Rayleigh wave dispersion data on Australia by Bolt and Niazi (1964) and Thomas (1969) are examined in detail in the period range of 20 to 40 sec. The dispersion characteristics correspond well to region 7 in Santô's (1965) classification. A further and similar inspection of the African mass, Brazilian shield, Canadian shield etc. reveals that the entire family of shield areas systematically belongs to Santô's regions 6 and/or 7. They are uniformly characterized by the absence of appreciable lateral velocity gradients in clear contrast to corresponding extremely steep gradients for high seismicity areas.

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