The pre-earthquake dilatancy, postulated to explain certain premonitory seismic velocity changes, may provide a preferred direction of groundwater flow persisting after the occurrence of a shallow earthquake. This direction is parallel to and nearly coincident with the compressive stress axis associated with the focal mechanism of the principal shock. Aftershocks may result from a redistribution of pore pressure and fluid flow not only along faults but also along this direction of relatively large permeability. This suggestion provides a possible explanation for certain observations of aftershocks aligned along the compressive stress direction.

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