JMA's early-warning system gathers data from a series of a thousand seismographs across Japan—200 operated by the agency and another 800 used by the National Institute for Earth Science and Disaster Prevention.
The seismic stations measure the intensity of a quake's primary wave, or P-wave, which travels through rock at about 4.3 miles (7 kilometers) a second. This data gets sent automatically to the agency.
Scientists can then use the P-wave to determine the scale of the quake's secondary wave, or S-wave. This wave moves slower—about 2.5 miles (4 kilometers) a second—but is what causes damage at the surface.
Prerecorded alerts are automatically broadcast on television and radio and are electronically transmitted to municipal governments, hospitals, construction sites, and railway operators.
The agency issues a warning only if the quake is predicted to be at level 5 or higher on the Omori Seismic Intensity Scale, the seven-point scale used widely in Japan.
Tetsuzo Fuyushiba, Japan's minister of land, infrastructure, and transport, launched the warning system in October 2007 amid high hopes for its effectiveness.
But on January 27, 2008, a level 5 quake struck in the early morning in Ishikawa Prefecture on Japan's northern coast.
No alert was issued, because the early-warning system predicted the tremor would be a level 4. No injuries were reported.
And on April 29 the agency issued a warning to residents of Okinawa Prefecture of a level 5.2 earthquake near the island of Miyako Jima—some four seconds after a level 4 tremor had shaken the area.
Once again no one was injured, but school buildings and airport facilities were damaged.
The agency believes the warning went out late because the quake's epicenter was very close to the island, so data on the seismic waves didn't arrive at the agency in time.
(Related: "Earthquake Fault Under Tokyo Closer Than Expected, Study Finds" [July 14, 2005].)
These glitches in the warning system highlight the difficulty scientists face trying to anticipate an earthquake's destructive power.
(Related: Read about technologies for predicting the next "big one" in National Geographic magazine.)
And even with advance notice, some temblors are likely to be too strong to avoid significant damages.
In 1995 a level 7.3 quake hit the city of Kobe just before dawn, killing more than 6,400 people.
"Even if we had received a warning a few seconds in advance, there would have been nothing we could have done," said Nobuo Nakai, 69, who helped pull his neighbors from their wrecked homes in the quake's aftermath.
"Nature is so powerful, and we cannot accurately predict events such as this," he said.
"Some people say that animals like frogs and catfish can sense when an earthquake is coming and that they act oddly," Nakai added.
(Related: "Pandas Sensed China Quake Coming?" [May 15, 2008].)
But for the Kobe quake "no one was watching them at the time."
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