ABSTRACT: A history of anomolous crustal motion precedes major earthquakes. Grustal movements prior to the Kanto (1930; M = 6.9), Nigate (1964; M = 7.0) and Izu-Oshima (1978; M = 6.8) earthquakes in Japan and the Tangshan (1.976; M = 7.8), Haicheng (1976; M = 8.0) and Songpan (1976; M = 7.2) erathquak.es in China are only a few examples. Similar movements have also been reported in Southern California. Experimental studies suggest rock deformation under simulated crustal conditions produces a combination of elastic and inelastic volume dilatancy. Field evidence for gravity decrease, water expulsion and radon emission in seismic regions suggests a mode- of volume dilatancy occur during precursory stages of major earthquakes. Preseismic cmstal movements can therefore be interpreted as surficial expressions of volume dilatancy at subcrustal levels in a `slip-deficient seismic zone.