Regarding the risk assessment of the possible occurrence of a massive earthquake in the Nankai Trough, the local governments concerned may struggle to deal with difficult problems.
A working-group panel of the Central Disaster Management Council has compiled a policy draft to deal with this possible major earthquake, whose epicentral region stretches from the seabed off the coast of Shizuoka Prefecture down to off the coast of the Kyushu region.
This is a massive quake with a 70 to 80 percent chance of occurring within the next 30 years. Assuming that the degree of danger can be estimated in advance, the new policy draft calls for residents and businesses in certain areas to evacuate as a precaution.
The most conspicuously abnormal phenomenon that could occur is presumed to be an earthquake of magnitude 8 or higher striking within one half of the Nankai Trough region.
Earthquakes cannot be predicted. Therefore, the only thing that can be done is to prepare for their sudden occurrence. The new policy draft does not correspond to this ironclad rule of quake-disaster management. Taking the unique nature of a massive Nankai Trough earthquake into consideration, this can be considered a realistic approach.
Quakes of this sort have occurred at intervals of 100 to 150 years. It has been confirmed that even when not all parts of the trough were shaken at once, there was more than one case in which the eastern side of the trough was rocked first. In 1854, the Ansei Tokai Earthquake was followed by the Ansei Nankai Earthquake just 32 hours later, causing enormous damage.
Given such a chain reaction of quakes, there is room to judge whether an earthquake that has occurred within half the entire region could lead to further quakes elsewhere.
Prepare for false alarms
The period for advance evacuations is said to be about one week. This was decided on the basis of quake-related data gathered both at home and abroad. During this period, crustal movements will be ascertained. It is significant to prevent damage from spreading through advance evacuations, while appropriating personnel and goods for rescue and relief activities in the affected regions.
The problem is that an advance evacuation often ends up being a false alarm.
Under the new policy draft, even when there is a possibility of an earthquake measuring lower than the magnitude-8 class occurring, the Japan Meteorological Agency will swiftly transmit relevant information, based on the risk assessment to be made by experts. But the probability of a massive quake actually occurring remains as low as around once in ten-plus occasions when it comes to the prediction of a magnitude-8 class quake, or at around once in several hundreds of occasions when a magnitude-7 class quake is predicted.
The principal entity to decide on which areas and residents will be in need of an advance evacuation is local municipalities of cities, towns and villages. Subject to an advance evacuation are coastal communities that are likely to be inundated by tsunami shortly after a quake occurs, and elderly people and others who would require a lot of time for evacuation.
The range of areas assumed to be affected by the quake includes 707 cities, towns and villages in Tokyo plus 28 prefectures. It won't be easy for these local governments, even if they are prepared for a possible false alarm, to decide whether to issue evacuation directives and the like.
It is reasonable that there were many local governments that said, in response to an inquiry by the Cabinet Office, that they cannot make such a judgment.
The government will draw up guidelines that will help local governments and businesses make their risk assessment. In that event, the government should also let local residents know that there is an ample possibility of an advance evacuation ending up being for nothing. To enhance the accuracy of the prior assessment, it is also essential to improve the observation network and accumulate relevant data within the anticipated areas.
(From The Yomiuri Shimbun, Dec. 13, 2018)
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