Japan's earthquake warning system: another miss, another excuse - July 24, 2008
From Nature’s Asia correspondent David Cyranoski.
Another major earthquake hit Japan just after midnight. The epicentre was along Japan's eastern coast in northeastern Iwate prefecture. Magnitude 6.8, depth of 108 km.
This is over 500 km from Tokyo, but your correspondent could feel the rolling 1 minute plus earthquake at his home 40 km south of Tokyo.
It was another miss for Japan's early warning system, and though they are still trying to figure out why it took so long, a new excuse/explanation is in the works...
The system is supposed to alert any region facing an intensity of "lower five" or more on Japan's 7 point scale that ranks the amount of shaking an earthquake brings to the surface. Since a given earthquake's impact on buildings will depend on its depth, direction, and on the geological structure of the earth, the intensity scale is related to but not directly correlated with the magnitude alone.
On several occasions since the system went into effect last fall, Japan has failed to warn or belatedly warned citizens of earthquake intensity of low 5 or more. The Japan Meteorological Agency has blamed this on a weak signal (an earthquake that barely registered lower five, and did so only in a limited area) that was difficult to pick up.
On the last such earthquake, it blamed the shallowness of the quake. Quakes that are too close to the surface leave not enough time between the p-waves (used to estimate the magnitude and depth of the earthquake so a prediction of intensity can be made) and the s-waves which travel more slowly and bring destruction.
This morning’s (00:26) earthquake, which came from a distant 108 kilometres depth and hit the epicenter with a "high 6" shaking, leaves recourse to neither of those excuses. Seismographs initially picked up the mostly-harmless p-waves at 00:26:35. The devastating s waves struck at the epicenter 10 seconds later. A warning did not go out until 21 seconds later at 00:26:56.
JMA officials told me that the reasons for the delay are now being analyzed. Initial analyses show that there might have been two earthquakes a couple seconds apart--the first, weaker one was estimated to threaten intensity of 3 or 4 and so did not trigger the warning system. The second, stronger magnitude 6.8 quake hit seconds later, might have confused the system. "It might have just taken time to untangle the signals from two quakes," says a JMA official, noting that even the idea of there being two quakes has not yet been confirmed.
Residents at a distance of 130 kilometres from the epicentre got 3-4 seconds warning. Rokkashomura, home to nuclear waste reprocessing and uranium enrichment plants, is 75 kilometers away. It was hit with an intensity of 4. No damage has been reported. So far their are reports of 15 seriously wounded and 76 others wounded.
Previously on Nature
Japan's monitoring system beaten by shallow quake – 16 June 2008
Japan's earthquake warning system fails again - 14 May 2008
Japanese nuclear reactor under-designed for earthquake? - 17 July 2007
Earthquake prediction: A seismic shift in thinking - 28 October 2004
Image: Jacques Descloitres, MODIS Land Science Team via NASA
Comments
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http://www.youtube.com/watch?v=cNVZcxKxPnk
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Posted by: Ladislav Mihaly | July 27, 2008 02:50 PM