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Yomiuri Shimbun file photo The Cabinet Office building in Chiyoda Ward, Tokyo
Jiji Press
16:14 JST, November 9, 2022
TOKYO (Jiji Press)—Japan’s Cabinet Office said Tuesday an alert will be issued against a subsequent earthquake with a moment magnitude of 8.0 or greater when an earthquake with a moment magnitude of at least 7.0 occurs along the Chishima Trench or the Japan Trench in the Pacific Ocean off the coast of northern Japan.
Issuing the alert, the office and the Japan Meteorological Agency, as well as local governments, will ask residents and businesses in Pacific coastal areas stretching from the northernmost prefecture of Hokkaido to Chiba Prefecture, east of Tokyo, to prepare against big shakes and tsunami for about a week after the first one happens off Hokkaido, Aomori Prefecture or Iwate Prefecture. Meanwhile, they will not request advance evacuations, traffic controls or school closures.
When the first earthquake’s moment magnitude is 8.0 or greater, the advisory will be given even if the focus of the quake is off Miyagi or Fukushima, both northeastern prefectures located south of Iwate.
Moment magnitude is a scale capable of evaluating large earthquakes more accurately than the magnitude scale used ordinarily by the JMA, although it takes longer to calculate.
Disaster response guidelines drawn up by the office specifically call on residents, firms and local governments to get ready for immediate evacuation in case that a subsequent massive quake occurs or a tsunami warning is issued. But at the same time, the guidelines ask them to maintain social and economic activities.
The new earthquake advisory system will start on Dec. 16.
In March 2011, the main temblor of the Great East Japan Earthquake with a moment magnitude of 9.0 happened along the Japan Trench off the Sanriku coast and unleashed mammoth tsunami two days after a quake with a moment magnitude of 7.3 struck the northeastern region.
Along the Chishima Trench, also known as the Kuril Trench, a quake with a moment magnitude of 8.5 occurred off Etorofu, one of the four Russian-held, Japanese-claimed islands off Hokkaido, in 1963, 18 hours after a shock with a moment magnitude of 7.0.
A forecast says a 9.0-moment-magnitude earthquake and subsequent tsunami is looming along the trenches. If such a quake actually happens, it would measure lower 6 or higher on the Japanese seismic intensity scale of 7 in the Pacific coastal areas, or tsunami with a height of at least 3 meters would hit there.
Still, only about one in 100 earthquakes with a moment magnitude of 7.0 or higher was followed within a week by a jolt with a moment magnitude of 8.0 or greater within areas up to 500 kilometers from the first one, according to global earthquake data for the past century.
The temblor advisory is expected to be issued about once in two years, and in most cases a subsequent quake is unlikely to take place.
On Tuesday, the office said in a report that the new system is expected to work to mitigate damage from an earthquake despite its high level of uncertainty, because it would help people become more vigilant.
