D. GROUND MOTIONS - EARTHQUAKES Background Information A wide variety of factors determine both the ground motions and structural responses trom“ucleardetonationeLé7—Ghétgy “9? the detonations, distance from ground zero, depth of the shot and depth of measurement, and the nature of the ground (hard rock, etc.). "Competent" rock such as granite couples and transmits more energy into seismic ground waves than does alluvium - a non-cohesive sedimentary deposit, Although ground waves will be more rapidly absorbed in alluvium, it is possible for waves to travel great distances along the surface with relatively large amplitudes (amount of motion) alluvium is very thick. if the However, these surface waves die out rapidly with depth into the ground. Because of the above factors, it is nec- essary to analyze each situation to predict 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 experi- ences 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 (1/1000) 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 Gi