This is pretty damned spectacular and cool. Tokyo and other areas of the country received alerts of the impending massive 8.9 quake before it could be felt.

In Tokyo, because of its distance from northern Japan, the warning message reached people before the quake actually jolted the capital.

When an earthquake occurs, seismometers near its source detect the first shockwave, which is usually mild and harmless. This wave is followed by another wave that travels more slowly and is much stronger and potentially destructive.

The agency’s computer system analyzes the first, faster-traveling wave and quickly estimates how powerful the second wave will be; if it is likely to be very powerful, the system immediately issues a warning message.

The message comes on TV and radio, and many Japanese cellphones can receive and display the message. Some power and gas facilities as well as railway and industrial systems automatically halt their operations the moment they receive the warning message, the spokesman said.

In places relatively far from the seismic center, people can receive the warning up to half a minute or so before the quake reaches those areas.

Japan is the only country that has such an early warning system, according to the meteorological agency. The agency developed the system with help from scientists and launched it in December 2007.

The system is designed to send out the warning only if the intensity of the quake is lower 5 or stronger on the Japanese scale. (The scale has three levels for each number, so there’s lower 5, 5 and upper 5.) Friday’s quake was 7 on that scale, and the system sent out the warning message 8.6 seconds after it detected the very first shock wave.

Note that this was developed by the Japanese equivalent to the American NOAA.

30 seconds might not seem like a lot - but that’s enough time to duck and cover or get others to a safer location. It can also help shut down chemical processes before the tremor hits - limiting damage further.