The Meteorological Agency issued a false alarm saying a strong quake was imminent in Tokyo and surrounding areas early Tuesday, causing subway and other transportation services to shut down briefly before the morning rush hour.
The quake warning was issued around 6:37 a.m., but only an imperceptible jolt occurred in the region at around 6:36 a.m., the agency said, adding that it is investigating the cause of the error, including a possible computer glitch.
The agency said in the afternoon that unnecessary updates by an assigned company the previous day caused the erroneous alert as the system transmitted excessive figures on the strength of the quake at a point in Chiba Prefecture.
The alert caused Tokyo Metro to suspend services on eight of its subway lines for two to nine minutes, but the timetables were restored by 8 a.m.
Other rail services were affected. However, the Japan Railway group, which includes bullet-train services, was not impacted because it operates its own alarm system.
A quake with a preliminary magnitude of 4.1 occurred around 6:36 a.m. about 20 km under the sea east of Chiba Prefecture, but it was imperceptible to humans, the agency said.