_4 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 present data show that seismic waves generated by nuclear detonations 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 duration 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 necéssary 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. The response of structures to earthquakes has been the subject of study for many years andsatisfactory 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 cracking or other minor effects. In the case of underground nuclear explosions the site is selected with safety in mind so that structures outside the test area will not ordinarily be subjected to ground motions of more than small amplitude. The pos- sibility that light damage may result, therefore, must be considered. ate The Data The maximum range at which seismic waves from the largest nuclear detonations to date at the Nevada Test Site are 28