Variations in phase velocity of Rayleigh waves from the Samoa earthquake of April 14, 1957 are reported for the United States. These variations are correlated with topography and Bouguer gravity anomaly on a continental scale, demonstrating regional iso-static compensation. The correlation of phase-velocity variations with crustal-thickness changes is justified, and permits specification of the mechanism of compensation as the regional Airy system.

Regional average crustal thicknesses are: Peninsular Ranges and Southwestern Desert, 40 km; Basin and Range Province, 48 km; Rocky Mountains, 47 km; Interior Plains, 35–41 km; Appalachian Mountains, 40km.

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