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:
The faults bordering the southern margins of the Callville Basin crustal block are the sources of several hundred small earthquakes which have been located since November, 1940, and the supposed sources of several thousand small shocks which have been felt or recorded since 1936.
Although certain earlier settlers may have noticed small shocks prior to the building of Boulder Dam, the preponderance of felt shocks was noticeable after the lake had reached its peak load for the year 1936; and thereafter felt and recorded shocks seemed to be more closely associated with seasonal increases or peak loads.
Other evidence which indicates a settlement of the area north of Callville Basin and elsewhere relative to areas where the bedrock is primarily granitic in character.
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.