• Subsequent quakes

New system must keep everyone vigilant after initial shaking subsides

It is necessary to keep in mind that a strong earthquake could be followed by another one that is even more powerful. It is important to remain alert even after the initial shaking subsides.

The Japan Meteorological Agency has decided to issue an advisory when a magnitude 7 or greater earthquake strikes along the Japan Trench or the Chishima Trench in the Pacific Ocean off the coast from Hokkaido to the Tohoku region, thus calling for vigilance against a potential subsequent tremor that could be even more powerful than the first.

In and around the trenches, there has been a case in which a strong earthquake was followed by an even more massive one: the 2011 Great East Japan Earthquake. A magnitude 7.3 quake occurred on March 9 of that year, causing no major damage. However, a magnitude 9 quake struck eastern Japan two days later.

Even if an initial earthquake is small, it is important to be prepared for a potential subsequent tremor like the one that occurred in 2011. In the event of an earthquake along the Japan Trench or the Chishima Trench, it is estimated that around 100,000 people could die in the worst-case scenario. It is meaningful that the agency will issue advisories for such potential events.

Since magnitude 7-class earthquakes happen relatively frequently in and around the trenches, an advisory on a potential subsequent quake is expected to be issued about every two years, according to the agency. However, it is believed that the probability of a magnitude 8 or greater quake actually occurring within a few days after an alert is issued remains as low as around once in 100 occasions.

Therefore, it can be said that most advisories will likely end up as false alarms. How information with such uncertainty should be handled, and to what extent preparations should be made for a potential subsequent tremor, are difficult questions. It is hoped that the government will educate the public on what this advisory means in an easy-to-understand manner.

The more time passes after an earthquake, the less likely a subsequent tremor becomes. When an advisory is issued, residents will be encouraged to take measures over the following week or so, such as fixing furniture in place and checking evacuation routes, while otherwise carrying on with normal life.

People will also be encouraged to pack clothes and personal belongings to keep by their side while they sleep so that they can evacuate immediately in case of an emergency. This may not be especially burdensome, as long as such steps are recommended for just several days following an initial quake.

A huge earthquake is also expected in the Nankai Trough, which stretches along an area off the Pacific Coast from the Tokai to Kyushu regions. If this quake happens and a second one is deemed imminent, residents along the coastlines will be urged to evacuate in advance.

Conversely, under the new post-quake advisory system for the Japan Trench and the Chishima Trench, residents will not be required to evacuate in advance but will be encouraged to maintain social and economic activities because magnitude 7-class tremors frequently happen there.

If advisories end up being false alarms over and over, some people may get used to them and consider that making preparations is meaningless. However, the guard should never be lowered.

With today’s science, it is difficult to issue an advisory that is accurate enough to be described as a forecast. Severe earthquakes without any precursors will surely continue to occur in the future. It is hoped that members of the public will make good use of the advisory system as an opportunity to help themselves become better prepared for an emergency, little by little.

(From The Yomiuri Shimbun, Jan. 6, 2023)