a review of shear wave splitting in the crack-critical crust · september 8, 2003 15:14...

September 8, 2003 15:14 Geophysical Journal International gji˙2037

Geophys. J. Int. (2003) 155, 221–240

A review of shear wave splitting in the crack-critical crust

Stuart Crampin∗ and Sebastien ChastinShear-Wave Analysis Group, Department of Geology and Geophysics, University of Edinburgh, West Mains, Edinburgh EH9 3JW.E-mails: [email protected]; [email protected]

Accepted 2003 May 14. Received 2003 May 8; in original form 2002 September 23

S U M M A R YOver the last 15 years, it has become established that crack-induced stress-aligned shear wavesplitting, with azimuthal anisotropy, is an inherent characteristic of almost all rocks in thecrust. This means that most in situ rocks are pervaded by fluid-saturated microcracks andconsequently 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 somecases with great accuracy, using anisotropic poro-elasticity (APE). APE is tightly constrainedwith no free parameters, yet dynamic modelling with APE currently matches a wide rangeof phenomena concerning anisotropy, stress, shear waves and cracks. In particular, APE hasallowed the anisotropic response of a reservoir to injection to be calculated (predicted withhindsight), and the time and magnitude of an earthquake to be correctly stress-forecast. Thereason for this calculability and predictability is that the microcracks in the crust are so closelyspaced that they form critical systems. This crack-critical crust leads to a new style of geo-physics that has profound implications for almost all aspects of pre-fracturing deformation ofthe 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 therock mass. There is some good news and some bad news for conventional geophysics. Manyaccepted phenomena are no longer valid at high spatial and temporal resolution. A majoreffect is that the detailed crack geometry changes with time and varies from place to place inresponse 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 andthe response predicted, opening the way to possible control by feedback. The need is to deviseways to exploit these new opportunities in the crack-critical crust.

Recent observations from the SMSITES Project at Husavık in Northern Iceland, gatheredwhile this review was being written, display the extraordinarily sensitivity of in situ rock tosmall changes at great distances. The effects are far too large to occur in a conventional elasticbrittle crust, and their presence confirms the highly compliant nature of the crack-critical crust.

Key words: anisotropy, compliance, crack-critical crust, fracture criticality, shear wavesplitting.

N O TAT I O N

∗Also at: Edinburgh Anisotropy Project, British Geological Survey, Murchi-son House, West Mains Road, Edinburgh EH9 3LA.

APE Anisotropic poro-elasticity: a model for the evolution of fluid-saturated microcracked rock (defined in Section 2.3.)Band-1 Double-leafed solid angle of directions making angles 15◦–45◦ either side of the average plane of stress-aligned vertical micro-

crack distributions (Section 2.3).Band-2 Solid angle of directions making angles 15◦ to the average plane of stress-aligned vertical microcrack distributions (Section 2.3).CW Clockwise vibrations of the DOV (q.v.) source (Section 3.5).

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CCW Counter-clockwise vibrations of the DOV (q.v.) source (Section 3.5).DOV Downhole orbital vibrator: a borehole shear wave source, previously known as the COV, the Conoco downhole orbital

vibrator (Section 3.5).EDA Extensive dilatancy anisotropy: the distributions of stress-aligned fluid-saturated microcracks in almost all in situ rocks

in the crust (Section 2.3).RVSP Reverse vertical seismic profile: seismic exploration source/receiver geometry where borehole source is recorded by

surface geophones (Section 2.2).SMS Stress-monitoring site: a source/receiver geometry using crosshole seismics to monitor stress-induced changes to

microcrack geometry (Section 3.5).SOC Self-organized criticality: a type of self-organizing system verging on criticality (Section 4.1).SWVA Shear wave velocity anisotropy: percentage of shear wave velocity anisotropy (Section 2.3).SWTD Time delay between split shear waves (Section 2.3).TIH Transversely isotropic anisotropic symmetry (hexagonal symmetry) with a horizontal axis of cylindrical symmetry

(Section 2.2).TIV Transversely isotropic anisotropic symmetry (hexagonal symmetry) with a vertical axis of cylindrical symmetry (Section 2.1).VSP Vertical seismic profiles: seismic exploration source/receiver geometry where surface source is recorded by borehole

geophones (Section 2.2).σ V , σ H , σ h Principal axes of vertical stress, and maximum and minimum horizontal stress (Section 2.3).sV , s H , sh Principal axes of differential vertical stress, and maximum and minimum horizontal stress, where

sV = (σ V −σ h), s H = (σ H −σ h) and sh = 0 (Section 2.3).

1 I N T RO D U C T I O N

Every 10 years or so we write a review of shear wave splittingand seismic anisotropy (Bamford & Crampin 1977; Crampin et al.1984a; Crampin & Lovell 1991). These last 10 years have been par-ticularly rewarding, and it looks as if at last we are beginning tounderstand what shear wave splitting means. It appears to moni-tor the low-level deformation or evolution of fluid-saturated rocksunder changing conditions. We can model, and even predict, theeffects of known changes in the complicated heterogeneous crust.This is an important advance, but the reason for this predictability iseven more important. The fluid-saturated cracks in the crust are soclosely spaced that they form critical systems, verging on criticalityand failure. This opens a new window into the behaviour of the crust,which appears to have implications for all solid-earth geophysics.Criticality is well recognized in other areas of geophysics and rockphysics, for example Turcotte (1992) and Sornette (2000), but theydo not identify the mechanism of fluid-saturated stress-aligned mi-crocracks nor the diagnostic of shear wave splitting.

The attitude of many geophysicists to shear wave splitting withazimuthal anisotropy, aligned with the stress field, is somewhatambiguous. Clearly, recognized in many, perhaps most, rocks inthe crust, shear wave splitting is observed, records are processed,modelled and interpreted as indicating propagation through someform of parallel vertical cracks or fractures. In the exploration in-dustry for example, these effects are taken as indicating stress direc-tions and the directions of hydraulic fractures so that the optimumdirections of water floods can be chosen. However, the petrologi-cal significance of the phenomenon is then usually ignored and itsimplications for the nature of the rock mass forgotten. Since moredetailed examination is not pressing, shear wave splitting, once iden-tified as an isolated phenomenon, is accepted without question andwith little interest in what it actually implies. This paper argues thatshear wave splitting provides a window into a new understandingof rock deformation in a crust pervaded by fluid-saturated stress-aligned grain-boundary cracks and pore throats. Observations ofshear wave splitting indicate that microcracking is so extensive andpervasive that almost all rocks may be thought of as critical systemsclose to levels of fracture criticality, which, when the stress field is

modified appropriately, lead to fracturing, faulting and earthquakes.It can be shown that the parameters that control shear wave split-ting also control low-level (pre-fracturing) deformation and that theevolution of fluid-saturated cracks under changing conditions canbe modelled by anisotropic poro-elasticity (APE). APE modellingmatches a huge range of phenomena. On those few occasions whenthe observational data at depth is accurate the match of APE is exact.More often with less accurate data, the match can only be approxi-mate. Nevertheless, the match means that shear wave splitting can beused to monitor, model and in appropriate circumstances predict theresponse of deep in situ rocks to changing conditions. This calcula-bility and predictability has numerous potential applications rangingfrom industrial hydrocarbon exploration and production to a hugevariety of natural hazard monitoring, management and mitigation.

Shear wave splitting (seismic birefringence) is the most diagnos-tic, informative and easily observable evidence of azimuthal seismicanisotropy (Crampin 1981, 1985, 1994). Azimuthal anisotropy isnow recognized as being characteristic of shear wave propagationthrough the fluid-saturated stress-aligned grain-boundary cracks andpore throats in almost all in situ sedimentary, igneous and metamor-phic rocks (Crampin 1994, 1996; Winterstein 1996). This impliesthat most rocks in the crust are pervaded by distributions of highlycompliant stress-aligned microcracks. Consequently, any interpre-tation of in situ rock, which does not allow for the presence of sucheasily deformed stress-aligned fluid-saturated microcracks is at bestincomplete, possibly flawed and at worst significantly in error. Thedeformation or evolution of fluid-saturated cracks under changingconditions has been modelled with APE (Zatsepin & Crampin 1997).Since APE modelling broadly agrees with all relevant observationsof cracks, stress, shear waves and shear wave splitting (there are noknown contradictions, Crampin (1999a), we claim that we are mak-ing some progress towards understanding in situ rock deformation(Crampin & Zatsepin 1997).

APE has been approximately calibrated in laboratory stress cells(Zatsepin & Crampin 1996; Crampin et al. 1997, 1999b). In siturock, however, is remote and difficult to access and, currently, thereis only one case study where the effects of APE have been effectivelycalibrated at depth (Angerer et al. 2000, 2002). These various resultsconfirm that APE is at least a good approximation to pre-fracturing

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Shear wave splitting in the crack-critical crust 223

rock deformation and suggest that the deformation of in situ rock canbe calculated and, in appropriate circumstances, predicted (Crampinet al. 1999a; Angerer et al. 2000, 2002). These various ideas havestimulated the design of stress-monitoring sites (SMSs) using state-of-the-art borehole instrumentation to monitor shear wave split-ting between boreholes in a convenient stress-oriented configuration(Crampin 2001). SMSs are designed to recognize the build up ofstress before earthquakes and volcanic eruptions and estimate (stressforecast) the time and magnitude of impending large earthquakes.

The implications of why the response of the immensely com-plicated heterogeneous crust, below 500–1000 m, say, should becalculable, monitorable with shear wave splitting, and in somecases predictable, is remarkable. The fluid-saturated cracks in thecrust are so closely spaced and so near fracture criticality atthe percolation threshold that the cracks behave as critical sys-tems (Crampin 1998, 1999a; Crampin & Chastin 2001). Fracturecriticality is the level of cracking when shear strength is lost andthe rock necessarily fractures (Crampin 1994; Crampin & Zatsepin1997). Note that a similar situation probably applies to the uppermantle, where the cracks would be films of liquid melt in grain-boundary cracks (Crampin 1995, 2003; Blackman & Kendall 1997;Mainprice 1997).

Critical systems of cracks lead to a new style of geophysics,which has profound implications for the detailed high-resolutionproperties of in situ rock (Crampin 1999a). However, if currentlevels of detail and resolution are satisfactory, then the crack-criticalnature of the crust can be ignored, but if greater detail and higherresolution are required, then the effects of the crack-critical crustinfluence almost every observation and measurement. In particular,there are likely to be profound effects on reservoir characterizationand oil recovery (Crampin 1999b; Crampin & Chastin 2001), andon stress forecasting earthquakes and other geological hazards.

The future of shear wave splitting and seismic anisotropy is arapidly expanding and developing field with many implications andapplications. Note that this is perforce somewhat of a personalizedaccount because there are few other papers relevant to the develop-ment we are presenting. The review is necessary to set the parametersof a complicated multifaceted rapidly evolving development.

