The crisis over the last few weeks at Oroville Dam was about how to handle large amounts of water flowing into the lake with both of the reservoir's spillways damaged.
But there also was a seismic element at play as well. As Lake Oroville swung from being at 41% of capacity to 101% in just two months, scientists are asking whether the filling of the reservoir at the fastest rate in at least a generation can produce a damaging earthquake.
No one knows for certain, but the question has been a source of debate since shortly after the nation's tallest dam was built, and a magnitude 5.7 earthquake occurred that some believe was related to an unprecedented lowering and rise of reservoir levels. Moreover, two small were detected last month soon after Lake Oroville began rushing over its emergency spillway.
About a day and a half after officials ordered mass evacuations of more than 100,000 people downstream of Oroville Dam — fearing the collapse of a hillside eroded by the overflowing lake — two tiny temblors were recorded. They struck in the early hours of Valentine's Day, just before 3 a.m.: a magnitude 0.8, and then a 1.0.
Those earthquakes were so small a human wouldn't be able to feel them at the surface. But given how close they 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 said of the need 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 some scientists believed happened at this reservoir in 1975, seven years after Oroville Dam was completed.
Lake Oroville had been filled before. What made the winter of 1974-75 unusual was that water levels had to be 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 that ended in June 1975.
Then the earthquakes started.
Instead of fewer than five earthquakes a month in a zone 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 in the sequence hit: a magnitude 5.7 on Aug. 1, 1975. It was strong enough to crack plaster and walls in Oroville and was felt as far away as San Francisco and Sacramento, where the Capitol's dome suffered minor damage. The 50-person staff of Treasurer Jesse Unruh was ordered to evacuate its office.
That August became a banner month for earthquakes around Oroville Dam — more than 3,000 temblors were recorded, before fading to more than 700 a month later and over 100 by October.
Not all scientists agree that the earthquakes were triggered by human activity. But two state seismologists who authored a study in the journal California Geology offered an argument that the events were influenced by the operations of the man-made reservoir.
That Oroville earthquake was similar to a well-established example of a human-triggered quake seen after the construction of the Koyna Dam in western India in 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 authors Tousson Toppozada and Paul Morrison Jr., who respectively worked for the California Division of Mines and Geology and the California Department of Water Resources.
The Koyna Dam is believed to be related to a magnitude 6.2 earthquake in 1967, which killed 200 people.
There is other well-established evidence for reservoir-caused earthquakes, Rubinstein said, such as the construction of Hoover Dam and the filling of Lake Mead. "People started feeling earthquakes in an area with no historical seismicity," he said.
Other cases have been more controversial. Some independent scientists have said the 2008 magnitude 7.9 earthquake in China, which killed nearly 90,000 people, was linked to the filling of the reservoir behind the newly built Zipingpu Dam. Chinese authorities have argued vigorously that the dam was not the cause, said Ross Stein, an earthquake expert and founder of the website temblor.net.
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 additional, increasingly larger earthquakes. That makes it improbable that the microquakes would be the first dominoes in a lead-up to a damaging future earthquake, Rubinstein said.
"By all accounts, the situation does not in any way look alarming," Stein said. "But we recognize in the earthquake business that things don't look alarming until the earthquake occurs."
Indeed, the 1971 Sylmar earthquake and 1994 Northridge earthquakes came as surprises, said Peggy Hellweg, operations manager at the Berkeley Seismological Laboratory.
"Anywhere in California, an earthquake could happen, and I would not be extremely surprised," Hellweg said.
There are two ways a reservoir can cause an earthquake. A rapid filling or emptying of a lake can change the weight pushing on a fault, which can make an earthquake more likely, said Bill Leith, acting associate director on natural hazards at the USGS.
"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."
On the other hand, Lake Oroville has been dealing with seasonal changes in water levels for nearly half a century, and for the most part, it has been seismically quiet. "It appears to be a relatively stable system," Leith said.
The second way a reservoir can cause an earthquake is from added pressure. Water trickling deep into the earth can increase pressure underground that makes it easier for faults to move, according to seismologist Lucy Jones.
"The pressure in the water sort of pushes the fault back apart," Jones said. The added water pressure underground essentially unclamps a fault — like loosening a vice that keeps two blocks of rocks stuck together — in a way that makes it easier for the earth to move.
It can take years for water to filter down into the deep crevices of the earth and add fluid pressure around the fault, Leith said. The dramatic fluctuation in reservoir levels in 1975 may have been the trigger point for the earthquake that year.
The adding of water deep underground also is what is causing human-induced earthquakes in Oklahoma and southern Kansas, as added fluid pressure unclamps the fault and triggers earthquakes, Leith said.
The 1975 Oroville quake may have released much of the seismic stress that had accumulated in the area. "But I wouldn't put my reputation on that — just because it's impossible to know whether there are faults nearby that are close to failure," Leith said.
"This is a problem that we have everywhere where there are human-induced earthquakes," Leith said.
It's possible there is a fault on the brink of failure in the Oroville area. Even a seasonal change in reservoir levels could be the trigger that unlocks an earthquake.
"We don't have ways of measuring how close to failure the faults are," Leith said. "It's one of the challenges of seismology."
The California Department of Water Resources' latest assessment of Oroville Dam indicates it would be stable under the current predicted level of seismic shaking, a statement from the agency said. The closest known fault is the Cleveland Hill fault, which ends about three miles southeast of the dam.
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, the dam would move up to 10 inches, which is minimal considering the dam's giant size, the state concluded.
Other dams regulated by the state have been identified as in need in seismic retrofit, and are either in the planning process, undergoing construction or have had strengthening completed. They include Perris Dam in Riverside County, Calaveras and San Pablo dams in Alameda County, and Anderson, Calero and Guadalupe dams in Santa Clara County.
Chart source: California Geology
Chart source: California Department of Water Resources