The world's first earthquake early warning system has begun operating in Japan. The government-developed system takes advantage of the precious seconds between the first indication of a powerful tremor and the arrival of the second, destructive terrestrial shock. Some people have reservations about broadcasting such alerts.

If a large earthquake were to strike Japan today, some people would receive advance notice.

This kind of public address alert, recorded at a Tokyo university, will automatically be triggered at certain locations seconds before a big tremor is felt. In other cases, such as trains, the alert will be transmitted digitally, or by radio.

The secret to the system lies in the anatomy of an earthquake, which actually generates two waves. The first is a smaller, primary wave that travels at seven kilometers per second. The second wave, the one that can cause damage, moves at a slower four kilometers per second.

The new system takes advantage of the gap between the two waves to issue a warning that a quake is coming. Japan's nationwide network of some 1,000seismographs, developed over several decades, is now linked to a computerized system that can trigger the alert. It became operational this week.

Kenji Kuwashiro of the Keihin Kyuko Railway says its commuter train network is already using the early warning system.

Kuwashiro says signals will be sent to all trains to allow engineers to stop quickly, possibly before a potentially damaging seismic jolt hits the tracks.

While proponents of the systemtoutits ability to save lives at constructions sites, aboard trains or in hospital operating theaters, others fear the system might pose a new hazard. They worry that people will panic if an alert is issued.

Makoto Saito, the senior coordinator for the warning system at the Japan Meteorological Agency, acknowledges this possibility.

Saito says national guidelines need to be drawn up and the public educated about the system so it will save lives rather than endanger them.

In the meantime, out of caution, Japan's television and radio stations are not yet planning to broadcast the alerts.

Japan is one of the most vulnerable nations to earthquakes in the world. Some one-fifth of all of the world's earthquakes of magnitude six or stronger originate below or in close proximity to Japan.