Japan's Meteorological Agency plans to start issuing alerts in February for long and slow tremors that would affect skyscrapers, as part of its early warnings for powerful earthquakes.

The current emergency warning system issues alerts when a strong earthquake of an intensity of lower 5 or more on the Japanese scale of zero to 7 is projected by monitoring certain types of seismic waves.

Alerts for seismic events called "long-period ground motions" will be added to the system. High-rise buildings are especially vulnerable to such prolonged shakings -- even in locations distant from the epicenter.

The March 2011 mega-quake in northeastern Japan caused skyscrapers to sway in faraway Tokyo and Osaka.

The Meteorological Agency has a four-level scale to rank the intensity of such long-period tremors.

It plans to issue emergency warnings to areas in which the two highest intensities of Class 3 and Class 4 are predicted. The second highest, Class 3, means people will find it difficult to remain standing. The early warnings will urge people on the upper floors of high-rise buildings to take protective action.

Agency officials say Japan has had 33 earthquakes accompanied by long-period ground motions of Class 3 or Class 4 since the year 2000.

They say the increase in urban skyscrapers means more people are likely to be affected by such tremors.