Shear-wave polarizations in the Peter the First Range indicating crack-induced anisotropy in a thrust-fault regime

Abstract

Three-component seismograms of small local earthquakes recorded in the Peter the First Range of mountains near Garm, Tadzhikistan SSR, display shear-wave splitting similar to that previously observed near the North Anatolian Fault in Turkey. The Peter the First Range is in a region of compressional tectonics, whereas the North Anatolian Fault is a comparatively simple strike-slip fault. Detailed analysis of the Turkish records suggests that the splitting is diagnostic of crack-induced anisotropy caused by vertical microcracks aligned parallel to the direction of maximum compression. Preliminary examination of paper records from Garm shows that most shear waves arriving within the shear-wave window display shear-wave splitting, and that the polarizations of leading shear-waves are consistently aligned in a NE/SW direction. The area is complicated and the tectonics are not wellunderstood, but the NE/SW direction is approximately perpendicular t o the compressional axis in many of the fault-plane mechanisms of the earthquakes. These earthquakes are usually at depths between 5 and 12 km, although there are some deeper events nearby.Parallel shear-wave polarizations, such as those observed, are expected to indicate the strike of nearly vertical parallel microcracks, which would be aligned parallel to the direction of maximum compression. Thus the shearwave polarizations in the Peter the First Range indicate that the directions of principal stress are reversed in the rock above the earthquake foci where thrust faulting is taking place.

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

“…The general match of APE-modelling in Table 2 is, at least, consistent with observations over an enormous range of frequencies (from MHz in the laboratory to kHz-5 Hz in the field (Holmes et al, 1993;Crampin et al, 1986b; respectively)), and dimensions from wavelengths of a few millimetres in the laboratory to 80 cm-hundreds of kilometres in the field (Holmes et al, 1993;Heffer and Bevan, 1990;respectively). The list is rapidly increasing (note the number of recent publication dates in the listed references).…”

Seismic imaging of the geothermal field at Krafla, Iceland using shear-wave splitting

“…To avoid potential ambiguity, it is worthwhile to mention that in an anisotropic medium, aligned fast shear-wave polarization orientations are independent of the initial polarization of the shearwave at the source and are mainly caused by the medium's anisotropy (e.g. Crampin et al, 1986;Peacock et al, 1988;Crampin and Lovell, 1991).…”

“…The phenomenon is very similar to optical birefringence, whereby light transmitted through an anisotropic crystal undergoes analogous splitting and polarization parallel and perpendicular to the alignment of atoms in the crystal lattice. In the seismic case, the polarization direction of the fast split shear wave parallels the strike of the predominant cracks regardless of its initial polarization at the source (Crampin et al, 1986;Peacock et al, 1988). The differential time delay between the arrival of the fast and the slow shear waves (typically a few tens of milliseconds) is proportional to crack density, or number of cracks per unit volume within the rock body traversed by the seismic wave (Hudson, 1981;Crampin, 1987;Crampin and Lovell, 1991).…”

Stress Field of the Earth’s Crust

“…Wave polarisation and the delay between the two phases vary with azimuth, allowing the symmetry of the anisotropy to be identified. Crampin et al (1986) suggested that stress aligned fluid-filled cracks (with dimensions of less than one wavelength of the S -wave) are the cause of shear wave splitting . Both Crampin et al (1986) and Peacock et al (1988) report observations of temporal variation in shear-wave splitting, over periods of years, interpreted as due to changes in the microcrack population induced by stress change.…”