TOKYO — Japan's weather agency issued an emergency earthquake alert on the 16th warning of a magnitude 9.9 quake with a maximum intensity of 7 near Hokkaido and the Kuril Islands, then canceled it eight minutes later.
The Japan Meteorological Agency issued the alert at about 12:38 p.m. that day, according to Japanese media including the Nikkei and the Asahi Shimbun. The cancellation came after the agency found it had vastly overestimated the quake's magnitude. The agency said it is investigating the cause in detail and ruled out a malfunction caused by a cyberattack.
Japan's emergency earthquake alerts are divided into advisories, issued when weak tremors are expected, and warnings, which give advance notice of strong shaking. A warning is issued when a strong quake of intensity 5-lower or above is expected, and is sent to areas where shaking of intensity 4 or higher is feared. The alerts are delivered through television, radio and mobile phones.
The agency's initial automated prediction system miscalculated not only the magnitude but also the time and epicenter of the quake. The agency first assessed that the quake struck near the Kuril Islands at about 12:36 p.m. and, based on that reading, simultaneously sent warnings of strong shaking across a wide area including Hokkaido, Tohoku, Kanto, Hokuriku, Koshin, Tokai and the Izu Islands.
In reality, the only place where shaking of intensity 1 or higher was observed was Hakodate in Hokkaido, at intensity 1. It is the first time since the system was introduced that an emergency earthquake alert sent to the public has been canceled as a false alarm.
The agency held an emergency news conference at 2 p.m. that day and bowed in apology. "We deeply apologize for issuing an emergency earthquake warning with a vastly overestimated magnitude, causing anxiety and great inconvenience to the public and relevant agencies," said Ayaki Ebita, head of the agency's earthquake and tsunami monitoring division.
On the cause, the official said: "A detailed investigation is still under way, but no record of abnormal external access to the system was confirmed at the time of the false alarm." The official ruled out an error caused by a cyberattack.
The largest earthquake ever recorded in human history was the 1960 Chile earthquake, at magnitude 9.5. The magnitude 9.9 in the false alarm would, if it actually occurred, be the largest on record in seismological history. Ebita said the figure of magnitude 9.9 is "an unrealistic number that is virtually impossible to occur in seismological terms."