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When an earthquake occurs, a certain amount of time elapses before destructive seismic energy hits nearby population centers. Though this time is measured on the order of seconds, depending on the proximity of the rupture to a given city or town, a new public safety program in Japan is taking advantage of the fact that seismic energy travels slower than electronic communication.In this program, the Japan Meteorological Agency (JMA) rapidly determines the hypocenter (earthquake epicenter and focal depth) and magnitude of the earthquake by using real-time data from stations near the hypocenter. The distribution of strong ground shaking is anticipated quickly, and then the information is delivered immediately to government officials, representatives from various industries, members of the news media, and individuals before strong ground shaking reaches them. For example, on receiving the warning, the control room of a railway company can send an emergency notice to all train drivers to stop their trains immediately, elevators in buildings can be triggered to stop at the nearest floor and open their doors automatically, and surgeons can temporarily suspend their surgical operations to avoid risk to patients on operating tables.This innovative new service, called Earthquake Early Warning (EEW), started nationwide in Japan and became fully operational in October 2007. This service is definitely different from earthquake prediction. Although it is currently impossible to be aware of earthquakes before their occurrence (earthquake prediction), EEW operates with the assumption that it is possible to warn people located at a certain distance from the hypocenter before strong ground shaking reaches them.Even though the interval between the delivery of EEWs and the time when strong shaking reaches people is relatively short (counted in seconds), EEWs can be a useful and powerful tool for mitigating an earthquake disaster by giving people enough time to take appropriate safety measures in advance of strong shaking. Determining Hypocentral Parameters and Anticipating Seismic IntensityEarthquakes occur when stressed rock moves through brittle rupture. Two types of seismic waves are radiated from the hypocenter: One is the P wave, which travels at about 7 kilometers per second, and the other is the S wave, which travels at about 4 kilometers per second.EEW technology not only takes advantage of the relatively slow velocity of the seismic waves as compared with instantaneous electronic communication, but it also uses the difference in arrival time between P and S waves. The S wave is slower than the P wave, but the amplitude of the S wave is usually 3-10 times larger than that of the P wave. This generally means that stronger shaking is observed along the S wave.The hypocenter and magnitude of an earthquake are determined as quickly as possible using only early parts of the P waves at a few stations close to the hypocenter. Using information about the hypocenter and magnitude, the arrival time of the S waves and seismic intensit...

INTRODUCTIONe earthquake early warning (EEW) information provided by the Japan Meteorological Agency (JMA) is designed to enable public o cials, key safety personnel, and the general public to take advance countermeasures against the e ects of earthquake strong motion. e goal of the early warning system is earliest -wave arrival time in each subprefectural area (about a quarter to a third of one prefecture) before the strong motion arrival. For the system to be e ective, it is essential that JMA publicize the principle and purpose of the warning messages, the technical limits of EEW, and the proper actions to be taken when a warning is received. is is particularly important given the very short warning times (a few to a few tens of seconds) so that EEW can be used e ectively without causing unnecessary confusion. In this article we outline the design of the EEW system in Japan and the necessary preparatory process to start providing EEW information to the general public, summarize the performance of the system since it was launched nationwide in DESIGN OF THE EEW SYSTEMe parameters that the EEW must determine are the estimated origin time, the hypocenter location, the magnitude (in the JMA intensity scale, http://www.jma.go.jp/jma/en/ Activities/earthquake.html#S_I), and earliest arrival time (in seconds) of the strong motion for each subprefectural area.In Japan, seismic intensity has been recorded by instruments designed speci cally for this task (called "seismic intensity meters") since 1996. Seismic intensity meters observe seismic intensity at representative sites for the purposes of disaster mitigation (i.e. collect are not used for hypocenter and magnitude calculation.the acceleration a er a lter with (1/ ) 1/2 amplitude response 1996). Physically, seismic intensity is proportional to the loga--tion site per unit time. JMA has been issuing seismic intensity subprefectural area within two minutes a er the earthquake occurrence when seismic intensity of 1 or over (in JMA scale) is observed. More detailed information (seismic intensity at each observation site) promptly follows this report. Although these are "post-disaster" reports, they have been used as trigger information to start emergency responses such as directing rescue resources to an area where strong motion was observed. To integrate strong motion disaster mitigation, EEW was developed to enable countermeasures of the strong motion arrival. For EEW, seismic intensities are evaluated at -out Japan. Estimation of the seismic intensity has three steps: 1) estimation of hypocenter, 2) estimation of magnitude, and -amplitude on engineering bedrock (Si and Midorikawa 1999), 2) multiplication by the ampli cation coe cient to account conversion from velocity to seismic intensity (Midorikawa 1999). e hypocenter is the starting point of the rupture and is the earthquake, i.e., the magnitude, is predictable at the starting time of the rupture is controversial (e.g., Iio Nakatani a method that is applicable in the real-time processing environment to forecas...

ABSTRACT:JMA is now developing Earthquake Early Warning system. In this issue, the methods used for prompt estimation of hypocenter locations, magnitude and seismic intensities, the warning dissemination criteria, and future plans are briefly introduced.

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