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official data (Japan Meteorological Agency press releases, Oct 16, first and third reports; JMA EEW issuance record)

Japan's earthquake warning system announced an M9.9 that never happened

Origin Lane: climateNear the Kamchatka Peninsula · 50.0°N 158.1°E (JMA hypocenter, about 160 km deep) · warning area Hokkaido to Chubu, JapanBy the Doom Signals desk

DATA: Japan Meteorological Agency, Oct 16 press release

For a few minutes on Friday afternoon, phones from Hokkaido to central Japan said a magnitude 9.9 earthquake was on its way. The ground barely moved. The quake was real, an M6.9 far to the north, and the number was the machine's.

Warning issued
12:40 PM JST Fri Oct 16 (11:40 PM EDT Thu Oct 15), Hokkaido through Chubu (JMA)
What the system computed
Source in the Kuril Islands at 48.2N 153.4E, 140 km deep, magnitude 9.9; strongest forecast intensity 5 lower to 7 in northern Nemuro (JMA EEW record)
What happened
Preliminary M6.9 near the Kamchatka Peninsula at about 12:38 PM JST, 50.0N 158.1E, about 160 km deep; maximum intensity 1 in Hakodate; no tsunami concern (JMA)
USGS
M6.9 (Mww), 135 km deep, 76 km NE of Ozernovskiy, Russia, 03:35 UTC Oct 16 (USGS us6000u2mq)
Cause
Stations far from the source were used to compute magnitude; a rule excluding stations more than 700 km away, added after the July 30, 2020 Torishima false alarm, did not work (JMA third report)
Fix
Emergency correction around 9:30 PM JST Oct 16; JMA says the system no longer issues a warning for this quake (JMA third report, 10:40 PM JST)

We apologize for the great inconvenience and burden caused to the public by this erroneous Earthquake Early Warning.

Japan Meteorological Agency, third report, Oct 16 (translated from Japanese)

At 12:40 PM Japan time on Friday, October 16, the Japan Meteorological Agency issued an Earthquake Early Warning for Hokkaido through the Chubu region of central Japan. Its automatic processing had placed the source in the Kuril Islands, at 48.2 North, 153.4 East and 140 km deep, and estimated the magnitude at 9.9. The forecast for northern Nemuro in eastern Hokkaido was seismic intensity 5 lower to 7. In Eastern time it was 11:40 PM on Thursday.

The earthquake that set it off was real but far smaller. JMA puts it at about 12:38 PM near the Kamchatka Peninsula, at 50.0 North, 158.1 East, about 160 km down, with a preliminary magnitude of 6.9. The strongest shaking anywhere in Japan was intensity 1, in Hakodate, and JMA said there was no tsunami concern. USGS rates the same quake M6.9 at 135 km depth.

By late evening JMA had a cause. Its third report says stations far from the source were fed into the magnitude calculation. After a false alarm from a quake near Torishima on July 30, 2020, the agency had added a rule to ignore stations more than 700 km from the source, and on Friday that rule did not work. JMA applied an emergency fix around 9:30 PM and says that, rerun with the fix, this quake no longer triggers a warning. It apologized and said its investigation continues.

The warning was wrong by three full magnitude units. Somewhere in the code, a filter stayed open.

Why it matters

Japan's early warning system is built to be trusted in the seconds that matter. On Friday it told much of the country to brace for a magnitude 9.9, because one safeguard quietly failed.

Sources

Primary + secondary attribution as listed.

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