The variation of delays in stress-induced anisotropic polarization anomalies

Abstract

Polarization anomalies in seismic shear wavetrains, diagnostic of propagation through anisotropic media, have now been observed in dilatancy zones in seismic regions. Stress-induced dilatancy will open cracks with preferred orientations, which will be effectively anisotropic to shortperiod seismic waves. The polarization anomalies are due to the shear waves splitting, in propagation through anisotropic media, into components with different polarizations and different velocities. This writes characteristic signatures into the shear wavetrains. The paper examines ways in which the differential shear-wave anisotropy (the delay between the split shear-waves) varies with direction by plotting stereograms of the relative delays, and their polarizations, for possible dilatancy symmetry-systems. It seems likely, that if sufficient observations of these anomalies can be obtained at each stage of the dilatancy episode, it w i l l be possible to estimate the symmetry directions of the dilatancy and the geometry of the stress-induced crack-system, as well as monitoring the progress of the dilatancy episode.

Shear-Wave Splitting in a Critical Crust: the Next Step

“…Since the TIV-anisotropy of shales and clays may be strong (up to 30% or greater), comparatively small dips to strata can result in significant azimuthal anisotropy (Crampin and McGonigle, 1981). Such dip-induced azimuthal anisotropy is almost certainly present in some of Winterstein's data in Anisotropists Digest, 147.…”

Investigation of upper crust anisotropy in Ghaen-Birjand region, east-central Iran

“…Crampin (1978) emphasized the role of aligned microcracks as a cause of crustal anisotropy and pointed out that for vertically aligned microcracks the direction of the faster shear wave polarization and the delay time between faster and slower shear waves are directly related to the direction of crack-alignment and its density, respectively. Crampin and McGonigle (1981) proposed an effective method using a diagram of horizontal particle motions. Shear wave splitting technique has been applied to observation of crustal anisotropy for various regions in the world (Savage, et al, 1990;Kaneshima, 1990;Gledhill, 1993), and to theoretical studies Liu, et al, 1993) during the last decade.…”

Shear-wave splitting observed above small earthquakes in a geothermal area of Japan

“…Such a type of crack distribution cannot cause a parallel alignment of the faster-split shear waves with small incident angles at the surface (Crampin & McGonigle 1981, however, observed the parallel patterns of the faster shear-wave polarizations in the Peter the First Range, near Garm, in the Soviet republic of Tadzhikistan, with a thrust-fault tectonic regime. The direction of the polarizations was perpendicular to the maximum horizontal compression derived from fault-plane solutions.…”

“…We shall, hereafter, consider only the case of liquid-filled cracks. Theoretical works (Crampin 1978;Crampin & McGonigle 1981) show that a distribution of liquid-filled vertical microcracks uniformly aligned in the direction of the horizontal maximum compressive stress can explain observed shearwave polarization patterns. We consider the case of thin circular cracks, in which a free parameter is the crack density (nu') where n and a are the number density and typical radius ot cracks, respectlvely.…”

“…Shear waves propagating through the medium split into two orthogonally polarized shear waves, one polarized in the crack plane, and the other in the plane of the seismic rays' propagation (Crampin 1978). If the cracks are vertical, whether they are dry or filled with watery liquid, the polarization directions of the faster-split shear waves are parallel to the strike of the cracks for seismic rays with small incident angles (a little under 40" for dry cracks, and 30" for liquid-filled cracks) (Crampin & McGonigle 1981). Locus of ground motion in the horizontal plane due to the faster-split shear wave abruptly changes its form as the slower shear wave with orthogonal polarizarization arrives.…”