsoons and tropical midlatitude teleconnection patterns in this sense the new feature offers an empirical framework for many of the well known features of the global climate system extensive comparisons show the feature to be observable in the raw data field and thus not a figment of the analysis technique the large slp signal may have its origins in a slp snow feedback loop that acts over the region from siberia to india it is suggested that the maintenance of the slp signal involves slp precipation feedbacks and more importantly the excitation of a major natural mode of climate variation the slp signal is only excited at time scales larger than about 18 mouths i e time scales characteristic of ocea

rock as a critical point system and the inherent implausibility of reliable earthquake prediction summary the statistical mechanics of critical point thermodynamic systems provide a powerful conceptual tool with which to examine the plausibility of reliable earthquake prediction the issue turns on how rock is heterogeneous if the correlation range of a spatially fluctuating rock property is bounded by a characteristic length then the spatial average of that property over dimensions greater than may be regarded as defining an effective continuum value for that property ideas about earthquake preparation in rock with an effective continuum mechanical strength can plausibly focus on failure that is initiated at small anomalous zones and grows to instability with access to a continuous volume of strain energy the dimensions and temporal evolution of the growing anomaly can then be expected to relate to the size location and timing of the impending seismic event borehole log evidence indicates however that rock heterogeneity correlation lengths are unbounded and hence that the effective continuum approximation for rock must be applied with discretion an infinite correlation length indicates a statistical mechanical critical point system in which the correlation length is related to a singularity in a physical property in thermodynamic critical systems the singularity occurs at the critical system temperature tc 1 t tc in rock the singularity occurs at the critical percolation probability pc 1 p pc statistical physics descriptions of critical point spatial correlations are based on nearest neighbour inter actions at every scale length nearest neighbour interactions at one scale say that of large failure structures the loci of large earthquakes involve minimal coupling to structures at other scales in particular small scale failure structures the loci of small earthquakes the mechanics of nearest neighbour interactions plausibly form a fundamental constraint on the role of small scale failure or related nucleating activity as a prelude to large scale failure in light of 1 the evidence against rock being an effective medium notionally supporting localized precursory activity that reliably grows to large scale mechanical failure and 2 the history of failure to identify reliable precursors to major earthquakes the onus should now be on advocates of earthquake prediction schemes to evaluate their claims for precursory activity in terms of rock as a critical point system

simulation of the 14 month chandler wobble in a global climate model the agent that generates and maintains the 14 month chandler wobble of the solid earth about its rotation axis has remained unresolved for a century with first the atmosphere later earthquakes and more recently the earth s fluid core proposed as candidates here we report that surface air pressure calculated in a coupled ocean atmosphere general circulation model gcm displays a 14 7 month signal whose amplitude is similar to that found by maksimov 1960 in station data we identify it as the atmospheric chandler wobble this result indicates that changes in atmospheric mass distribution excite and maintain the wobble of the solid earth and that neither earthquakes nor the fluid core are significant contributors another result is that in the gcm the amplitude of the wobble at high latitudes is a substantial fraction of the annual cycle and thus is an important factor in climate formation as maksimov 1960 suggested

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

a successfully stress forecast earthquake summary a m 5 earthquake in iceland has been successfully stress forecast by using variations in time delays of seismic shear wave splitting to assess the time and magnitude at which stress modified microcracking reaches fracture criticality within the stressed volume where strain is released local investigations suggested the approximate location of the forecast earthquake we report the criteria on which this stress forecast was based

shear wave anisotropy spatial and temporal variations in time delays at parkfield central california summary shear wave splitting is analysed on data recorded by the high resolution seismic network hrsn at parkfield on the san andreas fault central california during the three year period 1988 1990 shear wave polarizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress at some 70 to the fault strike whereas at station mm in the immediate fault zone shear wave polarizations are aligned approximately parallel to the fault normalized time delays at this station are found to be about twice as large as those in the rock mass either side this suggests that fluid filled cracks and fractures within the fault zone are elastically or seismically different from those in the surrounding rocks and that the alignment of fault parallel shear wave polarizations are associated with some fault specific phenomenon temporal variations in time delays between the two split shear waves before and after a ml 4 earthquake can be identified at two stations with sufficient data mm within the fault zone and vc outside the immediate fault zone time delays between faster and slower split shear waves increase before the ml 4 earthquake and decrease near the time of the event the temporal variations are statistically significant at 68 per cent confidence levels earthquake doublets and multiplets also show similar temporal variations consistent with those predicted by anisotropic poroelasticity theory for stress modifications to the microcrack geometry pervading the rock mass this study is broadly consistent with the behaviour observed before three other earthquakes suggesting that the build up of stress before earthquakes may be monitored and interpreted by the analysis of shear wave splitting