2 S H E A R - WAV E S P L I T T I N G

Shear wave splitting (seismic birefringence), where shear wavessplit into typically two approximately orthogonal fixed polarizationswith different velocities, is characteristic of propagation in mediawith some form of elastic anisotropy (Crampin 1981). Such split-ting writes easily recognizable signatures into the three-componentparticle motion of shear wave arrivals (visible in particle motiondiagrams or hodograms), so that shear wave splitting is the keydiagnostic phenomenon for investigating seismic anisotropy.

Note that shear wave splitting is controlled by small (typically lessthan 5 per cent) differences in the velocities of the two polarizedshear waves. The power and sensitivity of shear wave splitting isthat by rotating seismograms into the preferred polarizations, or byplotting polarization diagrams or hodograms, the time delay betweenthe two split shear waves can usually be measured with much greateraccuracy than most second-order measurements. Thus shear wavesplitting opens a window into the analysis of small variations ofthe rock mass that probably no other geophysical measurement canmatch, and is the underlying reason why shear wave splitting is thekey to monitoring the crack-critical crust discussed in Section 4,below.

2.1 TIV anisotropy will not be discussed

TIV anisotropy is transverse isotropy (hexagonal anisotropic sym-metry) with a vertical axis of symmetry where the shear wavessplit into strictly SH and SV polarizations (Crampin 1986). Suchsymmetry is characteristic of finely layered horizontal sedimentarystrata due to the interactions of reflections and transmissions throughthin layers. It is also characteristic of many shales, clays and mud-stones, where the anisotropy is caused by horizontal intergranularplatelets of mica and other minerals. Such TIV anisotropy has ver-tical and horizontal move-out velocities that may differ by 30 percent and cause severe problems in migration and in establishingwell-ties in industrial exploration seismics. These technical prob-lems can usually be accommodated by processing (Tsvankin 2001),although, like almost all examples of azimuthal anisotropy, the be-haviour in in situ rock has only recently being effectively calibrated(Winterstein & De 2001).

Thus, TIV anisotropy is comparatively well understood and prob-ably has few surprises left. However, almost the only real geophys-ical information it carries is that the rock was laid down in somesort of sedimentary process in some sort of fluid, and that gravity isvertical. We shall refer to TIV anisotropy only in passing.

2.2 Shear wave splitting with azimuthal anisotropy

This paper refers specifically to shear wave splitting varying az-imuthally, sometimes misleadingly called TIH anisotropy: trans-verse isotropy (hexagonal anisotropic symmetry) with a horizontalaxis of symmetry. It is misleading because strict TIH (caused bywholly parallel vertical cracks) is exceedingly uncommon. Almostall distributions involve significant elements of non-parallel cracks,although TIH symmetry is frequently a good first approximation.

Typically, below a critical depth of 500–1000 m, the polarizationsof the faster split shear wave are approximately parallel (within 20◦)to the direction of maximum horizontal stress. Such near-parallelpolarizations are observed routinely above small earthquakes andin a wide range of exploration configurations: reflection surveys,vertical seismic profiles (VSPs), reverse vertical seismic profiles(RVSPs), cross-hole seismics, etc. Table 1 lists evidence support-ing stress-aligned fluid-saturated grain-boundary cracks and porethroats as the source of azimuthal anisotropy.

Note that in general, the vertical stress, σ V , is zero at the free sur-face but increases with depth, and a critical depth is reached whenσ V

equals the minimum horizontal stress, σ h . Below this depth, cracksopen normal to the minimum stress, which is typically horizontal sothat the cracks are usually vertical striking approximately parallel tothe maximum horizontal stress, σ H (Crampin 1990) and gives thecharacteristic stress-parallel shear wave polarizations. Above thisdepth, crack distributions are controlled by stress-release and litho-logic phenomena, and may be very disturbed.

Below this critical depth, the polarization of the faster shearwave is parallel to the direction of maximum horizontal stress ina broad band across the centre of the shear wave window. Suchapproximately parallel polarizations are the characteristic, most di-agnostic, feature of observations of shear waves in most types ofrock (Crampin 1994). (The shear wave window is the cone of raypaths with angles of incidence to the free surface less than 35◦–45◦

(the actual angle depending on details of near-surface structure).Outside this window, shear waves are seriously disturbed and arecontaminated by S-to-P conversions. Only within the window canbe the waveforms of the incident shear wave be directly observedat a free surface (Booth & Crampin 1985).) The polarizations are

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Table 1. Summary of the basis for interpretating shear wave splitting with azimuthal anisotropy as the result of propagation throughstress-aligned fluid-saturated grain-boundary cracks and pore throats.

Observations of SWVA∗ Interpretation

(1) Fast shear wave polarizations within (a) Only transverse isotropy (hexagonal symmetry)20◦ of direction of maximum horizontal with horizontal symmetry axis (TIH anisotropy), or astress for nearly vertical propagation in minor perturbation thereof, has this property [2]most† crustal rocks below 500–1000 m depth [1] (b) The only common phenomenon possessing

such symmetry is stress-aligned fluid-saturatedcracks (specifically microcracks) [2]

(2) Observed with similar parameters (a) Parameters of anisotropy close to APE§

(minimum and maximum SWVA of ∼1.5 theory in model of crack evolutionper cent and ∼4.5 per cent, respectively) (minimum SWVA of ∼1 per cent to maximum atin almost all sedimentary, igneous, the percolation threshold of 5.5 per cent [3])and metamorphic rocks [1] (b) Only crack distributions consistent with all

observations are fluid-saturated grain-boundarycracks and pore throats [4]

(3) Temporal variations in time delays Temporal variations indicate both smallindicate increasing crack aspect ratios cracks and fluid-saturations supportingas stress builds up before earthquakes [4, 5, 6]. interpretation 2b, above

(4) Shear wave splitting occasionally seen Large aligned fractures also cause shear wavein heavily fractured beds [7, 8]. splitting [7, 8, 9]

(5) A large range (15+) of different static It is fundamental to the success of APEand temporal observations of cracks, modelling of the evolution of a crack rock massstress, and shear waves can be matched (see Table 2, below) that the modelled cracksby APE§ modelling. are highly compliant and this necessitates

distributions of micro as opposed to macrocracks.

∗Shear wave velocity anisotropy;†The only common exceptions are shales, clays, mudstones, which may have several tens of per cent of lithologically inducedtransverse isotropy with a vertical symmetry (TIV anisotropy), and oolites and coccoliths which have a highly constrainedmicrostructure and may have very little SWVA.§Model of anisotropic poro-elasticity [10].Percolation threshold is the theoretical crack density at which cracking is so extensive that statistically through-going fractures exist [3].[1] Crampin (1994); [2] Crampin (1981); Wild & Crampin (1991); [3] Crampin & Zatsepin (1997); [4] Crampin (1999a); [5] Crampinet al. (1999a); [6] Volti & Crampin (2003a,b); [7] Mueller (1991, 1992); [8] Meadows & Winterstein (1994); [9] Li et al. (1993); [10]Zatsepin & Crampin (1997).

only approximately parallel because microcracks have a range oforientations with crack normals averaged about the direction ofminimum compressional stress so that the anisotropic symmetry isonly approximately TIH (Crampin & Zatsepin 1997). See also thediscussion of the crack-critical crust in Section 4, below.

10 years ago, Crampin & Lovell (1991) published a review ofthe first decade since stress-aligned shear wave splitting was firstpositively identified in records within the shear wave window abovesmall earthquakes (Crampin et al. 1980). Crampin & Lovell tried toanswer three questions concerning shear wave splitting: what doesit mean, what use can we make of it and what should we do next?Substantial progress has been made in all three questions, reviewedbelow, but the questions are still relevant. We believe we are justbeginning to be able to answer them, and the answers are excit-ing, although they are not always the answers that were originallyexpected.

2.3 What does shear wave splitting mean?

Crampin (1994, 1996) and Winterstein (1996) reviewed all the thenavailable (∼80) examples of shear wave splitting recorded abovesmall earthquakes and in exploration surveys, which were princi-pally VSPs. With the exception of the TIV anisotropy mentioned

above, almost all sedimentary, igneous, and metamorphic rocks be-low 500–1000 m display azimuthal stress-aligned shear wave split-ting. There is a minimum shear wave velocity anisotropy (SWVA)of about 1.5 per cent and a maximum in ordinary unspecified rockof about 4.5 per cent. Higher values of SWVA are found nearer thesurface and in heavily fractured rocks and areas of high heat flow(Crampin 1994).

The polarizations of the faster waves are subparallel to the di-rection of maximum horizontal stress. Since the only phenomenoncommon to all rocks where such symmetry is observed is stress-aligned cracks, this strongly suggests that the shear wave splittingobserved in most rocks is caused by stress-aligned microcracks(Table 1)—the extensive dilatancy anisotropy (EDA) of Crampinet al. (1984b). Fig. 1 shows the classic schematic illustration ofshear wave splitting through distributions of microcracks alignedperpendicular to the direction of minimum horizontal stress. Fornearly vertical propagation, the polarization of the faster split shearwaves are parallel to the strike of the cracks, which is approximatelyin the direction of maximum horizontal stress.

Since, the percentage SWVA of parallel cracks is approximatelyequal to 100 times the crack density (Crampin 1994), the values ofobserved SWVA indicate crack densities of 0.015 ≤ ε ≤ 0.045 inostensibly intact rocks, where the crack density is ε = Na3/v, and Nis the number of cracks of radius a in volume v. Since grains in most

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Figure 1. Schematic illustration of shear wave splitting in distributions ofstress-aligned fluid-saturated parallel vertical cracks aligned normal to thedirection of minimum horizontal stress, where for nearly vertical propagationthe polarization of the faster split shear wave is parallel to the strike ofthe cracks, parallel to the direction of maximum horizontal stress. Suchparallel vertical crack orientations are typically found below the criticaldepth, usually between 500 and 1000 m, where the vertical stress is greaterthan the minimum horizontal stress.

Figure 2. Schematic interpretation of observed percentages of SWVA below the critical depth, interpreted as uniform (dimensionless) distributions of equallysized parallel penny-shaped parallel cracks with the given percentage of SWVA, where ε is crack density (approximately equal to a hundredth of the percentageanisotropy) and a is crack radius. (After Crampin 1994.)

rocks usually have a comparatively restricted range of dimensions,grain-boundary cracks have a similarly restricted range. Assuminguniform equally sized penny-shaped cracks, Fig. 2 gives a schematicillustration of distributions of cracks that have the observed rangeof SWVA. Crampin (1994) suggested there is a fracture criticalitylimit, 0.045 ≤ ε ≤ 0.1, separating ostensibly intact rock (ε ≤ 0.045)from rock that is disaggregating at the free surface (0.1 ≤ ε). Thefracture criticality limit in stressed fluid-saturated rock is now shownto be associated with the percolation threshold at about ε ≈ 0.055(Crampin & Zatsepin 1997).

