## Earthquake Hazard

The Auburn Dam Project, a component of the Central Valley Project, was halted in 1975 following a 5.7 earthquake near Oroville. Experts believe that the Oroville earthquake may have been a result of Reservoir-Induced Seismicity (RIS).

In 1996 USGS reviewed the seismic-hazards associated with the Auburn Dam Project. The Open-File Report is the most recent government report reviewing the seismic hazards of the Auburn Dam Project. The potential of a major earthquake induced by a dam near Auburn is real. If the dam were to fail due to stresses associated with an earthquake we would have a tragedy far exceeding the potential floods the dam is meant to protect us from.

Following the 1975 Oroville earthquake, the Engineering Department at UCLA conducted a study modeling a failure of Folsom Dam. The study predicted 250,000 fatalities resulting from a dam failure. Keep in mind this was in 1975 when Sacramento was considerably less populated. In 1980 the US Bureau of Reclamation (BOR) completed a study assessing inundation following a complete collapse of a dam at Auburn. If a dam at Auburn were to fail due to seismic activity, water would reach Folsom Dam in 5 minutes and peak in a little over an hour. The water from a failure of a dam would not cause a calm pool, but would carry debris from structures, streamside foliage and sediment deposition. Evacuation would be difficult due to the sudden onset and lack of forecasting. Residents evacuating to rooftops to await rescue would not be an option, as the water depth in many areas would reach 40+ feet. This potential flooding would be far worse than a 250 or 500-year flood event with more casualties and a larger inundation area than a natural flood event.

Considering the potential threats of reservoir-induced seismicity, resulting in a dam failure, it would be wise to ask the following questions?

1. How likely is it that the structure would induce an earthquake?
2. If an earthquake is induced, how large could that earthquake be?
3. How much potential surface displacement could an earthquake cause?

How likely is it that the structure will induce an earthquake?

Some background on earthquake faults is necessary. If a fault has shown activity or displayed movement within the last 100,000 years then geologists considered it to be â€˜Activeâ€™. The earthquake at Oroville in 1975 was a result of activity on the same fault system that the Auburn Dam would span.

In studies performed after the 1975 Oroville earthquake, paleosalts spanning the fault in question showed significant displacement across the fault. By aging the paleosalts geologists are able to discern when the displacement occurred. The fault shows activity within the last 100,000 years.

Another sign of seismic activity is significant bulging of the earths crust, five inches of movement per decade. In addition, there are microearthquakes along the edges of the bulging. According to Woodward-Clyde consultants (hired by the BOR following the Oroville earthquake), if the Oroville earthquake in 1975 were Reservoir Induced then a dam at Auburn would create a 30% probability of inducing an earthquake.

If an earthquake is induced, how large could that earthquake be?

Agencies responsible for assessing seismic hazard determine a Maximum Credible Earthquake (MCE). The Bureau of Reclamation (BOR) determined that building the Auburn Dam could result in a MCE of 6.5 magnitude (M) while the USGS believed that a 7.0M MCE was more accurate. The difference between a 7.0M and 6.5M earthquake is significant. A 7.0M earthquake is three times more intense than a 6.5M earthquake. The BOR chose to ignore the USGS findings and base their modeling and ultimately their dam design on a MCE of 6.5M.

The current design for the proposed Auburn Dam is a Gravity Dam designed to withstand the MCE of 6.5M. Considering how many lives are at risk it would be prudent to plan for a worse case scenario. Considering that the countries leading authority on seismicity determined a MCE of 7.0M it appears irresponsible for BOR to base their modeling on a MCE of 6.5M.

How much potential surface displacement?

“Estimates for surface displacement ranged from about one inch (BOR), to nine inches (Woodward-Clyde and the California Division of Mines and Geology-CDMG), up to a whopping three feet (USGS).”

- Earthquake Threat at Auburn Dam, Headwaters May/June 1990
Bill Cross and Tony Finnerty

Reports and additional links on Seismic hazards.