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A review of shear-wave splitting in the compliant crack-critical anisotropic Earth

Wave Motion
Publication Date
DOI: 10.1016/j.wavemoti.2004.05.006
  • Crack-Critical Systems
  • Earth’S Crust And Upper Mantle
  • Seismic Anisotropy
  • Seismic Birefringence
  • Shear-Wave Splitting


Abstract Shear-wave splitting due to stress-aligned anisotropy is widely observed in the Earth’s crust and upper mantle. The anisotropy is the result of stress-aligned fluid-saturated grain-boundary cracks and pore throats in almost all crustal rocks, and we suggest by stress-aligned grain-boundary films of liquid melt in the uppermost 400 km of the mantle. The evolution of such fluid-saturated microcracks under changing conditions can be modelled by anisotropic poro-elasticity (APE). Numerical modelling with APE approximately matches a huge range of phenomena, including the evolution of shear-wave splitting during earthquake preparation, and enhanced oil recovery operations. APE assumes, and recent observations of shear-wave splitting confirm, that the fluid-saturated cracks in the crust and (probably) upper mantle are so closely spaced that the cracked rocks are highly compliant critical systems with self-organised criticality. Several observations of shear-wave splitting show temporal variation displaying extreme sensitivity to small stress changes, confirming the crack-critical system. Criticality has severe implications for many Solid Earth applications, including the repeatability of seismic determinations of fluid flow regimes in time-lapse monitoring of hydrocarbon production. Analysis of anisotropy-induced shear-wave splitting is thus providing otherwise unobtainable information about deformation of the inaccessible deep interior of the Earth.

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