This indicates a very narrow range of crack size. There is a dif-ference in radius of less than a factor of 2 (∼1.8) between cracks inostensibly intact rocks having the minimum crack density usuallyobserved below 500–1000 m (1.5–4.5 per cent SWVA) and rocksthat are desegregating at the free surface (≥10 per cent). Such crackdistributions in intact rocks typically lead to normalized time de-lays of less than about 8–10 ms km−1 in most in situ rocks. Sincethe grains in any particular rock usually have similar sizes, the di-mensions of grain-boundary cracks and pore throats will also tendto have a narrow range. This means that the uniform microcrackdistributions in Fig. 2 are not too unrealistic.

Note that the mathematical derivation of SWVA is sensitive tothe ray path through the cracks, the velocities and Poisson’s ratio ofthe rock matrix, and to the properties of the pore fluid, as well asto the crack density (Crampin 1993). Consequently, SWVA is avariable quantity and does not refer to a fixed geometry of cracksbut depends on matrix and pore-fluid properties. For example, thevalue of SWVA is observed to be higher in areas of high heat flow(Crampin 1994), although the reasons for this are not fully under-stood. The variation of SWVA in three dimensions is also sensitiveto the crack aspect ratio and hence to principal axes of stress andthe crack geometry.

Fluid-saturated microcracks are highly compliant, and the evo-lution of fluid-saturated grain-boundary cracks and pore throatsunder changing conditions can be modelled by APE (Crampin &Zatsepin 1997; Zatsepin & Crampin 1997). The driving mechanismfor evolution (deformation) is fluid migration by flow or diffusionalong pressure gradients between cracks at different orientationsto the stress field. APE models fully 3-D distributions of cracks.Fig. 3 is a schematic but numerically accurate illustration of theeffect of the APE deformation mechanism on random distributionsof vertical cracks as the maximum horizontal stress is marginallyincreased. Hexagons have isotropic elastic symmetry so the two(solid) hexagons of cracks, at zero differential stress (top left), are

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Figure 3. Schematic illustration of the evolution of crack aspect ratios in aninitially random distribution of vertical cracks (solid lines) for four values ofincreasing maximum horizontal differential stress, normalized to the criticalvalue at which cracks first begin to close. Pore-fluid mass is preserved andaspect ratios are correct for a porosity of φ = 5 per cent. (After Crampin &Zatsepin 1995.)

a small selection of randomly oriented vertical cracks without anyhorizontal anisotropy. We define differential stress, si, as the stresscomponents, σ i , less the minimum stress which in this case, belowthe critical depth, is σ h , hence sV , s H , sh = σ V −σ h , σ H −σ h , 0,respectively. As differential stress increases, stress-induced pressuregradients cause fluid to move, by flow or diffusion, between adja-cent cracks at different orientations to the stress field (top right),but until the stress is sufficient to close normally oriented cracks,there is negligible anisotropy. At the critical stress (normalized tosH = 1 in the figure), cracks begin to close (bottom left) and thelevel of SWVA jumps from zero to a value close to the 1.5 per centSWVA minimum observed in the crust (Crampin 1994). As differ-ential stress continues to increase, the cracks increase in aspect ratioand begin to line up. It can be shown that the percolation thresh-old when the mechanism in Fig. 3 leads to through-going cracks atabout 5.5 per cent SWVA (Crampin & Zatsepin 1997), which canbe identified with the fracture criticality of Crampin (1994). (Sincestress-aligned shear wave splitting is almost always observed in insitu rocks, this implies that the critical horizontal stress is almostalways exceeded in rocks in the crust.)

APE theory shows that the parameters that control low-level de-formation before fracturing occurs are exactly those that controlshear wave splitting (Crampin & Zatsepin 1997). Note that the prin-cipal effects of marginal changes of stress and pore-fluid pressure,below the level at which fracturing occurs, are modifications to crack

aspect ratios. This has recently been confirmed by very different the-oretical techniques (Hudson 2000).

APE theory is highly constrained with no free parameters yetmatches a wide range of phenomena, some of which are listed inTable 2 (Crampin 1999a, 2000). The match of observations to APEmodelling in Table 2 can be very accurate, although typically it isdifficult to obtain accurate measures of crack behaviour at depthbecause the high temperatures and pressures make in situ rock es-sential inaccessible. Note that the crack density in Fig. 3 is lower atsH = 3 than at sH = 0, as increasing stress tends to close cracks andhence lessen the crack density and the associated SWVA. The onlyunambiguous change as stress increases is the increase in the av-erage crack aspect ratio (as noted above). The major observationaleffects of changing aspect ratios are comparatively subtle changesin a range of directions in the 3-D variation of time delays betweenthe split shear waves.

The effect of increasing the aspect ratio of parallel vertical crackson shear wave splitting is to increase the average time delay alongray paths in the double-leafed solid angle of directions (referredto as Band-1) making angles 15◦–45◦ to the plane of the cracks(Crampin 1999a). Time delays in Band-2 (directions within 15◦

of the crack plane) are sensitive only to crack density. Table 2 listsmatches of APE modelling in exploration seismology and behaviourbefore earthquakes (see the next section), as well as with laboratoryexperiments in stress cells.

It is this wide range of agreement of APE modelling with obser-vations that confirms that the shear wave splitting with azimuthalvariations observed in the crust is typically caused by stress-alignedfluid-saturated microcracks. Large cracks would be stiff and muchless compliant. Note that shear wave splitting is controlled by thecrack/pore throat geometry, and is largely independent of the non-crack porosity (equant porosity) (Crampin & Zatsepin 1997).

The large number of dynamic effects in Table 2 confirm that fluid-saturated cracks make the rock mass highly compliant. If internalconditions in the rock mass are changed in any way, the crack ge-ometry responds and the response can be monitored directly withshear wave splitting, and modelled or predicted by APE. Certainlylarge fractures, if they are aligned, will cause significant shear wavesplitting. However, the only confirmed observations, to our knowl-edge, are those of Mueller (1991, 1992) and Li et al. (1993) in theAustin Chalk, and Angerer et al. (2000, 2002) in Vacuum Field,New Mexico (who models a structure of large fixed fractures andcompliant microcracks). Large cracks are comparatively stiff, andthe reports in Table 2 suggest, and the results of Angerer et al. (2000,2002) confirm, that the dominant cause of shear wave splitting ispropagation through the compliant fluid-saturated grain-boundarycracks and pore throats (microcracks) that pervade most rocks. Thephenomena in Table 2 have been discussed in more detail elsewhere(Crampin 1997, 1999a,b, 2000).

Table 1 summarizes the evidence for interpreting the cause of az-imuthal shear wave splitting as propagation through fluid-saturatedmicrocracks, specifically grain-boundary cracks in low permeabilityrocks and pore throats in porous rocks. The evidence is overwhelm-ing that the source is microcracks, particularly the extraordinarymatch of APE modelling to observations in Table 2 and the mod-elling of Angerer et al. (2000, 2002), and yet the evidence is almostwholly indirect. In situ cracks are remote, essentially inaccessible,and are nearly transparent to everything except seismic shear waves(Crampin 1981, 1999a). Note that recent observations reported inSection 6, below, confirm the extraordinary compliance of the fluid-saturated cracked rock mass and sensitivity of shear waves to nearlynegligible changes in stress.

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Table 2. Match of APE* modelling to observations (Crampin 1999a, updated).

Ref. (Obs.) Ref. (APE)

Static effects

Field observations of SWVA† (below 500–1000 m depth)(1) SWVA in all rocks independent of porosity and geology [1] [2](2) Minimum SWVA in intact rock: observed ≈1.5 per cent; APE modelled ≈ 1.0 per cent [1] [2](3) Maximum SWVA in intact rock: observed ≈4.5 per cent; APE modelled ≈ 5.5 per cent [1] [2](4) Narrow range of crack density: 0.025 ≤ ε ≤ 0.045 [1] [2](5) Proximity of fracture criticality (at percolation threshold) ≈5.5 per cent [1] [1]

Other field observations(6) Fracture criticality limit specifies crack distributions with a range of dimensions [3] [4]of about nine orders of magnitude(7) π /2 shear wave polarization changes (90◦ flips) in overpressurized reservoirs [5, 6] [2, 5, 6]

Dynamic effects

Temporal changes in SWVA during production procedures(8) Changes before and after pumping tests [7] §

(9) Changes before and after high pressure CO2 flood in carbonate reservoir [6, 8] [6]

Temporal changes in SWTD¶ before earthquakes(10) Variations of time delays before earthquakes (with hindsight) [9, 10] [2](11) Successful forecast of time and magnitude of an M = 5 earthquake in SW Iceland [9] §

Temporal changes in SWTD before volcanic eruption(12) Variations in SWTD for some 5 months before 1996 September 30; Vatnajokulleruption, Iceland, at distances of: 230 km WSW; 170 km SW; and 240 km, N [11] §

Variations of shear waves in laboratory experiments(13) Variations of SWVA and permeability in uniaxial stress cell [12] [13](14) Variations of (isotropic) shear wave velocities to changes in confining [14] [14]pressure and pore-fluid pressure for oil-, water-, and gas- (dry) saturationsin stress cells of sandstone cores(15) Variations of velocity and attenuation from sonic (transducers) to seismic [15] [16](resonant bar) frequencies

∗APE, anisotropic poro-elasticity; †SWVA, shear wave velocity-anisotropy; §Effects compatible with APE; ¶SWTD, shear wavetime delays;[1] Crampin (1994); [2] Crampin & Zatsepin (1997); [3] Heffer & Bevan (1990); [4] Crampin (1997, 1999a); [5] Crampin et al.(1996); [6] Angerer et al. (2000, 2002); [7] Crampin & Booth (1989); [8] Davis et al. (1997); [9] Crampin et al. (1999a); [10]Booth et al. (1990), Crampin et al. (1990, 1991), Liu et al. (1997), Gao et al. (1998); [11] Volti & Crampin (2003a,b); [12] Kinget al. (1994); [13] Zatsepin & Crampin (1996); [14] Crampin et al. (1997, 1999b); [15] Sothcott et al. (2000a,b); [16] Chapmanet al. (1998, 2000).

3 S H E A R - WAV E S P L I T T I N G — C U R R E N TP O S I T I O N

Here we attempt to answer the second question of Crampin & Lovell(1991) ‘what use can we make of it?’. The polarizations of the fastersplit shear wave are generally assumed to indicate the alignmentof cracks and fractures. As noted above, the polarizations are fre-quently claimed, without proof, to indicate the alignment of largefractures permitting easy oil flow. Unambiguous demonstrations ofanisotropy due to large fractures are few (Mueller (1991, 1992)and Li et al. (1993) in the Austin Chalk, Texas, and Davis et al.(1997), Duranti et al. (2000) and Angerer et al. (2000, 2002) inVacuum Field, New Mexico; Angerer et al. (2000, 2002) analysedshear wave splitting in the presence of both microcracks and largefractures). There is also little direct evidence in other literature thatthe preferred directions of flow in oil fields are due to large fracturesrather than the preferred flow directions through highly permeablestress-aligned microcracks.

