Japan Issues Megaquake Warning After 7.1 Magnitude Tremors

An earthquake off the coast of Japan's southern island of Kyushu prompted the government to issue its first-ever warning for a potential megaquake originating from an undersea trough running southeast along the archipelago.

In response to the quake, bullet trains were slowed, the national broadcaster NHK displayed continuous cautionary graphics, and a semiconductor factory temporarily halted production.

The warning, triggered by a magnitude-7.1 earthquake on Thursday, concerns the Nankai Trough, a geological trench where the Philippine Sea plate subducts beneath the Eurasian continental plate. This area is known for producing megaquakes, which are defined as major tremors occurring in 100- to 150-year cycles and could potentially cause more damage than the March 2011 earthquake and tsunami that devastated Japan’s northeastern coast.

Japan's precautionary warning system, implemented several years ago, activates when a magnitude-6.8 or larger earthquake strikes a region where such megathrusts are expected, or when anomalous plate shifts are detected. The system allows authorities to issue varying levels of caution, including warnings for residents to evacuate from tsunami-prone areas for up to a week.

Although Thursday's earthquake did not result in significant damage or a tsunami, the Kyushu region, home to semiconductor factories operated by Sony Group Corp. and Taiwan Semiconductor Manufacturing Co., is taking precautionary measures. NHK interrupted its Olympics coverage to broadcast updates on the quake and the subsequent megaquake warning, with continued bulletins into Friday.

A 2019 government panel analysis estimated that a worst-case scenario from a Nankai Trough quake could result in over 230,000 deaths. Historically, major earthquakes in Japan have been preceded by smaller tremors; for example, foreshocks were detected two days before the 9.0-magnitude earthquake in 2011, which caused nearly 20,000 deaths and triggered a nuclear meltdown in Fukushima.

In light of the recent quake, Central Japan Railway Co. has reduced the speed of bullet trains between Tokyo and western Japan until the warning is lifted. Japan's regulatory body has issued alerts to all nuclear power plants in the country to take precautionary measures.

Local governments and companies are also responding to the advisory issued by the Japan Meteorological Agency. Chipmaker Rohm Co. has temporarily suspended operations in Miyazaki Prefecture, Kyushu, and is conducting safety checks. Chubu Electric Power Co. has established a communication network to strengthen coordination between headquarters and branch offices. The company’s Hamaoka Nuclear Power Plant, which has been offline since 2011, is under an "earthquake standby" alert, with 300 personnel on-site as of Thursday evening to monitor facilities handling hazardous materials and implement emergency safety measures

Also Read: Japan Earthquake: Tsunami Warning Issued After 7.1 Magnitude Tremors