Similar false alarms have occurred in the past. In July 2020, the JMA issued a warning to 15 prefectures, including Tokyo, after incorrectly estimating a quake’s location and overstating its magnitude.
In August 2013, noise from a seabed seismometer was mistakenly combined with data from a small earthquake, triggering widespread false warnings across the Kinki region.
No root cause for this one yet, but:
i misread “seismometer” as “sea monster”
Godzilla !