Scientists Are Asking if Rapidly Filling the Reservoir Can Produce a Damaging Earthquake
Despite years of warnings, nothing was ever done …
Californians dodged a bullet recently when the water from California’s Lake Oroville, a giant reservoir created by a dam 770 feet tall, was overwhelming a damaged spillway and eroding the hillside. Now massive weather systems have dropped huge amounts of precipitation, and new warnings are being issued about the entire structure failing. Officials also don’t believe there would be time for evacuations if the dam fails.
Regulators several years ago insisted there should be public-warning systems with better detection procedures because of the threat from the dam. Water topping the dam was channeling out rivers in the hillside below. A dozen years ago environmentalists warned the land below needed to be covered in concrete to stop erosion with water spilling over the dam.
It never was done, and during the recent rainfall, 200,000 people were evacuated from low-lying regions below the dam. Officials feared the emergency spillway could fail, unleashing a 30-foot wall of water.
The lake had filled quite suddenly after a years-long drought when billions of gallons of rain and melted snow flooded during recent weeks. Officials were releasing about 100,000 cubic ft/ second of water when the spillway eroded, leaving a chasm in the middle. When officials switched the water needing to be released to an emergency spillway, it dropped the water onto the dirt hillside, and the erosion began immediately. Officials were left with the no-win choice of sending the water down a broken main spillway or down an emergency path in which the erosion could threaten the stability of the top of the dam itself.
AP reported Sheriff Kory Honea said it would be “daunting” to give residents a warning to get out if the dam itself failed. “That is why we’re taking steps now to refine our notification plan and our evacuation plan, potential evacuation routes, in hope that we can give people more time to exit the area should that happen.”
Officials estimate there are nearly 9,000 people in the zone that could be inundated in short order if the dam ruptured. Oroville is only five miles away from the dam itself. With all the rainfall, the lake rose from 1.5 million acre/feet of water [acre/foot = 326,000 gallons] December 1, to capacity of 3.5 million acre/feet February 1.
The National Weather Service said the storm dropped six inches on Los Angeles County, and more than 10 inches on coastal mountain slopes. NRP reported many questions raised over the dam’s integrity earlier, and several environmental groups questioned the stability of the land below the dam in a flood situation. Evacuation orders for 200,000 downstream residents were made during the water onslaught.
Interestingly, Friends of the River, Sierra Club, and South Yuba Citizens League told authorities more than 10 years ago the main spillway, and the emergency route also needed to be covered with concrete.
At Oroville, all spending should have covered the hillside with concrete, and widening the valley road for quick exits. But neither was done. In an editorial, the San Jose Mercury News pointed out state officials decided in 2005 to ignore warnings that the massive earthen spillway could erode during heavy winter rains. It did erode, and possibly worse damage could be done by the end of winter.
Oroville is America’s tallest dam. Experts predicted if it failed, water could reach Oroville within an hour, and within 8-12 hours it could be in Yuba City. It was noted the flood could be 10 feet deep by then.
But now the Oroville Dam faces an even more daunting risk. Rapid rise in water levels can easily cause earthquakes. The previous damage to the crisis where large amounts of water was flowing into the lake with both spillways damaged. Lake Oroville changed from having only 41% of capacity to 101% in just two months.
Scientists are asking if the filling of the reservoir quickly can produce a damaging earthquake. Unknown, the question has been debated since shortly after the nation’s tallest dam was built, and a magnitude 5.7 earthquake occurred some believe caused an unprecedented lowering and rising of reservoir levels.
A day or so after officials ordered mass evacuations of over 100,000 people downstream of Oroville Dam, two tiny tremors, or temblors were recorded. They struck in the early hours of Valentine’s Day. Magnitudes of 0.8,and then a 1.0 were recorded. “It’s obviously something that needs to be carefully monitored,” U.S. Geological Survey geophysicist Justin Rubinstein said to carefully monitor dam sites for human-triggered earthquakes. “But in general, when you see seismicity induced by reservoirs, it’s generally sort of in the beginning of the lifetime of the dam,” Rubinstein said.
