Recapitulation
An upward thrusting of masses of granitic rocks with accompanying slumping and partial upending of intermediary sedimentary and volcanic rocks has formed some of the ranges and intermontaine valleys of the Colorado River region westward from the mouth of the Grand Canyon. The Hemenway-Las Vegas-Callville Wash valley now occupied in part by the Callville basin of Lake Mead, a few miles north of Boulder Dam, is one of these intermontaine valleys, which has settled on faults separating this valley from the Black Mountains to the east and southeast, and on suspected faults which possibly are buried under detrital material along the southwest margin of the valley. These faults probably have been quiescent during Pleistocene and Recent times. However, we have strong evidence that this activity is being revived under the weight of the recently created Lake Mead. This lake has caused the underlying crustal blocks to be depressed probably several inches. In particular, the lower basin of the lake, which contains nearly one-third of the water load, has caused the block thereunder to undergo a downward movement along the above-mentioned faults, while the granitic masses on the opposite sides of the faults have remained relatively stationary. The semimountainous country several miles north of the lake probably is being affected in the downward movement. Evidence to support this belief is based in part on the following observations:
Location of local earthquake epicenters has been possible by the employment of three seismological stations using temporary equipment and, later, small-model Benioff seismographs, placed in a triangular net about Lake Mead, and a fourth supplementary station at Boulder Dam. Seismological work is carried on by means of a coöperative program between the Bureau of Reclamation and the Coast and Geodetic Survey with the assistance of the National Park Service.
In an earlier report [Carder, 1945], maps showing the epicenters of nearly 500 small local shocks in the Lake Mead area, Nevada and Arizona, were published. They showed a concentration of these epicenters along the south border of the basin occupied by the lower lobe of the lake. It was concluded that many of these local earthquakes were caused by a downfaulting of the crustal block occupied by this arm of the Lake against the granitic masses to the southeast and southwest. It was mentioned that this downsettlement was probably a renewal, on a small scale, of pre‐Pleistocene activity under the stimulus of a suddenly added load of 12 billion tons of water. Supporting evidence for this conclusion is the subject of the present paper.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.