Reservoir Loading and Earthquakes

In June of 1967, Fairbanks experienced an earthquake swarm which included at least four events between magnitude 5 and 6 during the half hour. During the following weeks, aftershock activity tapered in typical fashion, and only an occasional small earthquake was felt (although many were being recorded by instruments).

In August, the notorious "thousand-year flood" struck the city forcing most families to evacuate their homes and move to higher ground. Packed together in places such as the university campus, people began commenting to each other that there seemed to be an increase in the number of earthquakes they were feeling. In fact, during the period that the flood was cresting, it seemed to many that there must have a hundred or more perceptible tremors.

Unfortunately, the Geophysical Institute was not able to gauge the accuracy of this figure, because the lower-lying local seismograph stations had been wiped out by the flood, and the data telemetry lines extending to the more distant stations in the state had been converted to communications channels for emergency use.

It is probable that there really were more earthquakes during the Fairbanks flood. There have been instances all over the world of earthquakes accompanying impoundment of water in reservoirs. Perhaps the most famous example in the United States is that of Hoover Dam, completed in 1935. Hundreds of earthquakes occurred as the water level rose. Since it reached its peak of 475 feet in 1939, the level of seismicity has fluctuated in direct response to water level. None of the shocks have been particularly damaging--the largest being about magnitude 5--but the area did not have a prior record of being seismically active. Another good example in the U.S. is that of Grand Coulee Dam in Washington state. A stable area prior to the completion of the dam in 1943, it is now characterized by frequent small earthquakes.

Other examples from around the world abound. One of the most widely publicized was that of the Vanont Dam in northeast Italy. From 1960 to 1963, while the reservoir behind the new dam (at 360 feet, the highest in the world) was being filled, some 250 tremors were felt as the water rose. The reservoir and dam are located in a deep, steepwalled canyon. Over a period of time, surveyors had noted a gradual downslope creep of the weak strata composing one of the canyon walls above the reservoir. On the night of October 9, 1963, when the reservoir was completely full, a huge slab of mountainside broke off and plunged into the lake, creating an immense wave which rose 330 feet over the top of the dam and rushed into the valley below. About 2,600 people were drowned. It can be argued that the self-induced earthquakes were not the sole agent responsible for the landslide (there had been a great deal of rain), but they were almost certainly a contributing factor.

Incredibly, the superbly engineered Vanont Dam held; the tragedy would have been much greater had it not. Although there have been many disasters related to dam failure throughout the world, surprisingly few have been directly related to load-induced earthquakes. No dam has actually given way because of quakes, although the dirt-filled Boca Dam in California nearly did so in 1966--a case of faulty engineering.

Because the proposed Susitna hydroelectric project would be situated in an area with a previous history of seismic activity, the earthquake engineering which has gone into its planning is probably unparalleled in history.