To predict the occurrence of earthquakes in the foreseeable future, remains elusive. However, if only to shake in the days before the earthquake to provide a few seconds to several seconds early warning to avoid the scourge of terror. In that case, Japan has already done!
Japan Meteorological Agency( JMA) constructed by the Earthquake Early Warning (EEW) have been in October 2007 on the line, extended to the whole territory of Japan. EEW through early warning, allowing high-speed trains slow down early to avoid derailment accident; or allow operation of the elevator stopped at an early close to the floor, and then opened the door to avoid being trapped personnel. Although this is only a short span of a few seconds, it was extremely valuable. At present, Japanese people can make use of simple electronic devices or mobile phones, receive real-time earthquake warning.
EEW is able to issue a warning, mainly in Japan must be attributed to intensive distribution of seismic stations (about one every 20 kilometers), and the computer can quickly calculate the location of the earthquake wave propagation direction and achievements. An earthquake occurs, the first since the source of the spread of the P wave, which will be about seven kilometers per second, the speed of advance; the same time the S wave came, it is about 4 kilometers per second pace. S-wave, although the slow pace of progress, but its amplitude is often the P wave of three to ten times against huge. EEW is therefore the basis of an earlier arrival of the P wave seismic parameters, and then against the S-wave arrival of early warning in order to avoid a greater disaster occurred.
When the earthquake occurred, the source of the earthquake in neighboring stations will be received under the P wave signal, first of all to determine the earthquake intensity. Once the earthquake intensity in more than four (according to the Japan Meteorological Agency earthquake grade), the equivalent of little seismic intensity (Mercalli intensity scale) when the 6-7, EEW warning will be issued. JMA has collected more than 1000 seismic stations of the seismic wave (wave form) record, the system will in the shortest possible time than the station received the P wave, that the location of the earthquake, and then calculating the amplitude larger S wave would arrive around the time in order to achieve the effect of early warning.
If follow-up to other stations in terms of the calculation is different from the initial estimate, to reach the level of the direction of error was 0.2 degrees, the vertical direction of error was 20 kilometers, seismic intensity than the original estimate of 0.5 large or small a level, EEW will be updated prior warning issued. Even possible when the station received the first shock wave signals, so that after EEW early warning, but no follow-up to other stations received a shock wave signal that information may be wrong, at this time will be canceled EEW warning.
JMA since February 2004, the continued testing the system for up to 29 months. To June 2006 until the test period, EEW a total of 855 times warning issued, of which only 26 false alarms. These false alarms are mostly due to an engine failure or caused by lightning, and are measured from a single point of information on early warning caused by.
EEW of course, is not omnipotent, if the earthquake epicenter is close to the ground occurred in the land where, as the S wave will soon arrive in the neighboring regions, resulting in an early warning system is too late to publish. In addition, if large-scale earthquake (Richter magnitude seven or above), because the fault ruptured a long time, EEW can not complete estimate the earthquake intensity. Or when two earthquakes occurred in the same location, EEW region can not be accurately separated from the two earthquakes occurred. But no matter how, EEW has become available to the Japanese people the importance of reaching an early warning system is very important progress.
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