http://bbs.wenxuecity.com/na/1213505.html
One minute prior to the effects of the earthquake being felt in Tokyo, the Earthquake Early Warning system connected to more than 1,000 seismometers in Japan sent out warnings on television of an impending earthquake to millions. This was possible because the damaging seismic S-waves, traveling at 4 kilometers per second, took about 90 seconds to travel the 373 km (232 mi) to Tokyo. The early warning is believed by the Japan Meteorological Agency to have saved many lives.[29][30]
http://en.wikipedia.org/wiki/2011_Sendai_earthquake_and_tsunami#Japan
