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

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

Early Friday afternoon, Japan's Earthquake Early Warning told Hokkaido through central Japan to brace for strong shaking from a quake it placed in the Kuril Islands. The alert itself showed no magnitude, but the system behind it had sized the quake at 9.9. 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 the public alert carried
The estimated source area and the regions expecting strong shaking. Under JMA rules the public warning gives no magnitude and no intensity values; the M9.9 was in JMA's detailed forecast data and its issuance record (JMA)
What happened
Preliminary M6.9 near the Kamchatka Peninsula, origin time 12:35 PM JST, 50.0N 158.1E, about 160 km deep; maximum intensity 1 in Hakodate; no tsunami concern (JMA earthquake information). JMA's first press release gives about 12:38 PM, when its stations detected the waves
USGS
M6.9 (Mww), 135 km deep, 76 km NE of Ozernovskiy, Russia, 03:35 UTC Oct 16 (USGS us6000u2mq)
Causes
Stations 700 km or more from the source were used to compute magnitude because a rule to exclude them, added after the July 30, 2020 Torishima false alarm, was not working; JMA calls this one of the causes (third report). Its second report also names a domestic magnitude formula applied to a very distant quake, and different source locations used for magnitude and for the shaking forecast
All facts (7)
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 warning the public gets, the one NHK must broadcast and carriers push to phones, does not carry a magnitude. Under JMA's rules it gives the estimated source and the regions where strong shaking is expected, without intensity values. The 9.9 was in JMA's more detailed forecast data, which goes to licensed providers, and in the issuance record JMA published afterward.

The earthquake that set it off was real but far smaller. JMA's earthquake information puts its origin at 12:35 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 found one of the causes. 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 leave out stations 700 km or more from the source, and on Friday that rule was not working. An earlier report that day named two more problems: a magnitude formula built for quakes in and near Japan was applied to one very far away, and the source location used for the magnitude differed from the one used to forecast shaking. JMA applied an emergency fix around 9:30 PM and says that, 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 strong shaking from a quake it had sized at magnitude 9.9, after a safeguard quietly failed.

Sources

Primary + secondary attribution as listed.

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