Japan identifies cause of false magnitude-9.9 earthquake alert that halted trains

On October 16, Japan's emergency alert system warned of a magnitude 9.9 earthquake — stronger than any recorded in history. Alarms sounded on phones, and high-speed trains temporarily stopped. Eight minutes later, the warning was canceled: the actual earthquake near Kamchatka had a magnitude of 6.9. Now the Japanese meteorological service has explained how several technical errors led to such a huge overestimation.

The Japan Meteorological Agency (JMA) determined that the system used data from a seismic station that was too far away, even though a special protective mechanism should have excluded it from the calculations. Additional errors occurred when determining the earthquake's location and choosing the mathematical formula to estimate its magnitude.

An emergency fix was implemented on the evening of October 16. However, the investigation continues: experts have yet to determine why multiple levels of protection failed to prevent the false alarm.

What happened in Japan on October 16

At around 12:38 local time, a magnitude 6.9 earthquake occurred near Russia's Kamchatka Peninsula. Its focus was at a depth of approximately 160 kilometers.

Two minutes later, Japan's emergency alert system issued a warning of a completely different scale. It determined an assumed epicenter in the Kuril Islands area and calculated a magnitude of 9.9.

The warning spread across a vast area from Hokkaido to central Japan, including the Tokyo region. Automated systems forecast extremely strong ground shaking — up to the maximum level 7 on the Japanese seismic intensity scale.

Alarms went off on mobile phones. At the same time, railway infrastructure safety systems were activated.

On the Tokaido Shinkansen high-speed line, traffic was temporarily halted in both directions between Shin-Yokohama and Toyohashi stations. Power to the affected section was cut off as part of protective procedures.

But no devastating earthquake occurred. The maximum actually recorded shaking intensity in Japan was only level 1 on the local scale — in the city of Hakodate on Hokkaido.

At 12:48, about eight minutes after the alert was issued, the meteorological agency canceled the warning. Train services were soon restored.

Why a magnitude 6.9 earthquake became 9.9

The cause turned out to be related not to a fault in a single seismograph, but to data processing by the automated system.

During the subsequent review, experts found that the algorithm used readings from a station located more than 700 kilometers from the calculated epicenter.

Such data should not have been included in the calculations. After a similar incident in July 2020, the Japan Meteorological Agency introduced a special mechanism that automatically excludes from calculations stations located 700 kilometers or more from the assumed epicenter.

However, on October 16, the protective filter did not work.

According to the Japanese publication ケータイ Watch citing the JMA, experts identified two more problems.

First, for an earthquake far outside Japan, the system applied a calculation formula intended for earthquakes within the country.

Second, when calculating magnitude and predicting shaking intensity, different assumed epicenter locations were used.

As a result, the algorithm combined unsuitable data and produced a fantastically inflated result. At the same time, the system did not recognize the value 9.9 as a possible error and automatically issued the warning.

How powerful would a magnitude 9.9 earthquake be

To understand the scale of the error, it is enough to compare it with the largest earthquake recorded by modern instruments.

In 1960, a magnitude 9.5 earthquake occurred in Chile. It remains the most powerful in the history of instrumental observations.

The Japanese system mistakenly calculated a magnitude of 9.9 — even higher than this historical maximum.

It is important to understand that magnitude is measured on a logarithmic scale. A difference of just one unit corresponds to approximately a 32-fold increase in released energy.

Between the actual magnitude of 6.9 and the mistakenly calculated 9.9, the difference is three units. This means that a hypothetical magnitude 9.9 earthquake would release approximately 31.6 thousand times more energy than the actual event near Kamchatka.

Of course, no such super-powerful earthquake occurred. This is purely an erroneous estimate by the automated system.

That is why the incident drew special attention: the warning announced an event that, in calculated strength, would surpass any known earthquake.

What has been fixed and could the mistake happen again

The Japan Meteorological Agency reported that on October 16 at around 9:30 PM, experts implemented an emergency software patch.

It restored the mechanism for excluding data from remote seismic stations. After the fix, experts reprocessed the information about the earthquake near Kamchatka and confirmed that the system no longer generates an erroneous warning for this event.

However, fixing one malfunction does not mean the investigation is complete. The weather service continues to examine the other identified problems and promises to take additional measures to prevent similar cases.

JMA apologized to residents for the false alarm and the inconvenience it caused.

Earthquake early warning systems operate under extremely tight time constraints. Only seconds may remain between detection of the first seismic waves and the arrival of dangerous shaking, so automatic calculations are performed almost instantly.

This case showed the downside of high speed: even a small error in processing input data can trigger a large-scale response from transportation and safety systems.

Now Japanese experts face a more complex challenge — to improve the reliability of algorithms without slowing the dissemination of warnings, on which human lives may depend during real earthquakes.

Based on materials from: Japan Meteorological Agency (JMA), third official statement, JMA, technical explanation of the error, ケータイ Watch, Central News Agency.