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

Summary. 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 P and first S -wave arrival times.