Japan issued alert for ‘largest recorded’ 9.9 magnitude earthquake, but ground didn’t shake: What happened

The ‘earthquake’ that never happened

Japan triggered widespread alarm on Friday after its earthquake early warning system “announced” a magnitude-9.9 earthquake alert, prompting emergency phone alerts, public loudspeaker warnings and television broadcasts urging people to take shelter, before the warning was withdrawn eight minutes later.However, no major shaking followed the alert, which the Japan Meteorological Agency (JMA) later attributed to errors in its earthquake magnitude estimation and location calculations. The agency is investigating the cause of the malfunction, reportd Japan Times.The alert was issued at 12.40 pm (local time), two minutes after a magnitude-6.9 earthquake struck off Russia’s Kamchatka Peninsula. The warning incorrectly placed the epicentre near the Kuril Islands and predicted seismic intensity of up to shindo 7, the highest level on Japan’s seven-point seismic intensity scale, across areas stretching from Hokkaido to the Tokai region.In Japan, however, the recorded shaking reached only shindo 1, a level at which vibrations may be barely noticeable. At 12.48 pm (local time), the JMA issued a cancellation notice, marking the first time an emergency earthquake warning had been formally withdrawn, according to the agency.The predicted magnitude of 9.9 also drew attention because it exceeded the magnitude of any earthquake recorded in history. The 1960 earthquake off Chile’s Biobio region, the strongest on record, measured magnitude 9.5. The JMA said 9.9 was the highest magnitude its system could communicate externally.Ayataka Ebita, head of the JMA’s earthquake and tsunami observation division, said the system had mistakenly estimated the earthquake’s epicentre and overstated its magnitude. The agency ruled out a cyberattack and said it was examining how the error occurred.At a subsequent briefing, the JMA said the system had applied a magnitude formula designed for earthquakes within Japan to a quake occurring far away. It had also used different estimated locations to calculate the magnitude and forecast shaking, although both calculations should have been based on the same location.The investigation is continuing. Ebita said lightning was not believed to have caused the error, adding that the processing of seismic sensor data could have contributed to the malfunction.In Tokyo on Friday, pedestrians paused to check their phones as warnings circulated, while users of the NERV Disaster Prevention app received a magnitude-9.9 alert accompanied by a countdown to shaking that never materialised.Lola Milroy-Tompkins, a 20-year-old British student at Waseda University in Tokyo, said she was eating lunch in a campus cafeteria when the alarm sounded.“I have never experienced an earthquake before and I expected the grand style shaking and everything to start falling around me,” she said.She recalled students worrying and talking among themselves as they tried to understand what was happening, but the anticipated earthquake never arrived.The JMA’s early warning system automatically combines readings from seismometers at observation stations to estimate an earthquake’s location, magnitude and expected intensity. Ebita said the reliability of such estimates depends on the number of observation stations and the quality of their readings, which can deteriorate when an earthquake occurs far from monitoring equipment.The incident also disrupted public transport. The Tokaido Shinkansen line was halted for 15 minutes after an automated safety system was triggered, Central Japan Railway said. Government officials reported no damage related to the Kamchatka earthquake.Chief cabinet secretary Minoru Kihara said the government took seriously the anxiety caused by the warning and the burden on people who evacuated or took precautionary measures, as well as those whose travel and business activities were disrupted.False alarms have occurred previously. In July 2020, the JMA issued a warning covering 15 prefectures, including Tokyo, after incorrectly estimating an earthquake’s location and magnitude. In August 2013, noise from a seabed seismometer was mistakenly combined with readings from a small earthquake, triggering widespread warnings across the Kinki region. Neither incident produced shaking of shindo 1 or higher, according to the JMA.The agency’s website noted that false alarms can result from sensor noise caused by accidents, lightning or equipment failures.