巨大地震注意「バランス」に課題［2024.8.16］

宮崎県沖で発生したマグニチュード（Ｍ）７・１の地震を受け、南海トラフ巨大地震が起きる可能性が平常時よりも高まっているとして臨時情報「巨大地震注意」が発表されてから１５日で１週間を迎えた。各地で備えの再確認につながった一方、社会経済活動に大きな影響を与えた臨時情報の在り方に関しては、課題を指摘する声もある。

「リスクの程度と社会の対応のバランスが取れていない印象を受けた」

南海トラフ周辺での地震活動に詳しい名古屋大の鷺谷威教授（地震学）は、臨時情報に伴う一連の状況に苦言を呈する。

■特急電車は運休、海水浴場は閉鎖

今回の地震により、巨大地震発生のリスクが高まったと言っても、発生確率は非常に低い。それでも特急電車は運休し、海水浴場は閉鎖されたほか、岸田文雄首相は外遊を取りやめた。「このような対応は昔の大震法（大規模地震対策特別措置法）の警戒宣言を引きずっているように感じた」と鷺谷教授は話す。

政府はかつて、南海トラフ沿いで想定される地震の１つで静岡県付近を震源とする「東海地震」（Ｍ８級）について、前兆現象をとらえれば直前予知が可能としていた。

これを前提として昭和５３年に成立したのが大震法で、首相が警戒宣言を発すれば対象地域の交通や生産活動を停止できるなどとしていた。

しかしその後、想定外だった東日本大震災（平成２３年）などを念頭に、現在の科学技術では「確度の高い地震予測はできない」と判断。大震法に基づく措置を転換し、国民に防災対応を促す臨時情報の運用が始まった。

■統計から得た一般論が根拠に

臨時情報の受け止め方が難しいとの声は、地震学者の間で多く聞かれる。今回の巨大地震注意は、世界中で起きた地震の統計から得た一般論が根拠で、南海トラフ沿いでの現象を解析した結果ではない。そのため、巨大地震が続いて発生する可能性については、科学的な精度が非常に低い。

一方で鷺谷教授は、想定震源域のプレート境界でＭ８・０以上の地震が発生した場合に出る「巨大地震警戒」の臨時情報については「（過去の発生履歴などから）理解できる」と指摘。臨時情報の在り方に関する社会的合意の必要性に言及した上で「随時見直しを行っていくべきだ」と話す。

　　
■英訳　

It has been one week since the emergency information "Warning of Major Earthquake" was issued, following the magnitude 7.1 earthquake off the coast of Miyazaki Prefecture, which raised the possibility of a massive Nankai Trough earthquake beyond normal levels. While this announcement led to a reassessment of preparedness in various areas, there have also been voices pointing out issues with the way the emergency information impacted socio-economic activities.

"I got the impression that the balance between the level of risk and society’s response was not well managed."

Professor Takeshi Sagiya, a seismologist from Nagoya University who is knowledgeable about seismic activity around the Nankai Trough, criticized the series of situations following the emergency information.

### Suspension of Express Trains and Closure of Beaches

Despite the increased risk of a major earthquake due to the recent earthquake, the probability of its occurrence remains extremely low. Nevertheless, express trains were suspended, beaches were closed, and Prime Minister Fumio Kishida canceled his overseas trip. "This kind of response felt reminiscent of the warning declarations under the old Large-Scale Earthquake Countermeasures Act," Professor Sagiya remarked.

The government used to believe that a precursor to a large earthquake near Shizuoka Prefecture, known as the "Tokai Earthquake" (estimated to be around magnitude 8), could be detected in advance, making short-term prediction possible.

Based on this assumption, the Large-Scale Earthquake Countermeasures Act was established in 1978, which allowed the Prime Minister to issue a warning declaration that could halt transportation and production activities in the target area.

However, after the unexpected Great East Japan Earthquake in 2011, it was determined that "highly accurate earthquake predictions are not possible" with current science and technology. As a result, the government shifted from measures under the Large-Scale Earthquake Countermeasures Act to the issuance of emergency information encouraging the public to take disaster preparedness measures.

### Generalities Derived from Statistics

The difficulty in interpreting the emergency information is widely acknowledged among seismologists. The recent "Warning of Major Earthquake" was based on generalities derived from global earthquake statistics, rather than an analysis of phenomena along the Nankai Trough. Therefore, the scientific accuracy regarding the likelihood of a subsequent massive earthquake is very low.

On the other hand, Professor Sagiya pointed out that the emergency information issued for a "Major Earthquake Alert," which would be triggered by an earthquake of magnitude 8.0 or higher occurring at the plate boundary in the expected source area, is "understandable" based on historical occurrences. He stressed the need for social consensus on the nature of emergency information and stated, "It should be reviewed continuously."

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