The last 2 years have seen several applications of shear wavesplitting.

3.1 Predicting the response of a reservoir to knownchanges during recovery processes

Angerer et al. (2000, 2002) analysed, interpreted and modelled withAPE (in effect predicted with hindsight) the response of the VacuumOil Field, New Mexico, to two CO2 injections resulting in pressureincreases of 1000 psi (6.9 MPa) and 200psi (1.38 MPa), respectively.These were Phases VI and VII of the Reservoir CharacterizationProject (RCP) of Colorado School of Mines (Davis et al. 1997;Duranti et al. 2000). The Phase VI injection of 17 MPa was anoverpressure.

The part of the Vacuum Oil Field accessed had a flat layer-cakestructure. Extensive 4-D 3-C reflection record sections suggestedthat changes in shear wave splitting were the most diagnostic ef-fects of both CO2 injection pressures (Davis et al. 1997; Duranti

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Figure 4. (a) Pre-injection waveforms of a multicomponent nearly vertical reflection survey near the centre of the Vacuum Field, New Mexico, carbonatereservoir (Davis et al. 1997). S1, S2 and P are record sections with mutually orthogonal polarizations, where the horizontals S1, and S2, have been rotatedinto the split shear wave arrivals parallel (S1) and perpendicular (S2) to the direction of maximum horizontal stress, respectively. The left-hand five traces areobserved waveforms at neighbouring receivers 17 m apart, and the right-hand three traces are synthetic seismograms modelled by APE to match the shearwave arrivals. Top and bottom of injection zone for shear waves (established by extensive analysis at Colorado School of Mines) are marked by arrows withtime delays in ms km−1. (b) Post-injection waveforms after a high pressure CO2 injection. Again, the left-hand traces are observations and right-hand tracesare synthetic seismograms modelled by APE with structure from (a) and an injection pressure of 2500 psi. (After Angerer et al. 2000, 2002).

et al. 2000). Angerer et al. (2000, 2002) processed the Phase VIrecords and determined an initial structure of large fixed faults withan internal microcrack structure with 2 per cent SWVA. Syntheticseismograms, calculated by ANISEIS (Taylor 2000), through theinitial macrocracked model reproduced the shear wave splitting ar-rivals in Fig. 4(a) from reflections from the top and bottom of theSan Andres Formation target zone, where the CO2 was injected.

The three sections in Figs 4(a) and (b) are the horizontal S1 and S2polarizations parallel and perpendicular, respectively, to the max-imum horizontal stress, and the vertical P-wave polarization. Thetime delays between the top and bottom reflections of the target zonein Fig. 4(a) are 176 ms for S1 and 178 ms for S2, indicating thatS1, polarized parallel to the maximum horizontal compressionalstress, is the faster split shear wave in the target zone. Note that

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other investigations at RCP identified the reflections from the targetzone, where the numerous other arrivals are various reflections andreverberations.

The five traces in the left-hand side of the record sections inFig. 4(b) show significant changes following the high-pressure CO2

injection. The three modelled traces to the right are calculated byinserting the specific injection pressure into the APE model of theinitial fracture model in Fig. 4(a). The match of observed to mod-elled arrivals is almost exact so that APE has correctly calculated(in effect predicted) the response of the cracked rock mass to theinjection pressure. (Note that the changes to the P-wave arrivals alsomatch, but P-wave arrivals contain little information and are easy tomatch (Crampin 1985).) APE shows that the difference in the shearwave response is largely caused by the increased aspect ratio as themicrocracks expand with the increased pore-fluid pressure (Angereret al. 2000, 2002).

A similarly satisfactory match was also found for the lower-pressure Phase VII CO2 injection (not shown), using the same initialcracked model and inserting the lower pressure in APE. The PhaseVII injection was in a similar well, 25 m from the Phase VI injectionwell, with varying conditions and a lower injection pressure. Thismeans that APE has in effect predicted the response of the microc-rack structure to two very different injection pressures. This is thebest in situ calibration of APE to date.

Fig. 4 shows another characteristic of shear wave splitting. Incontrast to Fig. 4(a), where S1 is the faster wave through the SanAndres Formation, the orthogonal polarization S2 in Fig. 4(b), isfaster with a time delay of 184 ms as opposed to 204 ms for S1. Thismeans that the polarizations of the faster split and slower shear waveshave interchanged. Angerer names this phenomenon a ‘90◦ flip’.Such 90◦ flips, together with large values of SWVA, are characteristicof shear wave splitting in overpressurized rocks. In overpressurizedhydrocarbon reservoirs, 90◦ flips have previously been observedin the Caucasus oil field (Crampin et al. 1996; Slater 1997). Such90◦ flips are also characteristic of shear wave splitting immediatelyabove active faults and have been observed at two places on theSan Andreas Fault (Crampin et al. 1990, 1991; Liu et al. 1997) andon the Husavık-Flatey Fault (HFF) in Iceland (Section 6.2, below).High pore-fluid pressures are necessary to relieve frictional forcesand explain fault slip with no observable frictional heat flow (Sibson1990; Gudmundsson 1999).

Thus, Angerer et al. (2000, 2002) show that, in some circum-stances at least, it is possible to predict the response of a reservoirto quantifiable oil-field operations. Since the response can be moni-tored by analysing shear wave splitting, this means that the responseof the reservoir to a known operation, such as a fluid injection,can, in some circumstances, be controlled by adjusting the injectionpressure to optimize the rock mass response. This means that theresponse can be monitored and controlled by feedback long beforeproduction rates would have indicated whether the procedures weresatisfactory or not.

3.2 Converted phases

One of the difficulties of analysing shear waves is the expense of us-ing shear wave vibrator sources onshore and the lack of an efficientshear wave source offshore. It has long been recognized that P–Sconverted phases are in many cases a cheaper alternative techniquefor generating shear waves (Garotta & Marechal 1987). Conver-sions at depth would have the advantage that shear waves would ini-tially have the higher frequencies associated with P waves. Thomsen(1999) calls such converted waves C waves and presents a strong

case for their use in studying shear wave anisotropy. The major diffi-culty as Thomsen recognizes is that: (1) results are highly direction-dependent. If the sagittal plane is one of the preferred split shearwave polarizations, the other split shear wave will not be generated,with possibly misleading results; and (2) the fixed P-wave polariza-tions almost never give the opportunity of giving two shear wavesource orientations. This means that C waves cannot give the sup-porting evidence of two shear wave splitting measurements alongthe same ray path to provide confirmation of the interpretation. Notethat a further cheaper optimum technique for monitoring shear waveanisotropy is suggested in Section 5.1, below.

3.3 Stress forecasting the times and magnitudesof earthquakes

It has long been suspected (Crampin 1978; Crampin et al. 1984b)that changes in shear wave splitting would monitor changes of mi-crocrack geometry caused by changes of stress before earthquakes.Peacock et al. (1988) observed comparatively consistent changes intime delays in Band-1 of the shear wave window above small earth-quakes, which they suggested indicated stress-induced changes incrack aspect ratios. The hypothesis that the major effect of increasingstress on crack distributions was indeed to increase aspect ratios wasconfirmed by APE some 10 years later (Crampin & Zatsepin 1997;Zatsepin & Crampin 1997). Crampin et al. (1990, 1991) showedthat the observations of Peacock et al. (1988) appeared to be causedby changes before the Ms = 6 North Palm Springs earthquake. Thistechnique was difficult to confirm elsewhere as suitably persistentswarms of small earthquakes to use as a source of shear waves arevery uncommon. Until 1997, changes in shear wave splitting inBand-1 directions before earthquakes had been observed only onfour occasions worldwide: before two earthquakes on the San An-dreas Fault in California (Peacock et al. 1988; Crampin et al. 1990,1991; Liu et al. 1997); one in Arkansas (Booth et al. 1990); and oneon Hainan Island, China (Gao et al. 1998).

The breakthrough came when shear wave splitting was monitoredin the European Commission funded PRENLAB-1 and PRENLAB-2 Projects (Stefansson et al. 2000) using Iceland as a natural labo-ratory for earthquake prediction studies. Iceland is above a highlyseismic offset of the Mid-Atlantic Ridge. Increases in time delays inBand-1 shear wave splitting were observed routinely before earth-quakes and before some volcanic eruptions in SW Iceland (Volti& Crampin 2003a,b). Observations before both earthquakes andvolcanoes confirm that changes of shear wave splitting are due tothe changes of stress along the ray path rather than associated withchanges in the immediate source zone of earthquakes.

The underlying assumption is that the rock mass is weak to shearstress. Fig. 2 indicates that all in situ crack distributions vergeon fracture criticality. This means that the stress released by alarge earthquake necessarily accumulates over an enormous vol-ume of rock, probably tens to hundreds of millions of km3 before anM = 8 earthquake. In Iceland to date, changes in shear wave splittinghave been observed before earthquakes with magnitudes betweenM = 3.5 and 5.6 at distances of 14 and 43 km, respectively (Volti& Crampin 2003a,b). (Note that Icelandic magnitude M is approxi-mately equivalent to body-wave magnitude, mb.) The time and mag-nitude of an M = 5 earthquake was successfully stress forecast fromthe data in Fig. 5 (Crampin et al. 1999a). This forecast assumed amore or less constant rate of deformation from the movement of theMid-Atlantic Ridge. If stress accumulates over a small volume, therate of accumulation will be fast for a comparatively short period oftime before fracture criticality is reached and the final earthquake

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Figure 5. Shear wave splitting time delays for 1996 January 1 to 1999 December 31, at Station BJA, SW Iceland. The middle and top diagrams show thevariation of time delays with time for ray paths in Band-1 and Band-2 directions. The time delays in ms are normalized to a 1 km path-length. The verticallines through the time delay points are error bars based on errors in hypocentral distance. The irregular lines are nine-point moving averages. The straight linesin Band-1 are least-square estimates beginning just before minima of the nine-point average and ending when a larger earthquake or an eruption occurs. Thearrows indicate the times of these larger events with magnitudes and epicentral distances indicated. The bottom diagram shows the magnitudes of earthquakesgreater than M = 2 within 20 km of the recording station. (After Volti & Crampin 2003a,b.)

would be comparatively small. Whereas if the stress accumulatesover a larger volume, the rate would be slower and over a longerperiod of time, but the resulting earthquake would be larger.

With this assumption, if relationships between the rate and du-ration of increase with magnitude can be estimated from previousearthquakes, as they were in Iceland (Crampin et al. 1999a), thetime and magnitude of an impending earthquake can be estimatedfrom the time that the increase reaches levels of fracture criticality.We call this process stress forecasting. The magnitude to durationrelationship is approximately linear over the small range of magni-tudes for which we have data in Iceland. These effects were recog-nized and the time and magnitude of an M = 5 earthquake weresuccessfully stress forecast (Crampin et al. 1999a). The locationof the impending earthquake cannot be estimated directly fromshear wave splitting, where effects are seen at over 40 km from anM = 5 earthquake. However, if it is known that a large earthquake isapproaching, other precursory activity can be interpreted correctly.Ragnar Stefansson at the Icelandic Meteorological Office correctlypredicted the location of the stress forecast event from the continuedseismicity following a previous earthquake forecast (Crampin et al.1999a).