That’s what scientists believed happened at this reservoir in 1975, seven years after the dam was completed. Lake Oroville had been filled before. What made the winter of 1975 unusual was water levels had to be reduced to their lowest level to repair intakes to the hydroelectric power plant. Months later, there was an unprecedented refilling of the lake that ended later that year. Then the earthquakes started. Instead of fewer than five earthquakes a month in a zone within 25 miles from the dam, June and July suddenly saw more than 10 earthquakes each month.
Given how close the earthquakes were to the dam, and the relationship between some man-made reservoirs and earthquakes, researchers gave the ‘temblors’ special attention. “It’s obviously something that needs to be carefully monitored,” U.S. Geological Survey research geophysicist Justin Rubinstein spoke of monitoring dam sites for human-triggered earthquakes.
That’s what scientists believed happened in 1975, seven years after Oroville Dam was completed, and Lake Oroville had been filled. What made the winter of 1974-75 unusual was water levels were reduced to their lowest level since the reservoir was first filled to repair intakes to the hydroelectric power plant. Months later, there was an unprecedented refilling of the lake in 1975. Then the earthquakes started. Instead of fewer than five earthquakes a month within 25 miles from the dam, as had been the case for the previous year, June and July suddenly saw more than 10 earthquakes each month.
Then the largest earthquake hit-a magnitude 5.7 in August of 1975. It was strong enough to crack walls in Oroville, and was felt as far away as San Francisco. August became the biggest month for earthquakes near Oroville Dam. More than 3,000 temblors were recorded, while fading to only 700 a month later, with an additional 100 in October.
All scientists don’t necessarily agree earthquakes were triggered by human activity. But two state seismologists who authored a study in California Geology, believed events were influenced by the man-made reservoir. The Oroville earthquake was similar to a human-triggered quake seen after the construction of the Koyna Dam in India,1962. “The major burst of seismicity did not occur upon initial filling, but occurred several years later following an unprecedented seasonal refilling in each case,” wrote author Paul Morrison Jr., who worked for the California Department of Water Resources.
The Koyna Dam was related to a magnitude 6.2 earthquake in 1967, which killed 200 people. One should consider other well-established evidence for reservoir-caused earthquakes, such as the construction of Hoover Dam and the filling of Lake Mead. People started feeling earthquakes in an area with no historical seismicity. Some independent scientists said the 2008 magnitude 7.9 earthquake in China, killing 90,000 people, was linked to the filling of the reservoir behind the newly built Zipingpu Dam.
Lake Oroville once was the poster child of the California drought. In late 2015, the lake dipped to 26% capacity but began its recovery about a year ago, reaching a high of 96% capacity in May before being drawn down in the summer and fall. What made this winter catch the eye of scientists, however, was how suddenly the reservoir filled up. Last year, it took five months to fill Lake Oroville; this winter, it took only two months. Oroville’s Valentine’s Day earthquakes have not been followed by increasingly larger earthquakes. Indeed, the 1971 Sylmar earthquake and 1994 Northridge earthquakes came as surprises, per Peggy Hellweg, Operations Manager at Berkeley Seismological Laboratory.
“Especially for a reservoir as large as Oroville, it’s a huge weight on the crust that’s basically being pulled up and down on an annual cycle. So it wouldn’t be surprising if there were earthquakes associated with that,” Leith said. “The rapid filling, I just think it increases the risk. … I would expect that a rapid rise or a rapid fall in the water level would be much more likely to trigger earthquakes.”
The 1975 Oroville quake likely released the seismic stress in the area. “This is a problem that we have everywhere where there are human-induced earthquakes,” Leith said. It’s possible there is a fault about to fail in the Oroville area. Even a seasonal change in reservoir levels could be the trigger unlocking an earthquake. The state’s latest seismic evaluation of the dam, completed in 2014, said the dam is not in need of a retrofit. Even if a magnitude 6.5 earthquake struck about three miles southeast of the dam, it would rise 10 inches. Nothing was done.
The history exists. Dams can cause earthquakes. The mess at Oroville Dam could have been avoided when regulators several years ago insisted there should be public-warning systems with better detection procedures.