Seoul, South Korea is home to 10 million people. Roughly 350,000 of them live in the Gwangjin District on the northern bank of the Han River, which itself bisects the city. One of the more notable buildings in the Gwangjin District is home to something called Techno Mart, a mall dedicated to everything electronic — from computers, printers, mobile phones, and even household appliances such as refrigerators. But Techno Mart’s offerings aren’t what makes it notable. Its size is. It is home to over 2,000 stores and is 39 stories tall.
So when it started shaking violently, people noticed.
Techno Mart, like many uber-malls, isn’t just stores. It has a food court, of course, but also a gym in case you want to work off the meal you just ate. There are open spaces overlooking the river, too, but if you’d rather some artificial entertainment, Techno Mart is home to one of the area’s larger movie theaters. Shutting down the complex for two days is a big deal, but that’s exactly what happened on July 5, 2011. Without immediate explanation, the building began shaking for ten minutes. There were no earthquakes nor heavy winds during that ten minute period, leaving officials befuddled. To be on the safe side, authorities shuttered the complex until they could determine if it was safe to occupy.
Those assurances came within 48 hours or so, but the explanation for the pseudo-seismic activity didn’t come to light for another two weeks. The cause, ultimately, involved a snap and the power, but not in the way you’d think. The “snap” in this case was “Snap!,” a German music group, and “The Power” was their hit song from 1990, which if you were alive back then, you’ll probably find familiar:
The culprit: Techno Mart’s gym, located on the 12th floor. A Tae Bo class attended by 17 to 23 middle-aged exercisers (reports differ) were guided through a more-intense-than-usual class, dancing aggressively to the song. As they worked out, the building shook. Chung Lan, a professor at Seoul’s Dankook University, explained how such a thing was possible:
It just happens to be that the vibration set up by the Tae Bo exercises coincided with the resonance frequency unique to the building. When an external vibration hits the resonance frequency of a certain object, the vibration is amplified and causes excess shaking even from slight movement.
And in this case, “excess shaking” means that there was enough shaking to make people think they were in the middle of an earthquake.
To test the theory, researchers had another class of Tae Bo dancers repeat their workout, to the same song, two weeks later. It yielded the same result, but this time, Chung and others were able to measure where the violent shaking was most noticeable. Only on the top floors could the vibrations be felt significantly, and all six researchers agreed that mechanical resonance was the likely culprit.
From the Archives: Early Warning Lemurs: How to detect an earthquake using zoo animals.
Related: The Tae Bo collection. For all your earthquake-making needs.