An elevator cable in 54-story Mori Tower complex in Tokyo snapped last October when a magnitude-6.8 earthquake hit Niigata Prefecture some 200 km away, according to the tower's operator.
Six of the building's 67 elevators, including the one with the broken cable, had to be halted Oct. 23 after they started swaying and hitting the walls of their shafts, according to Mori Building Co. at Roppongi Hills.
Since sensors on the six elevators were unable to detect long-oscillation seismic waves, the elevators failed to stop automatically when the earthquake hit, the officials said. Two people were trapped in two of the halted elevators, but were rescued two hours later.
The cable that snapped was about 1 cm in diameter, but officials said it didn't pose a danger to the elevator because each is supported by eight such cables.
Since the incident, Mori Tower has equipped 28 elevators serving the higher floors with new sensors capable of coping with temblors that have long and slow oscillations ranging from several seconds to over 10 seconds, the officials said. The height of a building determines its oscillation period during an earthquake. In general, the taller the building, the longer the oscillation.
It is believed that when the oscillation period matches a quake's wavelength, the amount of sway increases.
The Niigata earthquake, which resulted in the loss of 40 lives, also trapped about a dozen tourists in elevators at Tokyo City Hall.