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.