Japan's Meteorological Agency will start issuing alerts for tremors that cause high-rise buildings to sway slowly, as part of its early warning system for powerful earthquakes.

The agency will add alerts for long-period ground motions to its emergency earthquake warning system from Wednesday.

The March 2011 powerful earthquake that struck northeastern Japan triggered such ground movement, which caused high-rise buildings as far away as Tokyo and Osaka to sway.

The agency ranks the intensity for the ground motions on a four-level scale. It will issue alerts to areas where the two highest intensities, Class 3 and Class 4, are projected.
Class 3 means people will find it difficult to remain standing. Class 4 means people will have to crawl to move.

The agency says that after alerts are issued in areas near epicenters, additional alerts could be issued in other areas where long-period ground motions are projected.

Weather officials said that since the year 2000, Japan has had 33 earthquakes accompanied by such ground motions with intensities of Class 3 and Class 4.

They said more people are likely to be subject to such tremors as the number of urban skyscrapers is on the rise. They are calling on people in high-rise buildings to stay calm in case alerts are issued and take steps to ensure safety.