An Earthquake Early Warning is issued to warn the general public when an earthquake of 5 or higher on the Japan seismic scale is expected. An EEW forecast is issued to the National Meteorological and Hydrological Services when an earthquake of 3 or higher on the Japan seismic scale (or 3.5 or higher on the Richter magnitude scale) is expected, or when the amplitude of P- or S-waves measures more than 100 gals.The JMA has two EEW systems: one for the general public and another for the National Meteorological and Hydrological Services. When a P-wave is detected from two (or more) of the 4,235 seismometers installed throughout Japan, the JMA analyzes and predicts the approximate location of the earthquake's epicenter. This allows the JMA to notify people in affected prefectures by TV and radio if a strong earthquake is expected.
In Japan, the Earthquake Early Warning (EEW) (Kinkyū Jishin Sokuhō) is a warning issued when an earthquake is detected by multiple seismometers. These warnings are primarily issued by the Japan Meteorological Agency (JMA), with guidance on how to react to them.The system was developed to minimize earthquake damage and enable people to take shelter or evacuate dangerous areas before the arrival of its strong surface waves. It is used by railways to slow trains and by factories to halt assembly lines before the earthquake hits.
The effectiveness of the warning depends on the position of the receiver. After receiving a warning, a person has from a few seconds to a minute or more to take action. Areas near an epicenter may experience strong tremors before a warning is issued.After the 2011 Tōhoku earthquake and tsunami, the EEW system and Japan's tsunami warning system were considered effective. Although the tsunami killed over 10,000 people, it is believed that the casualties would have been much higher without the EEW system. In April 2011, the Chilean Subsecretary of Telecommunications said that their country hoped to establish a similar early-warning system.
The JMA announced the Earthquake Early Warning hit (accuracy) rate for the 2011 fiscal year on 31 May 2012. The hit rate is the percentage of warnings issued immediately on the detection of P-waves with a Japan Meteorological Agency seismic intensity scale (shindo) number (0 to 7) within one shindo number of the measured earthquake.For the fiscal years 2007–2009, the hit rate was 75 percent or higher (75 percent in 2007, 82 percent in 2008 and 76 percent in 2009). The 2010 hit rate fell to 28 percent, due to the number of aftershocks following the 2011 Tōhoku earthquake (which occurred near the end of the 2010 fiscal year); before the earthquake, the hit rate was 72 percent. Measurement techniques have since been refined to ignore minor earthquakes, and the hit rate for FY2011 increased to 56 percent. The JMA intended to increase the hit rate to over 85 percent by FY2015.
Technical improvements are being made to increase the hit rate, including the Integrated Particle Filter (IPF) and Propagation of Local Undamped Motion (PLUM) methods. The IPF method, introduced on 14 December 2016, obtains seismic source elements with a particle filter.The PLUM method, introduced on 22 March 2018, predicts the seismic intensity directly from observed intensity without estimating the hypocenter and scale. Accurate seismic-intensity forecasts can be made for large earthquakes or those whose hypocenter is unknown.
On NHK television channels and other Japanese TV broadcasters (ISDB, including 1seg), an alert is a message window on the screen with the earthquake epicenter and areas affected by strong tremors. Two sets of chimes sound, followed by a voice announcement in Japanese: "This is an Earthquake Early Warning. Please prepare for powerful tremors." , Kinkyū Jishin Sokuhō desu. Tsuyoi yure ni keikai shite kudasai.) In addition to NHK, the announcement is used by Fuji TV, TV Asahi and Tokyo MX. Nippon TV and TBS shorten it to "Kinkyū Jishin Sokuhō desu" ("This is an Earthquake Early Warning"). TV Tokyo sounds a set of chimes, without a voice announcement.The alerts also inform viewers of possible landslides or a tsunami caused by the quake in the affected area. If tsunami warnings are issued, the system utilizes 1seg to automatically turn on (and tune to NHK) all radios and televisions with 1seg technology in at-risk areas. In addition to Japanese, the warnings are broadcast in English, Mandarin, Korean and Portuguese.
On 30 May 2007, SoftBank announced development of an EEW broadcast system similar to NTT docomo's and au's. Deployment of the system was originally planned for FY2008, but was postponed for two years. On 25 August 2010, EEW service began on Shikoku, in the Kansai, Tōkai, Tōhoku (seven prefectures) and Chūgoku regions, and portions of the Kantō region. The EEW broadcast network has covered the whole country since 7 December 2010. The SoftBank 831N was the only model supporting EEW in March 2011, although more models had been expected to support the system after summer 2010.
Weathernews, a weather-information company, began a paid EEW service (The Last 10 Seconds) on October 15, 2007.The service requires a computer running Windows 2000 or later with an always-on connection to the Internet. The EEW application can be configured to receive information on earthquakes with a JMA magnitude of 3.5 or higher or with a seismic intensity of 3 or higher. Newer versions of the program allow for the announcement of lower-intensity earthquakes. The program announces the approximate location of the epicenter, the expected JMA seismic intensity and displays a countdown to expected major shaking.
ANET (ANET), a disaster-prevention technology company which is part of the Railway Technical Research Institute Group, released an application (EQMessenger) to receive ANET Alert on 7 July 2008. This deciphers and broadcasts EEW information on the epicenter, the estimated seismic intensity at the user's location, and the time remaining before the arrival of the S-wave. When the estimated seismic intensity exceeds the preset level, EQMessenger can sound a warning and display the epicenter, intensity estimation and the arrival of the tremor on a pop-up map. A similar, free Windows program, SignalNow Express, was made available by the Strategy Corporation after the 2011 Tōhoku earthquake. A free multi-platform program, that works on Windows, Mac, and Linux, JQuake, was created in February 16, 2020 as an inspiration to another program called Kiwi Monitor. JQuake tracks information in real-time, and reports any tsunami events that occur.