In Tokyo, because of its distance from northern Japan, the warning message reached people before the quake actually jolted the capital. When an earthquake occurs, seismometers near its source detect the first shockwave, which is usually mild and harmless. This wave is followed by another wave that travels more slowly and is much stronger and potentially destructive.Note that this was developed by the Japanese equivalent to the American NOAA.
The agency's computer system analyzes the first, faster-traveling wave and quickly estimates how powerful the second wave will be; if it is likely to be very powerful, the system immediately issues a warning message.
The message comes on TV and radio, and many Japanese cellphones can receive and display the message. Some power and gas facilities as well as railway and industrial systems automatically halt their operations the moment they receive the warning message, the spokesman said.
In places relatively far from the seismic center, people can receive the warning up to half a minute or so before the quake reaches those areas.
Japan is the only country that has such an early warning system, according to the meteorological agency. The agency developed the system with help from scientists and launched it in December 2007.
The system is designed to send out the warning only if the intensity of the quake is lower 5 or stronger on the Japanese scale. (The scale has three levels for each number, so there's lower 5, 5 and upper 5.) Friday's quake was 7 on that scale, and the system sent out the warning message 8.6 seconds after it detected the very first shock wave.
30 seconds might not seem like a lot - but that's enough time to duck and cover or get others to a safer location. It can also help shut down chemical processes before the tremor hits - limiting damage further.
Scientists have been working on developing alert systems for years - discerning trigger signals that act as precursors to the movement that causes the potential damaging shaking.
P-waves and S-waves. Both types leave the fault surface at the same time, but there the similarities end. P-waves, like sound waves, are compression waves. They travel relatively quickly, but they do not carry much power. During an earthquake, you feel the P-waves as a sudden, vertical thump. S-waves are more like ocean waves, slow movers that contain most of the energy and bring the strongest shaking. The ground motion is horizontal and vertical, and they can bat entire buildings around like they were dinghies in the surf.The Japanese meteorological agency managed to install their system, but the US has been sorely lagging in this department.
In addition, not all waves look alike; they take on different shapes depending on the size of the slip patch. The P-wave radiation for small slip patches has relatively low amplitude and high frequency—a small but sharp pulse. Bigger earthquakes rupture larger areas of a fault and have more slip, so the P-wave is larger in amplitude and lower in frequency. It is akin to the difference between the squeak of a small bird and the roar of a grizzly bear.
A single seismometer could estimate the magnitude of the earthquake based on just this information. Any P-wave with high amplitude and low frequency would trigger a warning. This single-station approach is the fastest way to give warnings near the epicenter. Yet the character of earthquake ruptures varies—not all magnitude 5.0 earthquakes look the same—and the specific sediments underneath the seismometer modify the P-wave. This variability increases the risk of both false alarms—warnings when there is no earthquake—and missed alarms when a damaging earthquake is under way.
The US, and particularly the West Coast and California, would benefit from such a system, but work on a similar system is well behind the curve. Even Mexico has a system in place, but the Japanese have taken it to another level with alerts that are broadcast not only on televisions and radio, but with text messages and email alerts for all manner of communications paths.
The Japanese system's merits were proven in earlier quakes, coincidentally hitting near Sendai (the epicenter of the worst damage in today's quake), when factories managed to shut down key systems reducing fire and other damage.
So, why has the US lagged? Funding - or the lack of same. Part of the problem is the deployment of thousands of sensors around California and the most quake-prone areas. There are significant gaps in the sensor placement. That's critical data that is missing.
And the cost for the installation? $80 million.
That's a small price to pay for the potential of saving countless lives, and reducing property damage.