Note that because of the difficulty in quantifying errors in estimat-ing both rates of increase of stress and levels of fracture criticality,stress forecasts are given in terms of a smaller earlier to largerlater (SELL) window. Note also that the largest mb = 5.6 (Ms =6.6) earthquake in SW Iceland since 1996 was not stress forecastbecause there were insufficient (shear wave source) earthquakes for7 weeks at the beginning of the increase and the increase was notrecognized (Volti & Crampin 2003b).

Note also that the large scatter of time delay measurements inFig. 5 is caused by the crack-critical nature of the distributions offluid-saturated stress-aligned cracks as discussed in Section 4.3,below.

3.4 Forecasting volcanic eruptions

The first implied increase of aspect ratios from increases of timedelays in Band-1 in Fig. 5, from 1996 May to September, ends at thetime of the Vatnajokull eruption at the beginning of 1996 October.(Note that data are sparse, and the least-squares line is only throughnine points. Consequently, the nine-point moving average does notshow the same increase as the least-squares line.) Similar implied in-creases are also seen at Stations GRI, KRI and SAU, Volti & Crampin(2003b). We interpret this as indicating increasing stress prior to thehydraulic fracture of the 10 km long fissure opened by the eruptionwhen the rocks reached fracture criticality. Following the eruption,a least-squares line fit to the time delays in both Band-1 and Band-2of the shear wave window show a comparatively linear decrease ofabout 2 ms km−1 yr−1 for about 2 years. Similar behaviour is seen inBands 1 and 2 at all four seismic stations in Iceland, BJA, GRI, KRIand SAU, where there were reliable measurements of shear wavesplitting, at distances up to about 240 km north and 240 km WSWof the Vatnajokull eruption (Volti & Crampin 2003b).

This slow decrease is interpreted as the relaxation in stress as theMid-Atlantic Ridge adjusts to the new stress regime following theeruption. We suggest these observations associated with the eruptionconfirm that: (1) changes in shear wave splitting are measuring theeffects of changes of stress in the rock mass rather than the details of

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seismogenic zones; (2) shear wave splitting is a sensitive diagnosticof small stress-induced changes to in situ rocks; and (3) the effectsof increases of stress may be seen at very large distances. Recently,several volcanic eruptions have disturbed the stress regime in SWIceland and no earthquakes have been stress forecast since 1998,although the M = 5.6 event (see the end of the last section) mighthave been forecast had there been sufficient source earthquakes.

It is possible that the effects reported for eruptions in Iceland maynot be typical of all eruptions. However, similar effects have been ob-served elsewhere. Miller & Savage (2001) reported 90◦ flips in shearwave polarizations following an eruption on Mount Ruapehu, NewZealand (the monitoring was not continuous). They interpreted theseas due to high-pressure magma injections in the comparatively shal-low crust. This is consistent with our observations at Vatnajokull.

We suggest that continuous observations at Ruapehu would haveshown changes in time delays as the fluid pressure increased as wesaw at Vatnajokull in Fig. 5. We note that phenomena associated withshear wave splitting are very pervasive, and are probably associatedwith the universality of critical systems discussed in Section 4.

3.5 Developing stress-monitoring sites

The above results indicate that changes in shear wave splitting canmonitor the effects of increasing stress before earthquakes and leadto stress forecasting the time and magnitude of impending events.However, reliable routine stress forecasting using earthquakes as thesource of shear waves is severely restricted because of the scarcityof suitably persistent swarms of small earthquakes to use as a sourceof shear waves. Routine stress forecasting away from such swarmsrequires the controlled source seismology of stress-monitoring site(Crampin et al. 2000; Crampin 2001). SMSs use cross-hole seismicsbetween boreholes to measure shear wave splitting in the same Band-1 directions that showed the changes above the small earthquakes inFig. 5. Fig. 6 shows the optimum SMS geometry. Stress monitoringneeds to analyse shear waves below the uppermost 500–1000 m ofthe crust, below the critical depth where σ V = σ h , so that cracks tendto be parallel and vertical. This also avoids the severe attenuationand scattering usually present in the uppermost layers (Leary &Abercrombie 1994; Leary 1995).

The optimum source for radiating split shear waves in SMSs is thedownhole orbital vibrator (DOV), which was recently commercial-ized by Geospace Engineering Resources Inc., Houston. The DOV,previously known as the Conoco Orbital Vibrator or COV (Liu et al.1993), is an eccentric cam swept both clockwise (CW) and counter-clockwise (CCW) to exert a rotating radial pressure on the boreholewall (Cole 1997). The sum and differences of the recorded CWand CCW signals can be combined to simulate radiation from or-thogonal point forces and hence simulate radiation of orthogonalshear wave polarizations oriented with respect to horizontal pilot-geophones mounted on the DOV casing (Daley & Cox 2001). Thisis exactly what is required to routinely monitor shear wave split-ting. Preliminary observations from the first SMS are discussed inSection 6. below.

4 S H E A R - WAV E S P L I T T I N GI N A C R A C K - C R I T I C A L C RU S T

4.1 Critical systems

The capability of APE to model, calculate, even predict, the re-sponse of fluid-saturated microcracked rock to changing conditionsin a highly complicated heterogeneous crust is remarkable and re-quires some explanation. Observations of shear wave splitting indi-

Figure 6. Specifications for a stress-monitoring site (protected by PatentApplication PCT/GBOO/01137), where D is the depth below which theminimum compressional stress is horizontal so that below that depth crackstend to be aligned vertically. The DOV source operates from a depth ofD + 300 m to D + 1000 m in the deeper well and receivers are at a depthD in (optimally at least two) vertical wells at an offset of 300 m. The azimuthsof the offsets should be within 45◦ of the direction of minimum horizontalstress, which for this illustration is taken to be north–south.

cate that stress-aligned fluid-saturated microcracks are a remarkablypervasive feature with similar parameters in almost all rocks in thecrust (Crampin 1994). The calculability is thought to be becausethe fluid-saturated cracks in the crust are so closely spaced that theyare critical systems. Critical systems involve dynamic interactiveprocesses that below criticality perturb only locally. Once systemsreach criticality, all members of the critical system influence allother members (Ma 1976; Jensen 1998).

The transition temperature of equilibrium thermodynamics is theclassical critical system, but critical systems are common in an enor-mous range of phenomena, including almost all complex systems innature (Bak 1996). Crampin (1998) and Crampin & Chastin (2001)suggest that stress-aligned fluid-saturated cracks in the Earth’s crustare also interactive critical systems. Similar schemes for the earthhave been suggested previously in the self-organized criticality(SOC) of Bak & Tang (1989) and Bak et al. (1988). The advancehere is that the microscale mechanism for deformation has beenidentified as stress-induced fluid movement along pressure gradi-ents between adjacent fluid-saturated grain-boundary cracks andpore throats. This is a quantifiable physical process that can be mod-elled, monitored and calculated by APE (Crampin & Zatsepin 1997;Crampin 1997, 1999a; Zatsepin & Crampin 1997).

The criticality is the reason APE modelling matches the hugerange of phenomena in Table 2. Bruce & Wallace (1989) showhow different critical systems have similar statistical behaviour at

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criticality, despite very different subcritical physics. This is knownas critical-point universality and implies the self-similar scalingseen in crack distributions (Heffer & Bevan 1990), the well-knownGutenberg–Richter relationship (Kagan 1992), and many other phe-nomena in the Earth and in rock physics (Crampin & Chastin 2001).

Critical systems are typically sensitive (the ‘butterfly’s wings’ sen-sitivity) to otherwise negligible variations in initial conditions thatcan lead to order-of-magnitude differences as the systems evolve(Bruce & Wallace 1989). This manipulation of fluid-saturated mi-crocracks makes in situ rock highly compliant (Crampin et al.2003a), where the behaviour at (fracture) criticality is the occur-rence of fracturing and earthquakes. Crampin & Chastin (2001)suggest that the reason for the match of the nearly parameterlessAPE modelling to deformation in a complex heterogeneous Earthis because the response of such critical media is controlled by thenon-linear behaviour near criticality. Consequently, the behaviourcan be modelled by critical-point universality of Bruce & Wallace(1989), and is a mean-field theory (Jensen 1998).

Crampin & Chastin (2001) note that similarities in critical be-haviour can be misleading. It is tempting to use simplistic modelsfor complex Earth processes and make claims for relevance merelybecause they produce similar statistics and similar self-similarity.Much quoted examples, as in Hergarten (2002), are the slider-blockmodel of Burridge & Knopoff (1967) and the cellular automataof Kadanoff et al. (1989) and Rundle & Klein (1993). Models withSOC are widely available but they are only phenomenological. Theyreproduce the statistics of SOC but offer little insight into the subcrit-ical physical process, which is the region of interest for geophysicsin understanding the behaviour before fracturing or faulting or earth-quakes. The perceived similarity with some highly simplified earthmechanism is arbitrary and largely irrelevant to a better understand-ing of earth processes. The power and relevance of APE modellingis that, because the small-scale physical behaviour is identified andmodelled (hopefully) correctly, APE modelling is satisfactory overa very wide range of different phenomena (Table 2).

4.2 The behaviour of a crack-critical system with SOC

A critical system of cracks with SOC has profound implicationsfor classical linear geophysics. Table 3 lists some of the directimplications for conventional geophysics (discussed more fully in

Table 3. Direct implications of distributions of cracks in the crust being critical systems with self-organized criticality, SOC (afterCrampin & Chastin 2001).

(a) The bad news(1) Spatial and temporal heterogeneities exist at all scalelengths(2) Gaussian statistics (averages) are only appropriate in limited situations(3) Inability to extrapolate reliably from place to place(4) Inability to extrapolate reliably from time to time(5) Hence the expectation that any measurement may degrade with time(6) Possibility of long-range interactions between hydrocarbon reservoirs(7) Possibility of long-range interactions between earthquakes in different regions(8) Possibility of long-term interactions in hydrocarbon reservoirs(9) Possibility of long-term interactions between earthquakes in different regions(10) Behaviour of crustal deformation may not correspond to or be explicable by conventional geophysics

(b) The good news when the rock mass is responding to slow changes(1) Current configuration of crack geometry within the deep interior of the rock

mass or reservoir can be monitored with shear wave splitting(2) Current configuration of cracks in rock mass or reservoir can be evaluated by APE(3) Response of rock mass or reservoir to known changes can be calculated by APE(4) Response of rock mass or reservoir can be controlled by feedback by

repeating b1, b2 and b3, above

Crampin & Chastin 2001). There is much bad news and many con-ventional assumptions are invalid for analysis at higher temporaland spatial resolution. For example, the self-similar scaling meansthat: spatial and temporal heterogeneities exist at all scalelengths;Gaussian statistics (averages) are valid only in limited temporal andspatial domains; and it is not possible to reliably extrapolate fromplace to place or from time to time so that any given measurementmay degrade with time. Similarly, the sensitivity of criticality to ini-tial conditions means that there is the possibility of long-range andlong-time interactions between hydrocarbon reservoirs and betweenearthquakes in different regions.

