Rowdy fans stomping and roaring when the Seattle Seahawks scored a touchdown last night (Dec. 2) shook the football stadium so hard that a nearby seismometer registered an "earthquake."
It's not the first time the seismometer, which monitors earthquakes, picked up ground-shaking vibrations from Seahawks fans. Nearly three years ago, on Jan. 8., 2011, a 67-yard touchdown run now known as the "Beast Quake" resulted in a fan frenzy as powerful as a magnitude-2 temblor. A 1988 showdown between Louisiana State University and Auburn University also registered on LSU's local seismometer, leading ESPN to dub it the "Earthquake Game."
This Monday night, with a Guinness World Record for loudest recorded crowd noise on the line, a seismologist from the Pacific Northwest Seismic Network watched the Earth shake in real time, with the game on one screen and the seismometer readings on another. (The crowd beat the sound record, registering a reading of 137.6 decibels.)
The play-by-play helped confirm that fans were triggering the seismic vibrations, said John Vidale, a seismologist with the Pacific Northwest Seismic Network.
"With all the hype, we wanted to be clear that every time the Seahawks scored a touchdown, we could see a characteristic signal on the seismometer," Vidale told LiveScience. The seismometer is about a block from downtown Seattle's CenturyLink Field, where the Seahawks play. [What If Everyone On Earth Jumped at Once?]
Not only did each touchdown create a clear signal on the earthquake monitor, but Vidale also thinks he can see the fans screaming. "It's not as clear as the touchdowns," Vidale said. "There's a lot of things that make a racket in downtown Seattle."
Though Vidale offered a comparison to earthquake size, vibrations from celebrating fans are different from actual quakes, he said. Instead of an earthquake's many distinct pulses, the vibrations have just two frequencies, reflecting the stadium's natural resonance.
As the fans jump in excitement, the stadium starts to shake and sway — similar to tall buildings swaying in the wind — a phenomenon called resonance. "The fans jump, and the stands start swaying, and what we're really seeing is the natural resonance of the structure," Vidale said. "I think the fans are self-synchronizing, and it's shaking the building so much that the entire neighborhood shakes a little bit."
The vibrations from the stadium traveled at about 2.5 hertz and 5 hertz and lasted about 30 seconds, and was equivalent to a magnitude-1 or magnitude-2 earthquake, Vidale said.
The stadium seismometer is part of an extensive network that monitors the Pacific Northwest's active earthquake faults and volcanoes for potential hazards.
Vidale said the network can also detect vibrations from powerboats in Puget Sound, and recorded the sonic booms that shook Seattle windows in 2010, when fighter jets were called in after a small private plane entered airspace restricted for a visit by President Barack Obama.