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Shortly after Lake Mead (the reservoir impounded by Hoover Dam) began to fill, local earthquakes were felt. The earthquakes reached a culmination in a magnitude 5 earthquake about a year after the reservoir had filled to 80 percent capacity. For a number of years thereafter, small local earthquakes showed a close correlation in numbers and energy release with seasonal peak loads. In due time the correlation was as close with unloading as with loading. Sometimes there was no direct correlation at all.

In recent years many instances of local earthquakes attributed to crustal unbalance have been noted. Noteworthy is the earthquake activity near Denver and associated downhole injections of waste chemicals; and earthquakes associated with reservoir loading at Kariba in Africa, Kremasta and Marathon in Greece, Vajont in Italy, Monteynard and Grandval in France, and Koyna in. India. In one or two instances the earthquakes may be a direct result of settlement under load. More likely they result from a triggering of stored strain energy. The crustal block is considered to be strained to near yield but in delicate equilibrium. This equilibrium may be upset by a sudden shifting of the load, or by lubrication or buildup of fluid pressure within fractures in the rock. This decreases frictional resistance between the walls of fractures. Many newly created reservoirs have no association with local earthquakes because there are no ready-made conditions in the crustal block containing the reservoir for the production of earthquakes.

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