How Japan's early warning system detected the earthquake

Here's how the world's first early warning earthquake detection system kicked into gear before an 8.9-magnitude quake struck Japan on Friday.

An 8.9-magnitude earthquake hit Japan on Friday, sending the area into immediate crisis mode.

But some residents knew it was coming before it happened.

A fantastic primer by Juro Osawa in the Wall Street Journal details how the world's first early warning earthquake detection system kicked into gear before the big one struck the nation's coast.

The system, developed by Japan's meteorological agency in 2007, was able to detect a shockwave near the 'quake's seismic center and relay that message over television, radio and mobile phone.

Here's how it works:

  • Seismometers detect the first shockwave.
  • Computers analyze the wave and estimate how powerful the second one will be.
  • If that wave is estimated to be more powerful than a certain threshold ("lower 5" on the local scale), an alert is issued.

It's simple enough, but it's a critical step so that companies -- think utilities, petrochemical plants, rail operators and others -- can shut down facilities and minimize damage.

Make no mistake -- the warning is issued mere seconds before the earthquake actually occurs. But it's just enough to make a difference for those further away from the seismic center -- as well as initiate a ripple effect across the globe.

More on Japan's preparedness via the New York Times:

In the country that gave the world the word tsunami, Japan, especially in the 1980’s and 1990’s, built concrete seawalls in many communities, some as high as 40 feet. In addition, some coastal towns have set up networks of sensors that can sound alarms in every residence and automatically closed floodgates when an earthquake strikes to prevent waves from surging up rivers. Ports are sometimes equipped with raised platforms.

Update: Here's a great Q&A on the subject with Brian Shiro at Popular Mechanics.

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