The system went into operation in 2007 and links more than a thousand seismographs across Japan. Each location is linked to a central computer that attempts to detect the weak but fast-moving primary waves that are generated by an earthquake and use them to triangulate the location of the quake and estimate its size.

The amount of warning provided depends on the distance from the earthquake's epicenter. Those closest to the source of the quake and susceptible to the strongest shaking get little or no warning. The warnings begin to make a difference around 50 to 100 kilometers from the epicenter of a strong quake, where up to 30 seconds warning can be provided.

That may not sound like much, but it's enough time to bring high-speed trains to an emergency halt, stop factory production lines and get schoolchildren under their desks.

On Friday afternoon, the system sprung into action and a warning flashed automatically on television screens. The first waves of shaking from the quake arrived in Tokyo about 10 seconds later, said people who had televisions on at the time.

A recording of the warning as it appeared during a live broadcast from Japan's parliament, . The warning appears at the 8-second mark, and the shaking can be seen beginning about 40 seconds later and continues to build for about 2 minutes.