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- Volume 3, Issue 3, 1985
First Break - Volume 3, Issue 3, 1985
Volume 3, Issue 3, 1985
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Evidence for aligned cracks in the Earth's crust
By S. CrampinThree-component records of shear waves from small earthquakes recorded within the shear-wave window at the surface and shear waves recorded below the surface in three-component VSPs almost routinely display shear-wave splitting indicating some form of effective anisotropy. This splitting appears to be the result of liquid-filled cracks throughout at least the brittle, top 10-20 km of the crust, where the cracks are aligned with respect to the direction of the principal axes of stress. Liquid-filled cracks with small aspect ratios have very little effect on P-wave propagation, but may have major effects on the polarisations of shear-waves. Fluids are a very common constituent of the crustal rock mass (Fyfe, Price and Thompson 1978). Water is contained in the pore space of sedimentary rocks when they are first deposited, and every prograde metamorphic process releases chemically-bound water into microcracks of the otherwise intact rock mass. Deep wells (Kozlovsky 1984) find water-filled fractures deep in the crust, and even upper mantle xenoliths are pervaded by fluid-filled microcracks, where the fluid is largely CO2 (Andersen, O'Reilly and Griffin 1984). We shall usually refer to such cracks in the top half of the crust as 'waterfilled' but recognise that they may be oil-filled in hydrocarbon reservoirs. The major difficulty in accepting the proposition that such cracks are common is that seismic investigations, which are the most direct way we have of evaluating the properties of rocks in situ, have previously displayed little evidence for widespread cracking. The reason most seismic investigations in the past, including the detailed surveys by the hydrocarbon industry, have not found evidence for widespread cracking is because these studies have usually been confined to P-waves, and water-filled cracks have little effect on P-wave propagation. In contrast, the behaviour of shear-wave particle displacements (polarisations) is very sensitive to crack distributions whenever the cracks display preferred orientations. This paper reports recent observations of shear waves in seismic zones by my colleagues and myself at the British Geological Survey (Crampin et al. 1980; Booth et al. 1985; Crampin and Booth 1985; Crampin et al. 1985b) and in stable continental areas by the hydrocarbon industry, that display a number of phenomena which suggest that distributions of aligned water-filled cracks are widespread in (at least) the brittle, top 10 to 20 km of the crust. Monitoring the behaviour of such extensive dilatancy anisotropy (EDA) by the analysis of shearwave polarisations has been suggested as a way of monitoring the build-up of stress before earthquakes (Crampin et al. 1984).
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Microcracks in the Earth's crust
Authors S. Crampin and B.K. AtkinsonThe presence or absence of cracks within in situ crustal rocks is open to wide misunderstandings because of the inaccessibility of the material and the difficulty of reproducing in situ conditions in the laboratory. There is now evidence from a wide range of results that most crustal rocks are pervaded by aligned liquid-filled microcracks, where the liquid is usually water but may be oil in hydrocarbon reservoirs. The recognition that there are aligned liquid-filled microcracks deep within the crust ties together a number of previously contradictory phenomena. Liquid-filled cracks in the Earth's crust, although generally recognised as being present (Brace 1972, 1980), are little understood and the implications of their behaviour have not been explored. This is partly because it is impossible to reproduce in laboratory conditions more than a few of the large range of independent phenomena controlling the existence and behaviour of cracks in rock in situ. The liquid in these cracks is usually a water solution but may be oil in hydrocarbon reservoirs. The other major source of misunderstandings about cracks in the crust is that the principal technique for examining the properties of rocks at depth in the crust has been the analysis of travel times of seismic bodywaves, and reflection and refraction experiments by the exploration industry yield consistent structural interpretations without any need to assume the existence of widespread cracking. We now recognise that this is because almost all seismic experiments in the past have used P-waves, and P-wave travel-times are very little affected by liquid-filled cracks with low aspect-ratios. In contrast, shear-wave splitting (shear-wave birefringence) is very sensitive to distributions of aligned cracks (Crampin 1978). Recent observations of shear-waves (Crampin et al. 1980; Crampin 1985a) suggest that parallel, vertical, water-filled micro cracks pervade much of the brittle, upper 10-20 km of the crust. We call such distributions of aligned fluid-filled cracks extensive dilatancy anisotropy (EDA) (Crampin, Evans and Atkinson 1984). EDA is a unifying concept that allows a variety of phenomena from geology, geophysics and the rock-mechanics laboratory to be correlated for the first time. The existence of such cracks has wide implications for all deformatory processes in the crust, and the ability to monitor crack geometry by shear-wave propagation has applications to many currently important activities ranging from determining preferred directions of flow in hydrocarbon reservoirs to earthquake prediction.
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)