above factors, it is necessary to analyze each situation in predicting possible ground motions and structural responses. One way to express the effects of ground motion is in units of “g.” This refers to the acceleration that a freely falling body experiences on earth, i.e., 32 feet per second change in velocity for each second that the acceleration occurs. As a “rule of thumb”—the threshold of ground motion that may be perceptible to humans is one-thousandth (‘/1000) of a “g.” Ground motions can be accentuated at higher places such as tall buildings. As another “rule of thumb,” one-tenth of a “g’”is frequently accepted as the criterion for threshold of property damage. However, this is based on damage from earthquakes and pres- ent data show that seismic waves generated by nuclear detona- tions and chemical high explosives result in less damaging effects than would be predicted for the same peak acceleration from an earthquake. Part of this difference may lie in the fact that ground motions from earthquakes persist for a longer period of time for each shock. Also, there are repeated shocks in most cases. Thus, structures are subjected to more damaging effects because of the number of shocks and greater dura- tion of each shock than would be the case for the same peak acceleration experienced as a result of ground motion from an underground nuclear explosion. Since nuclear detonations produce ground motions, it has been speculated that they may“trigger” a natural earthquake. It is not possible to have a natural earthquake, however, without prior storage of strain energy—a process that occurs over a period of years. It would be necessary to conduct an explosion several miles deep in an earthquake susceptible area to be near a zone wherethe stress might be great enough for an incipient quake to be triggered.4 The response of structures to earthquakes has been the subject of study for many years and satisfactory procedures have been developed for design of structures to withstand the effects of earthquakes. However, in these cases the interest is in significant structural damage, rather than plaster crack- ing or other minor effects. In the case of underground nuclear explosions thesite is selected with safety in mind so that struc- tures outside the test area will not ordinarily be subjected to ground motions of more than small amplitude. The possibility that light damage may result, therefore, must be considered. The Data The maximum range at which seismic waves from the largest nuclear detonations to date at the Nevada Test Site are 28 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 nowavailable for many underground nuclear explosions. Analyses of data and application of geophysical principles are resulting in a steady improvement - in methodsof prediction of ground motionsfor 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 approachis 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.