There is also very good news. In at least some circumstances,the response to given phenomena can be modelled and predicted, asexemplified by the APE modelling of Angerer et al. (2000, 2002)and the successfully stress forecast earthquake of Crampin et al.(1999a). If the response of the rock mass to known changes can bepredicted, in some circumstances it may be possible to control theresponse by feedback (item b4 in Table 3).

Note that these advantages are only valid if natural processes aregiven time to evolve. Jensen (1998) shows that SOC behaviour maybe expected only in ‘slowly driven, interaction-dominated thresholdsystems’. Consequently, one essential ingredient to SOC is that thesystem evolves slowly from a marginally stable state of metastabilitytoward a threshold. In the Earth, the critical state at the thresholdis fracture criticality at the percolation threshold, when fracturing,faulting and earthquakes occur. When the threshold is reached thereis fracturing and the system relaxes to another metastable state. Theslow drive is necessary in order for the intrinsic properties of thesystem to have sufficient time to control the dynamics (a continuousstream of sand on a sand pile would not have the discrete avalanchescharacteristic of SOC). The reader is referred to Crampin & Chastin(2001) and references therein for further discussion.

4.3 The scatter in measurements of time delays

One of the remarkable features of the measurements of time delaysabove small earthquakes is the exceptionally large scatter of the datapoints, which still show temporal variations when averaged by least-squares lines (Crampin et al. 1999a; Volti & Crampin 2003a,b). Thetemporal variations have geophysical significance (for stress fore-casting earthquakes, Crampin et al. (1999a) even though the scatter

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is greater than the variation. Similar scatter is observed in almost allobservations of time delays above small earthquakes (Peacock et al.1988; Crampin et al. 1990, 1991; Liu et al. 1997 and further ex-amples in Volti & Crampin 2003a,b). The sensitivity of shear wavesplitting (Section 2) means that the scatter is likely to be variationsof small second-order quantities. However, the scatter is much toolarge to be due to errors in location, errors in local structure, or tomisinterpretations or misidentifications (Volti & Crampin 2003a,b).Consequently, the scatter appears to be a fundamental feature of allmeasurements, reflecting what is presumably a fundamental prop-erty of the behaviour of stressed, fluid-saturated, microcracked rock(Volti & Crampin 2003a,b).

(The only exceptions where significantly less scatter are observedoccur when the earthquakes are used as the shear wave source signalsare above isolated swarms (Booth et al. 1990; Gao et al. 1998)typically less than about 1 or 2 km in diameter (Crampin 1991).The reduction in scatter may be because the source-to-receiver raypaths are more restricted so that one source of variation is morelimited.)

The suggested behaviour is that because the pervading fluid-saturated cracks are a critical system, the effects, the distributions ofincreased aspect ratios in Fig. 3, are necessarily heterogeneous andcluster in time and space (Volti & Crampin 2003a,b). The deforma-tion mechanism in Fig. 3 is very sensitive to even nearly negligiblechanges of stress (Crampin et al. 2003a). The variations in earthand ocean tides are well known to cause changes in water well lev-els (presumably also due to the opening and closing of stress-alignedcracks). So even when stress is slowly accumulating before an earth-quake, the stress tensor is continually disturbed by the variations oftides.

Thus the proximity to criticality implies that there is a continuoustemporal and spatial adjustment or re-arrangement of the clusters ofdistributions of larger aspect ratios. (Such variations of clusteringare typified by Ising models of critical behaviour (see, for example,Bruce & Wallace 1989).) This means that shear waves, particularlyfrom varying earthquake foci, are likely to propagate along slightlydifferent paths as clusters vary in time and space, and in particularpass through crack distributions with significantly different crackaspect ratios. Consequently, the splitting displays a range of shearwave time delays. These clusters of different crack parameters alsocause the variations in shear wave polarizations, which are typicallyobserved to vary by 15◦–20◦ about the mean value (Peacock et al.1988; Crampin et al. 1990, 1991; Liu et al. 1997; Volti & Crampin2003a,b).

As stress increases, the clusters of increased aspect ratios getlarger and their separation, the correlation length ξ , the distancebetween the clusters of larger aspect ratios, also increases. As stressbuilds up, ξ increases until the correlation length spans the par-ticular structural region as fracture criticality at the percolationthreshold is reached, and the most prominent line of weakness, usu-ally a pre-existing fault, slips and the impending large earthquakeoccurs.

Although difficult to quantify, with too much scatter and notenough data, there is some indication that the scatter of time delaysincreases, in Band-1, as the impending earthquake approaches. Thiswould be expected as the characteristic length, ξ , of the disturbanceincreases and the potential for greater scattering also increased.

Note that the recent reports from the SMSITES Project in Sec-tion 6.3, below, show that the scatter is indeed the result of criti-cality. The scatter is caused by the 90◦ flips in shear wave polariza-tions in the high pore-fluid pressures on all seismically active faultplanes.

5 S H E A R - WAV E S P L I T T I N G — T H EF U T U R E

Here we try to update the answers to the last question of Crampinand Lovell, with specific implications: what should we do next?Some of the direct implications of crack-critical systems with SOCare listed in Table 3 and their practical implications are listed inTable 4. These various results have profound implications for muchof conventional reservoir characterization and hydrocarbon recov-ery, and much other solid-earth geophysics. The implications aremostly detailed, largely but not exclusively, nearly negligible effects,which can only be monitored by shear wave splitting. Consequently,if conventional understanding and accuracy are satisfactory and suf-ficient, the effects of the crack-critical system in the Earth’s crust andmantle may be largely neglected. However, if better, more detailed,spatial and temporal resolution is required, then the implicationsof crack-critical systems need to be exploited. If we wish to obtainmore than the (typically) 30–40 per cent of the oil in place usuallyrecovered, or if we wish to predict the time and magnitude of earth-quakes, then we need to take advantage of the good news in Table 3and exploit the practical implications in Table 4.

Note that Crampin & Chastin (2001) do not claim that the lists inTables 3 and 4 are inclusive: merely that there is some, often indirect,evidence for each listed item, so that Tables 3 and 4 are valid in thepresent state of our understanding. There will certainly be furtherbad news, good news and practical implications in the future, whenwe understand the crack-critical crust rather better than we do now.

The next four sections list four applications directly leading fromthe recognition of crack-critical systems. There are a wide varietyof other possibilities.

5.1 Monitoring hydrocarbon recoverywith single-well imaging

The observation that many reservoir parameters display self-similardistributions, such as crack distributions (Heffer & Bevan 1990),and the 1/f -noise characteristic of well-logs (Bak et al. 1987; Leary1991; Bean 1996), means that the larger the data sample is thelarger the possible variation. Consequently, Gaussian averages are nolonger valid (Table 3) (except in particular limited circumstances).Similarly, critical systems imply that detailed measurements nolonger have temporal and spatial stability, but vary with time andplace with temporal and spatial heterogeneity. This means that thewhole basis of conventional reservoir characterization is no longerwell-founded. To improve performance we need to devise newstrategies.

Crampin (1998) and Crampin & Chastin (2001) suggest (Table 3)that measurements are valid only at the time and place they aretaken. Additionally, all surface-based measurements, source and/orreceiver, are limited in resolution by passage through the absorp-tion and scattering in near-surface structures (the uppermost 500–1000 m, at least) (Leary & Abercrombie 1994; Leary 1995). Thismeans that passage through near-surface layers is typically limitedto frequencies less than typically 80 Hz for P waves and 20 Hz forshear waves. Consequently, to obtain measurements with sufficientresolution to improve on current practice, in hydrocarbon produc-tion, for example, measurements need to be made at the time (duringproduction) and place (the particular location in the reservoir) theyare required.

One way, possibly the only way, to measure such properties is bya single- or dual-well imaging time-lapse configuration (Crampin

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Table 4. Some practical implications of critical crack systems with SOC for fluid–rock interactions within the Earth’s crust (afterCrampin & Chastin 2001).

(a) General implications(1) Fluid-saturated crack distributions are highly compliant and crack geometry responds to small

nearly negligible changes of stress, pressure and other phenomena(2) Since fluid–rock properties vary with time, and vary from place to place, measured fluid–rock

properties are only strictly valid at the place and time they are measured. Hence, the need formeasurements with single-well imaging if accurate specifications are required

(3) Since fluid–rock interactions have a dominant effect on almost all physical and chemicalbehaviour within the crust and mantle (see a2, above), these various effects apply to a hugerange of geophysical phenomena, particularly those associated with any deformation, includingalmost all processes during hydrocarbon recovery

(4) Behaviour of stress-aligned fluid-saturated crack distributions appears to be remarkablyuniform (within certain limits) even in very heterogeneous structures

(5) Pre-fracturing deformation of any given fluid–rock configuration can be monitored byobservations of shear wave splitting

(6) Pre-fracturing deformation can be modelled by anisotropic poro-elasticity (APE)(7) Response of fluid–rock systems to known changes can be calculated by APE(8) Response to calculated changes (a6, above) can be monitored by shear wave splitting (a5, above),

and the response controlled by adjusting changes to optimized response

(b) Specific implicationsImplications for hydrocarbon exploration and production

(1) Reservoir properties may change from place to place(2) Reservoir properties may change with time, even without production processes(3) Relevant properties need to be measured at the place and time they are needed(4) Response to known changes can be calculated and predicted (Angerer et al. 2000, 2002)(5) Response of a reservoir can be controlled, in the sense of b4, Table 3(6) Possibility of long-range and long-time correlations across and between reservoirs (Heffer et al. 1995)(7) There is a limit to the resolution of any measurement

(c) Implications for earthquake geophysics(1) Deterministic prediction of time, magnitude, and place of large earthquakes is likely to be

impossible (Geller 1997; Kagan 1997; Leary 1997)(2) With sufficient source seismicity (Crampin et al. 1999a), or appropriate crosshole SMS

observations (Crampin 2001; Crampin et al. 2003a), times and magnitudes of future largeearthquakes can be stress forecast. Other information may than indicate location (Crampin et al. 1999a)

(3) In presence of sufficient source seismicity, or appropriate crosshole SMS observations, timesof future volcanic eruptions can be stress forecast (Volti & Crampin 2003a,b)

(4) There is the possibility of long-range and long-time correlations between earthquakes

(d) Implications for rock physics(1) Much of the behaviour in stress-cells in the rock-physics laboratory can be modelled and

predicted by APE (Crampin et al. 1997, 1999b; Chapman et al. 1998, 2000)

et al. 1993; Peveraro et al. 1994; Crampin 1999b). In such imaging,a string of three-component geophones in a producing well would beinserted behind casing, or behind tubulars, to record signals froma source (for example the DOV) pulsed in-line in the same well(single-well imaging) or in an adjacent well (dual-well imaging).The signals recorded within a production zone will be scattered fromthe internal structure within the reservoir and, except in fortuitouscircumstances there will be no reflections from approximately pla-nar reflectors to interpret. It is suggested that the only way to monitorthe detailed fluid–fluid, fluid–rock interactions during hydrocarbonproduction, which are likely to be dominated by the behaviour andstatistics of the crack-critical systems discussed in this paper, is bysingle-well or dual-well time-lapse imaging: analysing the changesin scattering induced by movements of fluid–fluid fronts within theproducing reservoir. Appropriate instrumentation for strings of re-ceivers and borehole shear wave sources in a single- or dual-wellconfiguration has recently become available. We suggest that manyof the instrumental developments for well imaging have been solved,and single- or dual-well deployments would be much cheaper alter-

natives to 4-D three-component reflection profiles, as well as provid-ing high-frequency signals and resolution in the appropriate partsof the producing reservoir.

