Abstract

The anisotropic characteristics of stress-induced cracks in dilatant rock can significantly influence the observed values of ts/tp obtained using the Wadati diagram. Systematic vertical migration of hypocenters of foreshocks within a region of thrust tectonics can lead to variations in both ts/tp and orientation of the compressive stress axis of the same type as observed before several thrust-type earthquakes. In this model, the seismic velocities become considerably anisotropic due to preferential alignment of open cracks, but it is not essential to assume pre-earthquake flow of groundwater into the dilated zone. Observations of ts/tp versus depth of focus in the Garm region, USSR and some other results appear to support this possibility.

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