known to have been perceived by persons without benefit of instruments has been about 100 miles. These few persons were situated under conditions favorable to the amplification of the ground motions. No structural damage from ground motion has been experienced beyond about six miles from the site of the nuclear detonations. Evaluation Records of ground motion are now available for many underground nuclear explosions. Analyses of data and application of geophysical principles are resulting in a steady improvement in methods of prediction of ground motions for planned events. Since ground motions from underground nuclear explosions are different in some respects from those from an earthquake and there is a need to predict marginal damage to structures for such explosions a new approach is required. The analytical procedures for structural response generally are valid and can be applied. Additional direct test information is required and is being acquired by the AEC. Until more data are developed, conservative estimates of the effects may be made by comparison with damage which might be expected from the same amplitude of ground motion in an earthquake. 29