Observations of dilatancy-induced polarization anomalies and earthquake prediction

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

“…The breakthrough in studies of anisotropy in the Earth came when theoretical studies [1,[13][14][15][16] demonstrated that stress-aligned shear-wave splitting was the most diagnostic phenomenon for recognising anisotropy in cracked rocks, and such stress-aligned shear-wave splitting was commonly observed in the Earth [2,3,17,18]. Shear-wave splitting, analogous to the birefringence of light in crystals, occurs when shear waves enter a system of aligned cracks.…”

“…The underlying reason for those developments was that observations of three-component seismograms had shown evidence of shear-wave splitting [2,3], and the only possible source of anisotropy that satisfies all observations is fluid-saturated stress-aligned microcracks [19,38,39]. Almost all suitable three-component seismograms display shear-wave splitting in almost all in situ rocks.…”

“…Field observations of SWVA b (below 500-1000 m depth) (1) SWVA in all rocks independent of porosity and geology (2) Minimum SWVA in intact rock: observed ≈1.5%; APE-modelled ≈1.0% (3)…”

“…Other field observations (6) Fracture-criticality limit specifies crack distributions with a range of dimensions of about nine-orders of magnitude (7) π/2 shear-wave polarisation changes (90 • -flips) in over-pressured reservoirs Dynamic effects Temporal changes in SWVA during production procedures (8) Changes between measurements made before and after hydraulic pumping tests (9) Changes between measurements made before and after both high-pressure and low-pressure CO 2 Variations of shear-wave properties in laboratory experiments (13) Ultrasonic variations of SWVA and permeability in uniaxial stress cell (14) Ultrasonic variations of (isotropic) shear-wave velocities to changes in confining pressure and pore-fluid pressure for oil-, water-, and gas-(dry) saturations in sandstone cores in stress cells (15) Ultrasonic variations of velocity and attenuation from sonic (transducers) to seismic (resonant bar) frequencies a Anisotropic poro-elasticity. b Shear-wave velocity-anisotropy.…”

“…This present review reports the progress of over 20 years study of shear-wave splitting in an anisotropic Earth. Stress-aligned anisotropy-induced seismic shear-wave splitting (seismic birefringence or double refraction), first positively identified in 1980 in both the Earth's crust [2] and upper mantle [3], has now been widely observed and investigated. There are several thousand papers on the theory and observation of shear-wave splitting in the Earth, and shear-wave splitting is an anticipated phenomenon in almost all crustal and upper-mantle rocks.…”