Abstract
Since the reservoir behind Boulder Dam started to fill in 1935, several hundred small earthquakes have been felt in the adjacent area and several thousand have been recorded since the first seismograph was installed at Boulder City early in 1938. Epicenters of several hundred of the latter have been located, in large part along known or suspected faults on the southern margins of the large lower basin in the reservoir. It is believed that the crustal block upon which this section of the reservoir rests has been tilted downward several inches along these faults and that the granitic massif south of the faults has not partaken in the downward movement. It is known that settlement of areas of sedimentary and volcanic rocks near the lake is somewhat greater than similar areas of granitic rock. It is believed that this settlement is the cause of large numbers of the local shocks partly because the peak occurrence‐distribution is usually associated with peak seasonal loads in the reservoir.
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