main features of foraminiferal distribution reflecting estuarine hydrography in oslo fjord abstract foraminiferal assemblages have been analysed in sediment surface samples located along three profiles covering the depth interval 1 120 m in breidangen a wide basin forming the middle portion of oslo fjord the study concentrates on the distribution of major faunal features including suborders and diversities number of species shannon wiener index equitability in addition species distribution is recorded and q mode factor analysis has distinguished six assemblages measurements of hydrographical properties temperature salinity dissolved oxygen reveal the estuarine stratification of the water column which is reflected also by the faunal parameters on the basis of these features the following subdivision is proposed with increasing depth 1 seasonal layer dominated by calcareous species in the upper and agglutinated species in the lower part with low to intermediate diversities 2 transitional layer characterized by a maximum in diversity and a reduction in the agglutinated component 3 stable deeper water showing high diversity and a further decrease in the agglutinated component the generally higher frequency of calcareous foraminifera below the seasonal water layer on the eastern side of breidangen compared to the western side is mainly attributed to the influence of inflowing skagerrak water additionally it may be a result of increased supply of organic matter to the western side increasing the oxygen demand in this area

a four year study of shear wave splitting in iceland 1 background and preliminary analysis this is part 1 of a report of a four year study of seismic shear wave splitting in iceland designed to seek temporal variations before earthquakes shearwave splitting is observed routinely in iceland whenever shear waves arrive within the shear wave window of seismic stations and whenever adequate data are available temporal and spatial variations in shear wave splitting are observed before both earthquakes and volcanic eruptions shear wave splitting is caused principally by the stress aligned fluid saturated microcracks and pore throats in almost all in situ rocks fluid saturated microcracks are the most compliant elements of the rock mass and changes in splitting can be directly interpreted and modelled as the effects of changing stress on the microcrack geometry in the rock mass often at considerable distances from the immediate earthquake source zone such changes were found and are reported in paper 2 this paper presents the background preliminary observations and analysis of shear wave splitting in iceland

on the cause of 1 f power spectral scaling in borehole sonic logs borehole logs provide in situ information about the scaling of fluctuations in the elastic properties of rock a surprising finding has been the widespread 1 f type 0 5 1 5 scaling of borehole sonic log power spectra yielding no clear correlation with geological age type or history raising questions about the reliability and origin of these findings ubiquitous fractures for example have been evoked as the most likely explanation of these results firstly we show that the smoothing effects of the logging tool must be taken into account in order to obtain reliable values of we then investigate the scaling properties of lithology only in perfectly exposed metamorphic rock by explicitly excluding fractures we find clear 1 f scaling for reconstructed velocity data based solely on lithological variations although fractures are certainly important in contributing to 1 f fluctuations in borehole logs intrinsic geology may also play a significant role

processing modelling and predicting time lapse effects of overpressured fluid injection in a fractured reservoir summary time lapse seismology is important for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs we analyse two 4 d 3c onshore surveys from vacuum field new mexico usa where the reservoir of interest is a fractured dolomite in phase vi a time lapse survey was acquired before and after a pilot tertiary recovery programme of overpressured co2 injection which altered the fluid composition and the pore fluid pressure phase vii was a similar time lapse survey in the same location but with a different lowerpressure injection regime applying a processing sequence to the phase vi data preserving normal incidence shearwave anisotropy time delays and polarization and maximizing repeatability interval time analysis of the reservoir interval shows a significant 10 per cent change in shear wave velocity anisotropy and 3 per cent decrease in the p wave interval velocities a 1 d model incorporating both saturation and pressure changes is matched to the data the saturation changes have little effect on the seismic velocities there are two main causes of the time lapse changes any change in pore fluid pressures modifies crack aspect ratios additionally when there are overpressures as there are in phase vi there is a 90 change in maximum impedance directions and the leading faster split shear wave instead of being parallel to the crack face as it is for low pore fluid pressures becomes orthogonal to the crack face the anisotropic poro elasticity ape model of the evolution of microcracked rock calculates the evolution of cracked rock to changing conditions ape modelling shows that at high overburden pressures only nearly vertical cracks to which normal incidence p waves are less sensitive than s waves remain open as the pore fluid pressure increases ape modelling matches the observed time lapse effects almost exactly demonstrating that shear wave anisotropy is a highly sensitive diagnostic of pore fluid pressure changes in fractured reservoirs in this comparatively limited analysis ape modelling of fluid injection at known pressure correctly predicted the changes in seismic response particularly the shear wave splitting induced by the high pressure co2 injection in the phase vii survey ape modelling also successfully predicted the response to the lower pressure injection using the same phase vi model of the cracked reservoir the underlying reason for this remarkable predictability of fluid saturated reservoir rocks is the critical nature and high crack density of the fluid saturated cracks and microcracks in the reservoir rock which makes cracked reservoirs critical systems

