Photo/Illutration A poster explaining the introduction of a new alert that will be issued for subsequent earthquakes near Hokkaido or off the coast of Sanriku, seen on Nov. 6 at a school campus in Iwaizumi, Iwate Prefecture (Akina Nishi)

The central government decided on Nov. 8 that in the event of a huge earthquake along the Japan Trench or the Chishima Trench, it will issue an alert warning of the potential for follow-up earthquakes.

The government calls them “subsequent earthquakes,” a term coined to differentiate them from the weaker aftershocks typically felt after a big quake.

The new alert is part of the government’s preparations for a possible magnitude-9 earthquake that may strike one day, which experts estimate could cause as many as 200,000 deaths.

The new warning will come online on Dec. 16, officials said.

In the event of such an emergency, the alert will urge residents in affected areas to take stronger disaster-prevention measures for a week due to a risk of subsequent earthquakes. 

When a magnitude-7 quake strikes, with an epicenter in or around Hokkaido or off the Sanriku Coast, the Japan Meteorological Agency and the Cabinet Office will hold a joint news conference two hours after the quake and issue an alert on the ground that the possibility of subsequent earthquakes is higher than usual.

Municipal governments will then notify their residents through available media, including radio.

Once an alert is issued, municipal governments in Pacific coastal areas from Hokkaido to Chiba Prefecture will be asked to strengthen their disaster-prevention measures.

These areas could face the risk of tsunami 3 meters or even taller and a quake with seismic intensity of 6 or more.

Residents in the affected areas would be asked to prepare to evacuate the area immediately once a tsunami alert is issued, such as checking evacuation routes and packing emergency items to carry with them.

202211009-quake-G-L
The Asahi Shimbun

But issuing the alert would not necessarily mean that a big quake would follow.

Trains would not be asked to suspend their operations, and businesses would not be asked to close. Residents would be asked to make their preparations as they carry out their normal lives as usual.

Major quakes have frequently struck the area along the Japan Trench and Chishima Trench in the past. That includes the 2011 Great East Japan Earthquake, which was magnitude 9.

The notion that a new alert is needed was driven by some grim projections.

According to a recent estimate, if a magnitude-9 quake hits the Japan Trench in winter late at night, some 199,000 people would die. If it occurs in the Chishima Trench, it projected that 100,000 people would die.

In addition to the damage caused by tsunami, many people living in cold weather regions would also likely die from hypothermia.

But if disaster-prevention awareness is improved and more people evacuate to safety early on, the total number of deaths would be reduced to 47,000 from a Japan Trench quake and 44,000 from one in the Chishima Trench.

If people make use of buildings designated as emergency evacuation shelters in the event of a disaster like this, experts project deaths would be reduced by a staggering 80 percent.

The central government said the likelihood of a huge earthquake occurring in the Chishima Trench in the next 30 years ranges between 7 and 40 percent, so it sees the need to strengthen preventative measures as an urgent matter.

But it is incredibly rare for such subsequent earthquakes to occur within a week of a quake that is magnitude 7 or larger. There is only a 1 percent chance of that happening anywhere in the world.

Still, the central government has zeroed in on the fact that a magnitude-7.3 quake struck off the coast of Sanriku two days before the 2011 quake.

Not to mention that massive earthquakes can occur in unexpected ways.

A large quake, magnitude 7 or higher, occurs every two years or so in the estimated epicenter area, the central government said.

Related News