There was no seismic activity reported at the time of the tremors, which began about 10:10 a.m., according to a report in The Korea Times.
"I fled the building with everyone else while it was shaking up and down. It almost made me feel dizzy," Lim Joon-hee, who works on the 20th floor of the building, told the Yonhap News Agency.

Korean office tower evacuated after mysterious shaking - This Just In - CNN.com Blogs