How Japan's Earthquake prediction system worked

Japanese video shows the warning leading up to the quake.

 Japan has spent well more than $1 billion on earthquake prediction systems, including a network of more than 1,000 GPS-based sensors scattered around the country — and the payoff came today when Tokyo's residents were given up to a minute's warning that a Big One was on the way. That may not sound like much, but it's enough time for people to switch off their gas lines and get beneath a table or a door frame. [via]

"The system functioned well, because warnings were seen on television across the country," Hirohito Naito, a seismic expert at the Japan Meterological Agency, told AFP.

The agency is in charge of quake preparedness as well as weather forecasting, and researchers have invested decades of effort into Japan's early-warning system. It's considered a model for the rest of the world, and U.S. researchers are adapting it for a system known as the California Integrated Seismic Network.

The system capitalizes on the fact that a seismic event sends out two types of shock waves: primary or P-waves, which move up and down; and secondary or S-waves, which shake from side to side. The P-waves travel faster but are weaker, while the S-waves are slower but do more damage. When Japan's system picks up the P-waves, it calculates how far away the source of the shaking is and issues an alarm while the S-waves are still en route. A warning can be broadcast via TV, radio, cell phones and home alarms less than 10 seconds after the P-waves are detected.

The early warning system isn't that useful for those who are close to the epicenter, because the S-waves come quickly behind the P-waves. But because Tokyo is about 230 miles away, that city's residents could have taken action as much as 80 seconds before the serious shaking began. As noted in this Technology Review report, that amount of time can give people a chance to stop a train, lower a crane, pull a car over to the side of the road, stop performing surgery in a hospital or get off an elevator in an office building.

That's assuming that you get the alarm immediately, of course. Some reports from Japan suggested that the alarms provided somewhat less advance warning, in the range of 15 to 30 seconds. This webpage from the Japan Meteorological Agency explains the early-warning system in much more depth.

Tsunami warnings worked
It takes longer to issue a tsunami warning, because that's dependent on an analysis of wave propagation from an undersea seismic source. The Japanese government issued a local warning three minutes after the quake struck. Technology Review estimates that residents in the hardest-hit coastal areas had 15 minutes of warning, and that Tokyo would have had at least 40 minutes to prepare.

Meanwhile, experts at the National Oceanic and Atmospheric Administration's Pacific Tsunami Warning Center (in Hawaii) and West Coast/Alaska Tsunami Warning Center (in Alaska) issued their first alerts nine minutes after the earthquake. They used computer modeling as well as readings from ocean buoys to track the waves as they sped across the Pacific at jetliner speeds. The wave-monitoring system has been beefed up significantly since the 2004 Indian Ocean tsunami, which pointed up gaps in the network.
Tsunami forecasters and emergency officials called for an evacuation of coastal areas in Hawaii, which were hit by walls of water measuring as much as 7 feet high.
"We called this right," Gerard Fryer, a geophysicist for the Pacific Tsunami Warning Center, told The Associated Press. "This evacuation was necessary. There's absolutely no question, this was the right thing to do."

Longer-term predictions
Could today's quake have been predicted days in advance rather than seconds in advance? In retrospect, maybe so: A 7.4-magnitude quake that hit Japan on Wednesday is now thought to be a foreshock heralding the bigger quake to come.
Two years ago, researchers looked at the records from Japan's crustal movement sensors and determined that large quakes could be anticipated by analyzing the "pre-signals" in the seismic data.
Then again, it's always easier to predict an event in retrospect. Five years ago, The Washington Post's Joel Achenbach wrote that Japanese geologists were sure the next Big One would take place southwest of Tokyo. Today's quake certainly qualifies as that Big One ... but it happened to the northeast, not the southwest.

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