Reservoir Induced Seismicity and Other Potential Problems with the Auburn Dam

2006, Laird Thompson, PhD Geologist
e-mail: lbtfracs@mindspring.com

Question: What is Reservoir Induced Seismicity (RIS)?
Answer: It is the creation of earthquakes resulting from having built a dam and impounding water behind that dam. The weight of the water combined with its’ lubricating effects create conditions that initiate earthquake activity where no earthquakes are expected if there were no dam and reservoir.

Question: Do all dams create RIS?
Answer: Yes. Most dams and reservoirs create only very small earthquakes because they are built in seismically stable areas, but careful monitoring has shown that even these dams do produce RIS. These small earthquakes do not cause damage and may not be felt by people living near the reservoir.

Question: There are faults all over California. Isn’t this a problem that is the same everywhere in the state?
Answer: Faults occur in much of California, but not all faults are active. Maps of California Seismicity and Earthquake Hazards show that the Central Valley and western Sierra Foothills are much less prone to earthquakes than much of the state. In fact, from an earthquake hazards perspective, this is historically the safest part of the state.

Question: If the western Sierra Foothills are so seismically stable, then we don’t have anything to worry about, right?
Answer: Under normal circumstances, that is true. However, there is an old, relatively inactive fault system called the Foothills Fault System that does run through the area. It runs right under the proposed Auburn Dam and it also runs under the Oroville Dam which suffered a magnitude 5.7 earthquake in 1975.

Question: But there are many dams in place along that fault trend and none of them have had major earthquakes. Why would the Auburn Dam be any more problematic than these other dams?
Answer: There are about 8 dams that have been built along the Foothills Fault System. The Oroville Dam is the only one that suffered a large magnitude earthquake. However, if one takes the cross-section of the canyon in which the dam was built; the height of the dam; and the weight of the impounded water just behind the dam (say within 2,500 feet of the dam) and compares all these dams, you get the following trend:

Oroville Dam (the largest weight of water): 5.7M earthquake in 1975. The lake holds 3.5 million acre-feet water.

New Bullards Dam: about 60% of the Oroville Dam weight. The lake holds 1 million acre-feet of water.

Don Pedro Dam: about 28% of the Oroville Dam weight. The lake holds 2 million acre-feet of water.

Folsom Dam: about 28% of the Oroville Dam weight (the Folsom Dam is also not along the main trend of the Foothills Fault System). The lake holds 1 million acre-feet of water.

New Exchequer Dam: about 23% of the Oroville Dam weight. The lake holds 1 million acre-feet of water.

New Melones Dam: about 21% of the Oroville Dam weight. The lake holds 2.4 million acre-feet of water.

Pardee and New Hogan Dams: only about 2% of the Oroville Dam weight. The lake behind Pardee Dam holds 0.2 million acre-feet of water.

Clearly, none of the other dams and reservoirs have created conditions of equal scale to the Oroville Dam. Thus it is perhaps not surprising that none of these other dams has suffered such a significant earthquake.

Question: Using this “yardstick”, how would the proposed Auburn Dam compare to the Oroville Dam?
Answer: The Auburn Dam would create about 80% of the Oroville Dam and reservoir weight. This is based upon water storage of 2.3 million acre-feet, which is the size reservoir under construction when the 1975 Oroville earthquake occurred.

Question: There may still be geologic differences between the possible faults at the Auburn Dam and the Oroville Dam. Can you say that the Auburn Dam would be 80% as likely to create a major earthquake?
Answer: No – this argument is too simplistic. The area of the proposed Auburn Dam has been extensively studied from a geological perspective and numerous small and medium-scale faults have been mapped there. Additionally, there are some rock units in the dam area that are very mechanically weak and create an extra hazard of significant slipping should an earthquake occur. With all these obvious geological features, it would make sense to perform a comprehensive engineering and earthquake hazard study of the Auburn Dam site, especially considering the original study was completed almost 30 years ago and the science of seismology has made significant progress since then.

Question: O.K., so there is an earthquake hazard at the Auburn Dam site, but we can still build a safe dam there, can’t we?
Answer: Maybe. The original dam design would have withstood a magnitude 1.6 earthquake. After the Oroville 5.7M earthquake, the Auburn Dam was re-designed to withstand a 6.5M earthquake. However, certain assumptions were made about expected offset on the dam should it suffer such an earthquake. One assumption was that there would be only about 9” of offset or movement in such an earthquake. New studies, however, have raised the significant possibility that the dam might suffer 2-3 feet of offset. Should this happen, no engineer could guarantee that the dam would remain intact. The most recent look at the earthquake safety issues posed by Auburn Dam was issued 10 years ago in 1996 by the U.S. Geological Survey in Open File Report 96-0011.

Question: So, if the dam did fail with a 600 foot deep reservoir behind it, how bad could this be for Sacramento?
Answer: A U.S. Bureau of Reclamation report completed in 1980 says that within 5 minutes of the Auburn Dam failing, the Folsom Dam would see a surge of floodwater that would crest at 12 feet deep. The Nimbus Dam area would eventually be under 70 feet of water and even the State Capitol building would be standing in 40 feet of water in 6.5 hours.

Question: But all this is speculation. We could also speculate that the Auburn Dam would survive such an earthquake. If the dam did survive, then earthquakes aren’t really a problem are they?
Answer: The earthquake hazard creates some “hidden taxes” with the Auburn Dam project for those living in Auburn and other near-by communities such as Granite Bay. The Auburn Dam would change the earthquake hazard rating for the area and local residents would have to buy earthquake insurance for their homes. Insurance companies would raise the rates for this insurance based on the elevated hazards. Additionally, local schools, hospitals and emergency facilities would have to be retro-fitted for the increased earthquake hazard. Increases costs such as these could easily run into the millions of dollars.

Question: But if the Auburn Dam is not built, Sacramento would be subjected to flooding by the American River anyway, right?
Answer: When completed in the near future, the present work program by the Army Corps of Engineers and the U.S. Bureau of Reclamation at Folsom Lake will provide flood protection for any storm system equivalent in size to any that has occurred since they started keeping weather records in Sacramento. In fact, it will protect against a storm 50% larger than has been experienced in the area – including the one in 1986 that resulted in significant flood damage in the Sacramento Area.

Question: Then why do we really want the Auburn Dam?