Strenuous group exercise has been fingered as the culprit for the mysterious tremors that led to the evacuation of a shopping mall in Seoul earlier this month. On Tuesday, investigators probing the incident noticed that the vibrometer on the 38th floor of the TechnoMart building in eastern Seoul suddenly started moving violently as a group of 23 men and women in the fitness center on the 12th floor had started jumping energetically around.
An office worker in the 38th floor of the building said, "When we were evacuating on July 5, that's how much the building trembled." The Architectural Institute of Korea and Prime Center, the owner of the Techno Mart building, on Tuesday held a press conference to explain what caused the tremor. "It is most likely that unusually intense exercise movement in the fitness center caused the building to shake," a spokesman said.
Based on a hypothesis by the Korea Infrastructure Safety and Technology Corporation, the institute recreated the situation in the fitness center on July 5. It asked 23 people to engage in Tae Bo exercise, a dynamic aerobic exercise that combines taekwondo and boxing movements. As predicted, the vibrometer on the 38th floor detected tremors 10 times more intense than usual.
Professor Chung Lan of Dankook University who led the inspection team said, "After experiments, we concluded that the Tae Bo exercise caused the building to vibrate and tremble on July 5."
The institute explained that the effects of the 4D cinema and winds, which had also been suggested, were unlikely to be the reason. "The 4D cinema is quite a long way from the office building where vibration was most keenly felt, and the vibration frequency range from 20 to 25 Hz is irrelevant to the building's own," Chung said. "Plus, as people in the building felt a vertical vibration, wind is unlikely to be the reason as winds tend to shake buildings horizontally."
The institute and Prime Center will reach a final conclusion in two or three months after detailed vibration measurements of the building in different weather conditions. Park Jong-yong of the Gwangjin district office, said, "Although it didn't come up with the final conclusion, we have a visible result at the moment, so we will decide soon whether to reopen the fitness center."
The CGV 4D Cinema resumed service on July 16 as an inspection last week revealed that it was not the cause of the vibration.