Dam’s leakage may have caused untold quakes
The earthquakes hit just days after last year’s near-catastrophe at Oroville Dam, when the spillway cracked amid heavy rains and 188,000 people fled in fear of flooding.
The timing of the two small tremors about 75 miles north of Sacramento was curious, and frightening. Were the quakes part of a seismic hot spot that caused the giant concrete spillway to tear? Was the weight of the water behind the dam triggering the quakes? Could all of Lake Oroville be prone to slipping?
A group of seismologists, summoned from the
U.S. Geological Survey’s research facility in Menlo Park, was put on the case. Their findings were published in the Bulletin of the Seismological Society of America on Monday.
Fortunately, the researchers concluded, the nation’s tallest dam is not in imminent danger. They don’t think earthquakes caused the fracture. Just the opposite may be true, they said.
Leaks in the dam’s spillway likely caused the seismic activity and had been generating tiny earthquakes for years — thousands of them, according to the new research. And if not fixed properly, the spillway could continue to produce chronic shaking, although posing no future threat.
“We’ve never seen anything like this,” said the report’s lead author, USGS seismologist Robert Skoumal. “It’s the first case of a spillway causing an earthquake that we’re aware of.”
Large reservoirs have been associated with tremors before. A few years after the 770-foothigh Oroville Dam was built in the late 1960s, creating California’s second-largest reservoir, the area was rocked by a magnitude 5.7 quake. Scientists now know the creation of a large lake can induce fault movement.
However, the discharge of water from a dam, out of a spillway, hadn’t been known to trigger earthquakes until now.
Skoumal and his colleagues discovered the oddity by investigating the two quakes that occurred at Oroville Dam on Feb. 14, 2017, a week after the half-mile-long spillway cracked.
The location and seismic fingerprints of the 0.8 and 1.0 magnitude events were traced, and the research team sorted through old earthquake records, learning that similar movement was frequent in the Oroville area of Butte County.
Since 1993, as far back as the records went, a total of 19,291 tiny seismic events had taken place, the team found. While plentiful, none of the quakes exceeded a 1.0 magnitude, making them too small to detect without measurement tools.
The likely cause of the activity, they concluded, was water pushing through crevices in the dam’s spillway and putting pressure on the underlying rock. The leaking water filled pores in the ground and made them expand before receding back into position, causing the earth to move.
A team of engineers that investigated the dam’s near failure determined this year that water had long been infiltrating the concrete spillway. Ultimately, the same pore pressure that Skoumal identifies as the cause of the quakes prompted a large chunk of the spillway to lift off, setting the stage for the ensuing failure.
“The power of the water is just amazing,” Skoumal said.
Managers at Oroville Dam turned to an emergency spillway for releases after noticing the spillway’s fracture Feb. 7, 2017. But the emergency spillway similarly began to erode. The fear that water would have no good way out of the reservoir and pour uncontrollably downstream prompted mandatory evacuations.
Water levels in the lake eventually dropped, and the threat cleared.
Skoumal said he was relieved to find that earthquakes didn’t cause last year’s problem. He added that knowledge of the seismic activity could have even helped detect issues at the dam.
“If we had had the information and found these events before the spillway failure, we could have hypothesized that there could have been something wrong,” he said.
He and his colleagues say they’re open to investigating other spillways for seismic activity — and possible leaks.
The spillway at Oroville Dam as well as the emergency chute have been completely rebuilt since last year, at a cost of about $1.1 billion, which includes the bill for February’s emergency response. The new spillway hasn’t been used yet, but state officials say the leaks are gone.
“The new spillway is designed to 2018 standards, including 30-inch thick structural concrete slabs with two layers of reinforcing steel,” said Erin Mellon, spokeswoman for the Department of Water Resources, who hadn’t seen the study. “Water stops are also included between the slabs and walls to stop any water from getting under the spillway.”
Skoumal and his colleagues report that there hasn’t been much seismic action since the main spillway was reconstructed. But because no water has flowed down the chute, the real test will come when the spillway is deployed again, perhaps this winter.