temporal changes in shear wave splitting at an isolated swarm of small earthquakes in 1992 near dongfang hainan island southern china summary an isolated swarm of small earthquakes occurred in 1992 near dongfang on hainan island southern china the institute of geophysics state seismological bureau of china monitored the swarm with five dcs 302 digital accelerometers for three months from 1992 june 1 18 earthquakes with magnitudes m l ranging from 1 8 to 3 6 were well located by five stations and shear wave splitting varying azimuthally was analysed on 27 seismic records from these events the mean polarization azimuth of the faster shear wave was wnw time delays between the split shear waves at two stations varied with time and space the time delays at one station fell abruptly after earthquakes of magnitudes 3 1 and 3 6 but did not change significantly at the second station this behaviour is consistent with the delay time changes being caused by changes in the aspect ratio of vertical liquid filled eda cracks thus the variation in shear wavesplitting time delay could be due to changes in crustal stress related to nearby smallmagnitude earthquake activity the connection between earthquake activity and crustal stress variation measured by shear wave splitting leaves the door open for possible observations of crustal stress transients related to the onset of an earthquake however our data cannot be considered as definite evidence for such precursors

seismic anisotropy the state of the art ii summary the theory causes observations and possible applications of seismic anisotropy in the earth have developed considerably since the previous state of the art paper was published in 1977 the behaviour of waves in layered anisotropic media is now much better understood and the evidence for seismic anisotropy indicates that anisotropy is likely to be present throughout much of the crust and upper mantle the top few hundred kilometres of the mantle appears to be anisotropic with the orientations aligned by the present or palaeo stress field the upper part of the crust is frequently anisotropic probably due to cracks differentially aligned by the non lithostatic stresses the possibility of being able to monitor crack geometry by seismic techniques opens a wide range of applications in currently important activities

using shear waves to predict lateral variability in vertical fracture intensity during the last five years the use of shear wave splitting in geophysical exploration has made the transition from research project to very promising field technique the measurement of the azimuth of the fast shear wave and analysis of the s wave splitting or birefringence are now commonplace furthermore a very timely practical application is suggested by the recent proposal that reflectivity on post rotation s1 and s2 data sets should indicate lateral variation of anisotropy this could be of great value because the determination of lateral variation in fracture intensity in a vertically fractured sedimentary layer is necessary to insure high efficiency in a horizontal drilling program designed to drain the fractures

the orbital vibrator a new tool for characterizing interwell reservoir space beginning in the early 1980s conoco spent over 10 years developing new seismic methodologies based upon deliberate generation of circularly and elliptically polarized shear waves at the earth s surface and in the wellbore those efforts were successful orbital vibrator technology is now available to assist understanding reservoir geology between wells and further promising research is underway at several locations

