Published online 16 June 2008 | Nature | doi:10.1038/news.2008.893

News

Japan's monitoring system beaten by shallow quake

Earthquake takes nine lives as destruction arrives ahead of warning.

Japan earthquake zoneRegions close to the quake's epicentre got less than a second's warning.Japan Meteorological Agency

A magnitude-7.2 earthquake that hit northern Japan on Sunday morning has claimed at least nine lives, left another dozen people missing, and destroyed houses and factories throughout the region. The affected areas were too close to the earthquake-generating (seismogenic) zone, a mere 8 kilometres deep, for Japan's controversial early warning system to be of use.

The warning system takes advantage of the different speeds of two waves emanating from the seismogenic zone: 'p-waves', and slower-moving 's-waves' that carry the destructive force. The early warning system signalled a powerful quake 3.5 seconds after detecting the p-waves, but at places such as Oshu within 30 kilometres of the epicentre, the s-waves had already arrived. Residents of Kurihara, one of the cities hardest hit, received only 0.3 seconds of warning.

Farther away, at a distance of 50 kilometres, the warnings were issued 5 seconds before the violent shaking; residents at 80 kilometres' distance were given 15 seconds. Those relying on television, radio and mobile-phone systems to relay the message would have had to have waited an extra second longer than those with an independent terminal that can receive the broadcast warnings directly, says Toshiyuki Matsumori, senior coordinator for earthquake early warning at the Japan Meteorological Agency.

False intensity

The system has failed in its first three challenges since its introduction last year (see 'Japan's earthquake warning system fails again'). Earthquakes that shake the ground with an intensity of a low 5 or above on Japan’s intensity scale (which specifically measures ground shaking, rather than quake magnitude per se, and has values that go up to 7) are supposed to trigger warnings. In each of the first three such cases, the earthquake shaking was borderline and the system initially underestimated the quake, delaying the warning.

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The latest earthquake pummelled some places with a shaking intensity of 7. It was quickly recognized as a powerful earthquake (although the first seismic readouts estimated a magnitude-5.7 earthquake rather than a 7.2).

The meteorological agency is carrying out a survey to see just how well the system worked and how much warning people had. Newspapers report children who were playing at a school running for shelter after an alarm sounded, a subway train slowing down, and an electronics-factory line properly stopping in time.

But the sad truth is that those closest to the quake’s origin, who are usually hit hardest, will have the least warning. Even a system in perfect working order cannot prepare people for nearby earthquakes. “There was a lot of damage and many people were hurt or killed, so we can’t really say it was a success, but the system did work as expected,” says Matsumori. 

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