By Maria Medina

OROVILLE (KPIX 5) — As the main spillway failed at Oroville dam last February, researchers at the U.S. Geological Survey were focused on another event happening at the dam that couldn’t be seen or heard without special equipment.

USGS research geophysicist Ole Kaven said they noticed two earthquakes: one a magnitude 1.0 and the other 0.8.

“We were wondering what was going on,” said Kaven.

Kaven and other scientists took a closer look, going back to records as far as 1993.

“And found close to 20,000 of these tiny events, more or less in the same location,” he said.

But now the questions were: what was causing them? Could it be the dam itself? And could the quakes be the cause of the spillway?

“What we ended up finding is that all of these 20,000 events coincided with the use of the spillway,” said Kaven.

Researchers found more than 99 percent of the quakes happened when the spillway was in use. The strongest quakes, which occurred during the failure, happened when the emergency spillway was opened and massive amounts of water were released.

oroville 1 USGS: 20,000 Tiny Man Made Earthquakes Shook Oroville Dam

Aerial shot of the Oroville Dam. (CBS)

But Kaven doesn’t believe the earthquakes caused the spillway failure. He said they believe the tremors are caused by water seeping into the bedrock underneath the spillway.

“The most likely culprit is that water actually gets under the spillway into cracks in the bedrock,” he said. “These cracks open and close and they generate these seismic signals.”

Earlier this year, a team of independent experts found it was the design and other complex factors that were the actual cause of the failure.

A spokesperson at the Department of Water Resources, which oversees the dam, told KPIX 5 that they just received the findings by the USGS and could not say if the quakes are a concern for the structure.

But if the research by the USGS is correct, it appears the earthquakes at Oroville dam are man-made. The spokesperson added that these quakes would not cause a larger, destructive earthquake.

Kaven said they will have a better idea if their theory is correct when the new spillway is used for the first time.

“It’s the first time we’ve seen this, but in all fairness it’s the also the first time we’ve looked,” he said.

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