Tokyo residents were rattled Thursday morning after an alert broadcast to smartphones across the capital warned of an impending magnitude 7.3 earthquake about to hit the capital — only for no quake to arrive.

The alert at 9:38 a.m. was a false alarm, caused by a system designed to give residents precious seconds to protect themselves before an earthquake hits, which overstated the magnitude of a smaller temblor, the Meteorological Agency said.

The system mistook the epicenter of a magnitude 5.8 earthquake off Japan’s coast, calculating it to be about 450 kilometers (280 miles) further away. That caused it to overestimate the size of the quake and trigger the alert. The warning was issued to Tokyo and 14 other prefectures — areas that account for about half of nation’s total population of 126 million — and halted trains and subway lines in the capital for hours.

The quake was far enough from shore that no shaking was felt in Tokyo or the surrounding regions.

The early warning system is designed to issue alerts when an earthquake is first detected. When a temblor of a certain size is detected, warnings are issued to smartphones, TV networks and factory lines, and trains are automatically halted, to give people time to prepare before more damaging secondary-waves strike.

Smartphones in the country generally have the alert enabled by default, which will override mute settings with a loud warning noise. Tokyo residents, who have long expected the arrival of a big quake directly hitting the capital area — are also on edge due to the growing coronavirus pandemic.

The alert is fully automated. A network of seismographs spans the country, measuring the initial P-waves when an earthquake strikes. Computers then collate the data and issue estimates of the potential size and likely location of the quake.

“There are many examples of where the alert has been helpful in the past, but these incidents reduce trust in the system,” a JMA official said at a briefing to explain the alert.

The system has issued false alarms before. In 2018 an alert was triggered when the system mistook two smaller quakes for one larger one, while in 2016 some operators mistakenly warned of a magnitude 9 quake that had struck Tokyo Bay.

In a time of both misinformation and too much information, quality journalism is more crucial than ever.
By subscribing, you can help us get the story right.