Earthquake warning system alerted people in Tokyo before they felt the quake

It's the world's first early warning system for earthquakes. It was developed by Japan meteorological agency, and as the earthquake shook the country on Friday, the warning system sent a warning message that went on national TV, radio and mobile phones. People in Tokyo got the message before they felt the earthquake.

From the Wall Street Journal:

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 cell phones 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.

Japan is the only country with this kind of warning system and it had never really been used in a big earthquake before. If you live near the epicenter of the quake, you wouldn't get the warning in time.

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Even though Japan is the only country to have a natilonal earthquake warning system, many stand-a-lone seismic detectors are in place in California. Working with Seismic Warning Systems, Inc. the Palm Springs and Riverside County Fire Departments have 12 seismic warning detectors installed in our fire stations here in Palm Springs and the Coachella Valley. We can receive up to 12 seconds warning before the significant shaking starts and that is enough time to automatically open the station apparatus room doors, turn on lights and radios and alert the firefighters through the PA system. We do not want our fire engines trapped in the station following the earthquake. We will have fires to fight and lives to save.

In 2010, a Public/Private partnership was developed between the Coachella Valley Association of Governments (CVAG), the Coachella Valley Emergency Managers Association (CVEMA) and Seismic Warning Systems, Inc. to equip and network over 120 sites including all 80 schools in three shcool districts (80,000 students). The system is currently being implemented as funding becomes available.

Once the regional network is stood up, each site will communicate via internet with all the seismic devices effectively doubling our warning time to up to 30 seconds on a 7.8 scenario starting near the Salton Sea on the Southern Section of the San Andreas Fault. This model is being mimicked in San Diego County, Imperial County, Baja Mexico, the High Desert Region and the San Francisco Bay area. Soon, each region will be able to alert each other with enough time to really make a difference in saving lives and property. We have the technology which has been proven over the past 11 years in the fire stations, now it's time to alert more than just our firefighters.
Blake Goetz, Fire Chief (Ret.)
Palm Springs Fire Department

There are also other precursor indicators being examined by geophysicists, ones derived from so-called nucleation events, ones which could give a longer warning. However, this science is very new, and very incomplete. See the research article, SCIENCE 18 February 2011: Vol. 331 no. 6019 pp. 877-880, DOI: 10.1126/science.1197341
"Extended Nucleation of the 1999 Mw 7.6 Izmit Earthquake",
Michel Bouchon, Hayrullah Karabulut, Mustafa Aktar, Serdar Özalaybey, Jean Schmittbuhl, and Marie-Paule Bouin. Also, that issue of SCIENCE has a nice overview of the research report's significance: SCIENCE 18 February 2011:
Vol. 331 no. 6019 p. 836, DOI: 10.1126/science.331.6019.836, "A Quake May Have Hinted That It Was on the Way", Richard A. Kerr.

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