Earthquake Early Warning in Japan: Warning the General Public and Future Prospects

The earthquake early warning (EEW) information provided by the Japan Meteorological Agency (JMA) is designed to enable public officials, key safety personnel, and the general public to take advance countermeasures against the effects of earthquake strong motion. The goal of the early warning system is to provide the maximum expected seismic intensity and the earliest S -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 effective, 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. This is particularly important given the very short warning times (a few to a few tens of seconds) so that EEW can be used effectively 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 October 2007, and discuss future prospects for the system. The parameters that the EEW must determine are the estimated origin time, the hypocenter location, the magnitude of the earthquake, the expected maximum seismic intensity (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 specifically 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. , in the middle of populated areas); the data they collect are not used for hypocenter and magnitude calculation. Seismic intensity is calculated from the maximum amplitude of …