5.2 Likelihood of greater hydrocarbon recovery at slowerproduction rates

The crack-critical crust has many implications for the behaviour ofthe reservoir. To take advantage of the good news in Table 3(b),a crucial requirement is that any induced change during produc-tion procedures must be sufficiently slow to leave time for naturalstress-relaxation phenomena to occur: for example, fluids must beallowed to percolate unforced. Calculable APE behaviour dependson SOC, and a necessary requirement for SOC is that the processshould be driven slowly (Jensen 1998). Fast changes, which do notallow stress relaxation, would deliver the bad news in Table 3(a)(the tendency for the reservoir to behave chaotically, irregularly andunpredictably) without any of the mitigating advantages (regular-ity, calculability and predictability). This is typically the current

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situation where maximum production rates are sought, and the sys-tem is overdriven so only a relatively small proportion of oil in anyreservoir can be extracted. Such overdriven fluid–fluid and fluid–rock interactions are irregular and unpredictable, and surface-basedseismics do not have sufficient resolution to monitor the progressin detail, hence the need for the single- or dual-well imaging ofthe previous section. There is likely to be true chaos in fast drivenprocesses, not merely calculable deterministic chaos. This suggeststhat producing reservoirs are likely to behave in more regular ways(and are more likely to achieve their expected production targets),if production rates are sufficiently slow to allow stress relaxation asproduction proceeds.

There are many examples of overmature reservoirs where noddingdonkeys continue to produce acceptable oil-to-water percentages atvery low production rates. This review suggests that such fieldsmay well produce a larger proportion of their initial reserves at slowproduction rates than fields with more aggressive production strate-gies. It is also suggested that to take advantage of a slower higherproduction requires slower production to begin from the outset ofproduction. It seems unlikely that a slower regime would be effec-tive once a field has been rapidly produced, and water breakthrough,and other probably irreversible processes had disturbed the initialfluid–rock structures.

Note that we do not know just how slow this slower productionneeds to be to take advantage of the good news in Table 3(b). Itmay be that even small (possibly marginal) decreases in productionrates could lead to substantially higher overall production. The high-pressure CO2 injection in Angerer et al. (2000, 2002) was matchedby APE modelling some 2 weeks after the injection had been com-pleted, which since APE depends on subcritical physics, presumablymeans it had reached an approximately steady state. Thus ‘slow’ inthis case may mean a comparatively short period of time (days orweeks rather than months or years). However, it is expected that op-timum rates will vary from reservoir to reservoir and from field tofield. To optimize production rates while maximizing oil productionrequires new production strategies while using single-well imagingto monitor the progress of fluid movements within the reservoir dur-ing production. To the best of our knowledge, such strategies havenot yet been attempted.

5.3 Stress forecasting (not predicting) earthquakeswith stress-monitoring sites

Mankind has been seeking ways to predict the magnitude, time andplace of earthquakes for over 120 years (Milne 1880) with a singularlack of success. Much of the research has been directed at seekingsome form of precursor, and a very large number of possibly precur-sory phenomena have been identified before earthquakes. However,no particular precursor has been consistently observed, and when aprecursor has been identified, it typically bears no quantifiable rela-tionship with the magnitude, time or place of the particular earth-quake with which it appears to be associated. It is now recognizedthat the earth is so complex and heterogeneous that the magnitude,time, and place of earthquakes are unlikely to be predicted deter-ministically (Geller 1997; Kagan 1997; Leary 1997, amongst manyothers).

The hypothesis that makes stress forecasting the time and mag-nitude of earthquakes possible is that rock is so weak to shear stress(see the discussion of Fig. 2), that the necessary stress increasebefore an earthquake builds up over an extensive volume of rock.Eventually a large volume of rock approaches fracture criticalityand the earthquake occurs at the weakest point which will typically

be on an existing fault plane. The evidence presented here sug-gests that the stress build-up, and the approach to criticality, canbe recognized at substantial distances from the eventual epicentre.The duration and rate of the increase are proportional and inverselyproportion, respectively, to the magnitude of the impending event,which occurs when the increase reaches fracture criticality (Volti &Crampin 2003a,b). This led to the successful stress forecast of thetime and magnitude of an M = 5 earthquake using swarm earth-quakes as the source of shear waves (Crampin et al. 1999a). Localseismicity correctly indicated the location.

SMSs use controlled-source crosshole seismics (Fig. 6) to moni-tor the state of the in situ crack distributions and their progress to-wards fracture criticality without the need for persistent swarms ofsource earthquakes (Crampin 2001). In principle, controlled-sourceseismology is capable of great precision in measuring shear wavesplitting (Li & Crampin 1992), so we can expect the source of thescatter of normalized time delays in Fig. 5 to be resolved, and theSELL windows to be much more tightly defined.

5.4 Other applications of shear wave splitting

There are two main types of application for shear wave splitting.Those involving: (1) static effects—the measurement and inter-pretation of the initial state of the rock mass; and (2) dynamiceffects—the measurement of changes in the rock mass by time-lapsetechniques.

(a) Static effects. It has been recognized for many years that thepolarizations of the faster split shear waves are aligned approxi-mately parallel to the direction of maximum horizontal stress andhence parallel to the maximum horizontal permeability in the rockmatrix through which the shear waves pass (Crampin 1981; Alford1986; Zatsepin & Crampin 1996). This means that the orientationsof hydraulic fractures, directions of water floods and other hydrocar-bon production strategies can be optimized by analysing shear wavesplitting. This strategy appears to be recognized by the oil industry,although not always adopted, as there are alternative techniques forobtaining similar estimates.

(b) Dynamic effects—monitoring toxic-waste and nuclear-wasterepositories. The case studies in Sections 3.1, 3.3 and 6.1, below,show that shear wave splitting is sensitive to otherwise negligiblechanges in rock mass conditions. This means that leakage or othersite instabilities in toxic-waste and nuclear-waste repositories couldbe detected by monitoring shear wave splitting with cross-hole time-lapse techniques. The optimum technique would be to set up bore-hole instruments to bracket the repository with ray paths from a DOVsource to three-component geophones. CW and CCW sweeps of theDOV would again generate orthogonal shear wave polarizations.Note that the ray paths should be far enough from the repository toavoid interface waves and to separate the most informative waves,the direct shear wave arrivals, from reflections and refractions fromside walls of the repository.

(c) Dynamic effects—monitoring slope and other near-surface in-stability. It is suggested that the APE mechanism for pre-fracturingdeformation (fluid migration along pressure gradients betweenneighbouring microcracks), while proven at depth, is also validfor near-surface deformation. The items in Table 2 suggest thatAPE deformation is based on a fundamental relationship betweenthe evolution of fluid-saturated microcracks and stress, which willbe present in all in situ materials, even poorly consolidated soils.It is expected that fracturing or failure only occurs when fracturedistributions reach fracture criticality. This means that monitoringshear wave splitting in appropriate near-surface geometries will also

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allow the approach of fracture criticality and local instability to berecognized.

A number of small-scale shear wave sources have been devel-oped, ranging from horizontal pistons, to a variety of oriented weightdrops and swinging hammers. These could be used to monitor thenear-surface microcrack structures for hillside and dam site stabilitytesting, as well as stability of foundations for buildings, dams andtunnels. There are many other examples of local instability problemswhere analysing shear wave splitting would allow the approach tofracture criticality and fracturing to be monitored.

6 O B S E RVAT I O N S F RO M T H E F I R S TS T R E S S - M O N I T O R I N G S I T E :C O N F I R M AT I O N O F T H EC R A C K - C R I T I C A L C RU S T

The first stress-monitoring site (Section 3.5) was developed in theEuropean Commission funded SMSITES Project at Husavık inNorthern Iceland, where the Husavık-Flatey Fault of the TjornesFracture Zone of the Mid-Atlantic Ridge runs onshore (Crampinet al. 2000, 2003a; Crampin 2001, www.smsites.org; see alsowww.glg.ed.ac.uk/∼scrampin/opinion).

The SMSITES Project has recently yielded four significant re-sults, which provide strong suppport for the arguments in thisreview.

(1) A data set of eight simultaneous measurements showing greatcompliance and sensitivity of in situ rock to small remote distur-bances (Crampin et al. 2003a).

(2) Evidence that seismically active fault planes are pervaded bycritically high pore-fluid pressures (Crampin et al. 2002).

(3) Explanation of the ±80 per cent scatter (as in Fig. 5) observedin measured time delays above small earthquakes (Crampin et al.2003b).

(4) Increases in time delays before large earthquakes show pre-cursory decreases immediately before earthquakes occur (Gao &Crampin 2003).

6.1 Eight simultaneous measures displaying stresssensitivity to small disturbances

A detailed calibration of the recording system at SMSITES, in whichthe highly repeatable downhole orbital vibrator borehole source waspulsed repeatedly every 12–20 s for 13 days, happened to coincidewith the start of a 2.5 day burst of low-level seismicity at 70 kmdistance (Crampin et al. 2003a). We recorded seismic traveltimespropagating horizontally at 500 m depth between boreholes 315 mapart. Stacked records yielded a resolution of ±20 µs. The boreholesare approximately parallel and about 100 m south of the major WNWto ESE striking Husavık-Flatey Fault, a transform fault of the Mid-Atlantic Ridge. The following traveltime anomalies in were recorded(Crampin et al. 2003a).

(1) P-wave traveltimes showed initially an abrupt 5 ms (6 percent) increase which then decayed linearly back to the original levelover 10 days.

(2) SH-wave traveltimes showed a 2 ms (2 per cent) reductionover 4 days in an ‘S-shaped’ relaxation curve typical of measure-ments of a phenomena relaxing after some disturbance.

(3) SV -wave traveltimes where 2 ms later than SH waves, butshowed a similar 2 ms (2 per cent) ‘S-shaped’ relaxation curve over4 days.

