A magnitude 1 or 2 quake was recorded during Michael Bennett's 22-yard fumble return for a touchdown in the first quarter. The game, a 34-7 victory that clinched a playoff berth for the Seahawks, registered on nearby seismic instruments several other times.
John Vidale, director of the Pacific Northwest Seismic Network, said fans get the oval stadium moving back and forth by stomping their feet and jumping up and down. "It's sort of what you see when a car rocks back and forth," said Vidale, also a University of Washington earth and space sciences professor.
Vidale said a small quake wasn't all that surprising in the stadium, which is built on pilings dug through soft ground in earthquake country.
Structural engineers who plan buildings in Seattle often do so with room for movement, so when a natural earthquake happens the buildings aren't so stiff that they crack and crumble.
Vidale watched the game from his office to compare seismic reports with game action because he wondered if there would be a repeat of the 2011 "Beast Quake," when Marshawn Lynch ran for a 67-yard, fourth-quarter touchdown as Seattle upset the Saints to advance to the second round of the NFL playoffs.
After the 2011 quake, Vidale looked at the whole season and found that that one Lynch run shook the stadium more than any other moment during that season.
He said it's possible quakes occur in stadiums in other cities, but he suspects Seattle's stadium is more flexible than most and may flow with the fans more.
"People unconsciously get the stadium moving, sort of like the wave," he said.
Seattle structural engineer John Hooper said similar quakes can happen anywhere a large number of people are moving in unison, but the difference in Seattle is that there were instruments nearby to record the quake and people were paying attention.
The same thing could happen next weekend in San Francisco, when the Seahawks play the NFC West-rival 49ers, said Hooper, director of earthquake engineering at Magnusson Klemencic Associates.
But he also acknowledged Seahawks fans are unique.
"They're very enthusiastic and it shows," Hooper said.
Seattle fans also made record noise Monday, setting the Guinness World Record for loudest outdoor sports stadium noise at 137.6 decibels.
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