Surface motion from large underground explosions

Abstract

Seismic effects of several underground nuclear explosions were measured in terms of ground surface motion by suitable seismographs from 1200 ft to nearly 10 miles from the source and with teleseismic instruments at great distances. Prior to the Rainier explosion (a 1.7-kt nuclear shot detonated 900 ft underground) empirical formulas were developed which predicted ground effects from the Rainier shot and several of the larger HARDTACK II shots with fair accuracy but with certain limitations. The limitations were (1) that at distances greater than a few thousand feet, observed displacements were somewhat larger than the formula predicted, necessitating revision of the formula; (2) that frequencies of ground waves did not exceed 20 cps; and (3) that the source conditions and material were in fair duplication. Ground amplitudes on deep alluvium were, as expected, more than twice the amplitudes at nearly the same distance on rock. Velocity response spectrums of one of the shots have been made and reproduced. The magnitude of the Rainier shot was about 4.0, based on the assumption that the source was contained in a volume of rock comparable to that of an earthquake having the same magnitude. Local travel-time data indicate that the subbasement rock associated with a speed of about 6.2 km/sec is about 3600 ft beneath the shot points area.