(4) SV–SH traveltime anisotropy showed a 0.2 ms (10 per cent)increase in the ∼2 ms time difference between the split shear wavesover 6 days.

(5) A continuously monitored water well on the island of Flateyimmediately above the HFF showed an abrupt 1 m reduction in levelfor 5 days superimposed on 40 cm ocean tide oscillations.

(6) Global Positioning System (GPS) measurements in a NS di-rection across the WNW to ESE trending HFF showed an abrupt7 mm extension, which decreased to the original level over about11 days.

(7) GPS measurements across HFF in an EW direction showeda similar 3 mm right-lateral extension which decreased to the initialposition after 4 days, and then increased to a permanent 4 mm EWextension in a displacement characteristic of the dextral HFF.

(8) The start of all these anomalies coincided with a 2.5 day burstof low-level swarm-type seismicity on the Grımsey Lineament, aparallel transform fault 70 km from SMSITES. The release of stressat these small earthquakes is thought to be the driving mechanismfor all the anomalies.

This data set is probably a unique imaging of the effects of low-leveldeformation on the rock mass. We were fortunate to record them.Continuous well level measurements at Flatey showed that the 1 m5 day ‘pulse’ in water level was the only such pulse in 15 months ofrecords.

The data set has not yet been fully interpreted and will be dis-cussed more fully elsewhere. In brief, some measurements appearto be compatible. Splitting into SH and SV waves, rather than othershear wave polarizations, is expected because propagation parallelto the fault is a symmetry direction. The 1 m drop in water levelis approximately what is expected from a 7 mm NS GPS exten-sion over microcracks and pores in a 200 m thick sandstone at thetop of which the seismic waves are propagating. However, there arepuzzling features: in particular, the different relaxation times of thevarious measurements.

Note the earthquakes mark abrupt releases of stress on theGrımsey Lineament. Such sensitivity to small disturbances at 70 kmdistance, the energy release of which is probably equivalent to oneM = 4 earthquake, is not expected in a conventional brittle elasticcrust. Thus the sensitivity is a direct confirmation that the crust ofthe Earth is a crack-critical system verging on fracture criticalityand failure.

Note that SMSs monitor changes within the interior of the rockmass. In particular, shear wave splitting monitors the approach of therock mass to fracture criticality when rocks are so heavily fracturedthat shear-strength is lost and fracturing, faulting, and earthquakesoccur. In contrast, GPS measurements monitor surface displace-ments. These are necessarily consistent with the overall movementsof tectonic blocks, as Michel et al. (2001) demonstrated in theiranalysis of GPS measurements before and after an earthquake inSumatra, However, GPS measurements do not monitor the approachof fracture criticality and earthquakes.

6.2 Critically high pore-fluid pressures on seismicallyactive fault planes

Seismic stations immediately above major faults, the San Andreasfault in California (Peacock et al. 1988; Liu et al. 1997) and nowthe Husavık-Flatey Fault in northern Iceland (Crampin et al. 2002),display 90◦ flips in shear wave splitting polarizations approximatelyorthogonal to the direction of regional tectonic stress and result infault parallel directions. Crampin et al. (2002) model these effects

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with APE and show that they can be interpreted as the effects ofcritically high pore-fluid pressures pervading the immediate vicinityof the fault.

The suggestion is that, as frequently noted, seismically activefault zones require high pore-fluid pressures to relieve friction andseparate asperities before slip can take place. Such 90◦ flips are onlyvisible above major faults because it is only above major faults thatmore than half of the ray path of a shear wave to seismic stations atthe surface is close to the fault in rocks pervaded by high pore-fluidpressures. Most earthquakes occur on small faults where 90◦ flipsonly occur near the fault, so that the majority of the ray path is innormally pressurized rock with normal not flipped polarizations.

6.3 Explanation of ±80 per cent scatter in measured timedelays above small earthquakes

The time delays in shear wave splitting recorded in the shear wavewindow above small earthquakes typically display a scatter of ap-proximately ±80 per cent about the mean (as in Fig. 5). It should benoted that observations of time delays in exploration seismics awayfrom earthquake source zones do not show significant scatter. Thismakes the scatter difficult to explain by conventional geophysics orerror analysis (Section 4.3; Volti & Crampin 2003a). Such smallearthquakes are typically generated by slip on comparatively smallfaults. The suggestion is that although 90◦ flips are present, causedby the high pressures, when propagating close to the fault, the ma-jority of the ray path to the surface is in normally pressurized rockswhich do not show flipped polarizations. Since in general, the nor-mally pressurized ray path is longer than the high-pressure segmentin the vicinity of the fault, the typical tectonic-stress aligned polar-izations are observed at the surface.

The time delay at the surface is the difference of the effects of thehigh-pressurized ‘flipped’ segments and the normally pressurizednormally polarized segments of the ray path. Crampin et al. (2003b)model these effects and show that small differences in triaxial stressand pore-fluid pressure can cause a ±80 per cent scatter. Since everyearthquake modifies the triaxial stress and pressure fields by somestress release, even small changes in triaxial stress and pore pressurecan easily cause the observed ±80 per cent scatter (Crampin et al.2003b). The widespread universal observations of this ±80 per centscatter suggests that, not surprisingly, all earthquakes are generatedby slip on critically high pressurized fault planes.

Although we have identified the source of the scatter, the scattercannot be eliminated. To calculate the scatter requires extremelydetailed knowledge of the geological and geophysical structure, de-tails of the irregularities on the fault plane, and the stress and porepressure in the earthquake source zone and along the whole of theray path to the recorder. Clearly, we are never able to model theseeffects in sufficient detail.

6.4 Stress relaxation in the earthquake source zone beforethe earthquake occurs

We have established in the main text that increasing time delays inBand-1 of the shear wave window monitor the effects of accumu-lating stress on crack aspect ratios. Although several anomalies hadbeen recognized, previously it had been thought that time delays inBand-1 increased until stress was released at the time of the earth-quake (Crampin 1999a). Reappraisal of the data sets shows thatwhenever there is adequate data to record the phenomenon there isa precursory decrease in time delays before the earthquake occurs.This decrease has only been observed within a few fault diameters

of the source and is believed to be caused by some form of stressrelaxation in the source zone.

The duration of the precursory decrease varies with earthquakemagnitude from a few tens of minutes before mb ≈ 2 events in north-ern Iceland to over 2 months before the 1988 Ms = 6 North PalmSprings earthquake in California. The plot of magnitude against logof the time duration is linear. Similar precursory decreases are alsoseen in samples in laboratory stress cells before fracturing occurs(Gao & Crampin 2003).

6.5 Interpretation

Taken together, these four results are a major confirmation and exten-sion of the ideas advanced in this review. In particular, they indicatethe relevance and universality of the APE model for the evolution offluid-saturated microcracked rock to changes of stress in a compli-ant crack-critical system. The results confirm that in situ rock, in atleast the crust of the Earth, is extraordinarily compliant as a resultof the critical system of fluid-saturated microcracks, as well as con-firming the science and the technology of stress-monitoring sites forstress forecasting the times and magnitudes of large earthquakes.

An explanation for the ±80 per cent scatter in time delays abovehas been sought for some 15 years since the phenomenon was firstrecognized by Peacock et al. (1988). It is encouraging that such along-standing problem seems to have been satisfactorily resolved.

It appears that we are beginning to understand shear wave split-ting. We suggest that these recent advances in shear wave splitting,together with the suggestions in the main text, present several newopportunities for monitoring, calculating and predicting rock massdeformation.

7 C O N C L U S I O N S

We have shown how the effective elastic constants of fluid-saturatedmicrocracks are extremely sensitive to fluctuations in the stress field.The response is strongly non-linear and much more sensitive thanlaboratory tests on small samples. There are four major conclusionsof this survey of seismic shear wave anisotropy.

(1) Shear wave splitting is highly sensitive to changes to the fluid-saturated stress-aligned grain-boundary cracks and pore throats per-vading almost all in situ rocks in the crust (and upper mantle).

(2) The detailed internal geometry of this microcracked rockmass is very sensitive to small nearly negligible changes in in situconditions, which cause observable changes to shear wave splitting.

(3) The evolution of stress-aligned fluid-saturated microcrackscan be modelled, sometimes with great accuracy, by APE, where themechanism of deformation is fluid movement along pressure gradi-ents between neighbouring grain-boundary cracks and pore throatsat different orientations to the stress field. APE modelling matches ahuge range of static and dynamic phenomena. Since APE is highlyconstrained with no free parameters, the response of the rock massto known conditions can be calculated and in some circumstancespredicted.

(4) The underlying reason for the calculability and predictabilityof a complicated heterogeneous crust is that the rock mass is so heav-ily cracked that it can be considered as a critical system. This hasprofound effects on many aspects of conventional solid-earth geo-physics, some of which are listed in Tables 3 and 4. In particular,many parameters of the rock mass are controlled by the behaviourof fluid-saturated microcracks, rather than the complicated hetero-geneous subcritical rock.

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Taken together these four conclusions amount to a new under-standing of low-level rock deformation, where the deformation of insitu rock in response to changing stress is controlled by the changesin the geometry of the stress-aligned fluid-saturated microcracks.If current usage in resolution and accuracy is sufficient, the abovefour items above can be ignored. However, if we wish to understanddetailed fluid–rock interactions (including shear wave splitting), toextract more oil from a reservoir, or to forecast the time and mag-nitude of earthquakes, the good news in Table 3 must be exploited,in the ways suggested in Section 6.

We suggest this could be a turning point for solid-earth geo-physics. The ideas outlined in this review are believed to be a majoradvance in understanding rock mass pre-fracturing deformation,with some disadvantages but with some important new propertiesto exploit. This is believed to be a fundamental new understandingof rock mass deformation with totally new applications. This is ageophysical (and geological) renaissance as Davis et al. (1997) oncesuggested for a small part of this revolution. We suggest that thesedevelopments could lead to the practice and techniques of both ex-ploration and earthquake geophysics being significantly different ina few years time.

However, the interior of the Earth is remote, toxic, subject tohigh temperatures and pressures and is remarkably difficult tounderstand—witness some of the results in this review which are,we suggest, new and surprising, even after half a century of inten-sive geophysical investigations, including some 20 years of analysisof shear wave splitting. Some of the applications in Sections 5 and6 are (observation-based) speculations. We should be interested inany information or plans that may add data to help to realize thesespeculations and advance this revolution in solid-earth geophysics.

A C K N O W L E D G M E N T S

This paper was partially supported by the European Commis-sion SMSITES and PREPARED Projects, contract numbers EVR1-CT1999-40002 and EVG1-CT2002-00073, respectively. The vari-ous ideas in this paper have developed over many years in manydiscussions with many persons too numerous to mention individu-ally. However, we should like to acknowledge our immediate col-leagues Yuan Gao, Peter Leary, Xiang-Yang Li, Enru Liu, DavidTaylor, Theodora Volti and Sergei Zatsepin, and thank them for theinvaluable part they have played and are playing in the developmentof these ideas.

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