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

three dimensional structure of low frequency pressure variations in the tropical atmosphere abstract results of a recent study show eastward propagation of information in the low frequency variations of the tropical sea level pressure slp field the current work extends that analysis to investigate the vertical structure of this signal it is found that the propagating signal exists to a height of at least 850 mb at 500 mb the signal is not so clear while at 200 mb there is little evidence of propagation of information instead the low frequency variations in 200 rnb height appear coherent for the tropical belt around the entire globe the analysis suggests that the anomalies discussed here appear first at the surface and later at higher levels in the atmosphere

stress transfer in earthquakes hazard estimation and ensemble forecasting inferences from numerical simulations abstract observations indicate that earthquake faults occur in topologically complex multi scale networks driven by plate tectonic forces we present realistic numerical simulations involving data mining pattern recognition theoretical analyses and ensemble forecasting techniques to understand how the observable space time earthquake patterns are related to the fundamentally inaccessible and unobservable dynamics numerical simulations can also help us to understand how the different scales involved in earthquake physics interact and influence the resulting dynamics our simulations indicate that elastic interactions stress transfer combined with the nonlinearity in the frictional failure threshold law lead to the self organization of the statistical dynamics producing 1 statistical distributions for magnitudes and frequencies of earthquakes that have characteristics similar to those possessed by the gutenberg richter magnitude frequency distributions observed in nature and 2 clear examples of stress transfer among fault activity described by stress shadows in which an earthquake on one group of faults reduces the coulomb failure stress on other faults thereby delaying activity on those faults in this paper we describe the current state of modeling and simulation efforts for virtual california a model for all the major active strike slip faults in california noting that the working group on california earthquake probabilities wgcep uses statistical distributions to produce earthquake forecast probabilities we demonstrate that virtual california provides a powerful tool for testing the applicability and reliability of the wgcep statistical methods furthermore we show how the simulations can be used to develop statistical earthquake forecasting techniques that are complementary to the methods used by the wgcep but improve upon those methods in a number of important ways in doing so we distinguish between the official forecasts of the wgcep and the research quality forecasts that we discuss here finally we provide a brief discussion of future problems and issues related to the development of ensemble earthquake hazard estimation and forecasting techniques

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

a review of shear wave splitting in the crack critical crust summary over the last 15 years it has become established that crack induced stress aligned shear wave splitting with azimuthal anisotropy is an inherent characteristic of almost all rocks in the crust this means that most in situ rocks are pervaded by fluid saturated microcracks and consequently are highly compliant the evolution of such stress aligned fluid saturated grain boundary cracks and pore throats in response to changing conditions can be calculated in some cases with great accuracy using anisotropic poro elasticity ape ape is tightly constrained with no free parameters yet dynamic modelling with ape currently matches a wide range of phenomena concerning anisotropy stress shear waves and cracks in particular ape has allowed the anisotropic response of a reservoir to injection to be calculated predicted with hindsight and the time and magnitude of an earthquake to be correctly stress forecast the reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems this crack critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre fracturing deformation of the crust and for solid earth geophysics and geology we review past present and speculate about the future of shear wave splitting in the crack critical crust shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass there is some good news and some bad news for conventional geophysics many accepted phenomena are no longer valid at high spatial and temporal resolution a major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes however at least in some circumstances the behaviour of the rock in the highly complex inhomogeneous earth may be calculated and the response predicted opening the way to possible control by feedback the need is to devise ways to exploit these new opportunities in the crack critical crust recent observations from the smsites project at husavik in northern iceland gathered while this review was being written display the extraordinarily sensitivity of in situ rock to small changes at great distances the effects are far too large to occur in a conventional elastic brittle crust and their presence confirms the highly compliant nature of the crack critical crust

changes in shear wave splitting at anza near the time of the north palm springs earthquake changes in shear wave splitting are observed at knw before and after the m 6 north palm springs earthquake of july 8 1986 knw is a station of the anza seismic network monitoring the anza seismic gap on the san jacinto fault southern california the gradual increase in the delays between the split shear waves over 3 years at knw reported by peacock et al 1988 ended in june 1986 the further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the north palm springs earthquake 33 km north of knw peacock et al demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress aligned fluid filled inclusions and speculated that this increase might be the result of a buildup of stress before an impending earthquake the new data appear to confirm this speculation but the temporal variations require a more complex interpretation although they still suggest that the changes in shear wave splitting are caused by earthquake induced stress changes to the fluid filled inclusions throughout the rockmass central to interpretation of temporal changes in shear wave splitting is the well established existence throughout at leastmore the uppermost 10 to 20 km of the crust of small fluid filled cracks microcracks and pores the existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock the authors term these distributions of inclusions extensive dilatancy anisotropy or eda and the individual inclusions eda cracks because although they may include a wide range of shapes many of the seismic properties can be simulated by distributions of thin parallel cracks less