The effects of crack anisotropy on the hypocentral locations of local earthquakes

Abstract

Exact modelling of the earthquake location process in regions of dilatancy-anisotropy shows that failure to take account of the velocity anisotropy in the determination of local hypocentres can result in erroneous and misleading locations. In particular, the locations can indicate spurious migrations of foci from the true epicentral positions and the true depths of foci. These spurious locations may indicate planes of hypocentres deviating from the true fault plane. Observations of such phenomena have been noted several times in the literature. However, once the anisotropic model is known, a simple location program, incorporating the anisotropic velocity-variations, permits accurate location of local earthquakes using Pand first S-wave arrival times.

Shear-wave polarizations near the North Anatolian Fault -I. Evidence for anisotropy-induced shear-wave splitting

“…However, it must be noted that the focal depths of earthquakes located mainly by P-wave arrivals from the upper focal sphere are not well controlled as there is a trade-off between origin time and focal depth. In addition, Doyle, McGonigle & Crampin (1982) show that the focal depths of local earthquakes in effective anisotropic structures appropriate to the T D P events can be in error by up to 2 km if they are calculated under the assumption that the structure is isotropic. Such mislocations in depth (epicentres are better controlled) will have little effect on measurements of the split shear waves, but they will affect the estimated angle of incidence .…”

Wave velocities in a pre-stressed anisotropic elastic medium

“…Otherwise, the stress is difficult to measure, in situ even in the most accessible parts of the crust. • The anisotropic group velocity observations can be used to determine more accurate hypocentral locations of local earthquakes (Doyle et al 1982). • The extent of fracturing in a region of a borehole is a vital factor in the extraction of oil and of geothermal heat.…”

Seismic anisotropy -- the state of the art: II

“…On a smaller scale, Doyle, McGonigle & Crampin (1982) show that, if local earthquakes in an anisotropic structure are located by programs assuming isotropy, both focal depth and epicentre may be systematically mislocated while still given apparently satisfactory residuals and error estimates.…”