Japan has switched on a nationwide advance warning system for earthquakes. The system gives any person tuned to the radio or television, seconds of warning before a large quake.
There is also a provision for purchasing for homes or businesses special receivers that deliver emergency notices by loudspeaker.
Scientists say this will ensure that warnings are received during the night too.
According to a Nature report, the system is designed to give people roughly 10 to 20 seconds of warning, depending on their distance from the quake's epicentre. Warnings are triggered for a given area if the earthquake intensity is expected to be magnitude 5 or greater.
Earthquake engineer Makoto Saito of the Japan Meteorological Agency in Tokyo said the warnings work because they are transmitted by telecommunication systems faster than a quake itself propagates outwards from its epicentre.
A network of 1,000 land-based seismometers throughout Japan is geared to detect the first rumblings of a quake through the pressure waves (p-waves) that are first to hit the surface, he said.
He said a quick analysis of these p-waves together with any other available seismic information gives an estimate of the magnitude and place where the earthquake originated. From this data, computers estimate the intensity of shaking that this quake will produce in various areas when slower-travelling s-waves reach the surface, he added.
While the system has been in use in Japan since 1989 to stop trains in the event of strong earthquakes, and has also been rolled out to other facilities, such as power stations, as well, this is the first time it has been extended to the general public.
Officials hope the warnings will help people get to safety. Although there will not be much that some people can do, it should prevent people from getting trapped in elevators, allow surgical procedures and other sensitive work to stop, and give schools time to prepare students.
However, there is concern that the warnings could lead to panic and dangerous overreactions. "We are trying to teach people exactly what to do so that they know how to react," said Saito. Saito said while other nations have also investigated or installed similar systems, none is as extensive as Japan's.
"As far as a nationwide system with this kind of coverage goes, ours is the first," said Saito.