#1877 予測されていた大津波：869 Tohoku tsunami pararalles stun Mar. 13, 2012

  標題は3月11日のジャパンタイムズ1面下段に載った記事の見出しである。
　「869年に起きた東北津波との類似点は私たちを驚愕させる」
　地質学からのアプローチによる学術的な3.11大津波検証である。

　URLを記しておくので、ダウンロードして読んでほしい。
http://www.japantimes.co.jp/text/nn20120311a1.html
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Sunday, March 11, 2012

869 Tohoku tsunami parallels stun

Research team's efforts set precedent to add history to other quake-study disciplines

By REIJI YOSHIDA

Staff writer

When the Great East Japan Earthquake hit Tohoku on March 11 last year, quake researcher Masanobu Shishikura grabbed a tablet computer and called up the website of the U.S. Geological Survey in Virgina to search for information.

Seeing projections of a preliminary magnitude of 8.8 and its location off Miyagi Prefecture, he immediately realized that what his team had long dreaded had finally become reality: a recurrence of the 869 Tohoku megaquake.

He also knew what was coming next — massive tsunami that would engulf coastal areas in Iwate, Miyagi and Fukushima prefectures in 30 to 60 minutes, just as the previous disaster did more than 1,100 years ago.

"I knew that the people would not be saved unless they immediately started running," Shishikura said in a recent interview with The Japan Times.・・・・・

“I knew that the people would not be saved unless they immediately started running,” Shishikura said in a recent interview with The Japan Times. “I just kept praying, wondering if people there were evacuating properly.”

Shishikura, who heads a quake research team at the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukuba, Ibaraki Prefecture, now regrets he was unable to save any lives despite his team’s findings.

But the March 11 tsunami and his team’s research have been influential in initiating a paradigm shift in Japanese seismological studies over the past year.

The disaster lent credibility to his team’s research methodology, prompting others to launch interdisciplinary studies integrating seismology, geology and philology to examine the history of earthquakes over hundreds of years.

Shishikura’s team took soil layer samples along the Tohoku coast and checked them against disaster accounts recorded in ancient chronicles. This helped them conclude that major temblors and tsunami repeatedly slam Tohoku over a cycle of between 500 and 1,000 years.

Shishikura had planned to visit the Fukushima Prefectural Government last March 23 to explain this danger.

Shishikura’s team even drew up maps of the areas flooded by the 869 tsunami and planned to distribute copies to residents along the coast. The maps turned out to be nearly identical to the areas that were inundated on March 11.

“We could have saved some lives if the tsunami had come just a month later,” he lamented.

Shishikura said his team’s approach — studying both soil layers and historical accounts to extrapolate quake cycles over hundreds of years — was regarded as a “minor” technique by seismologists.

Before March 11, mainstream seismologists had mostly focused on watching and analyzing real-time data from observation equipment tracking tectonic plate and ground movements.

This led them to focus only on temblors from the past 100 years or so, because that’s as far back as the available data went, Shishikura said.

But in the wake of March 11, a panel under the government’s Central Disaster Management Council in June adopted a new interim report signaling a clear departure from traditional seismology.

The report recommended that the government prepare for a “worst case” earthquake and tsunami combo that could strike in a cycle extending over hundreds or even thousands of years.

“To confirm occurrence of gigantic tsunami over thousands of years, it’s important to strengthen integrated research from not only seismology, but also archaeology, historical science and surveys of tsunami sediment, soil and animal fossils,” the report said.

Prompted by this shift, many seismologists are now using interdisciplinary methods to determine the worst-case quake-tsunami scenario.

Among them, Shishikura and other experts are particularly concerned about gigantic tsunami that could be striking Hokkaido’s east coast about every 400 years.

Far before its Tohoku surveys, his research team at AIST had taken soil layer samples from Hokkaido’s coastline and found traces of multiple tsunami that were probably caused by powerful earthquakes that originated in the Kuril Trench off Kushiro.

The most recent one is believed to have occurred in the 17th century and to have spawned tsunami at least 10 meters high, Shishikura said.

Since about 400 years have passed, the same thing could happen again at any time, he warned.

The eastern coast of Hokkaido has been sinking at a rate of roughly 1 cm a year, an unusually fast pace. This may be a sign of an impending quake.

“The tsumami could be the same size as (those on) March 11,” he said.

Shishikura also believes the March 11 quake changed the distortion balance of the plates near Tohoku to a great extent, possibly increasing the risk of another major temblor occurring in the region.

For example, after the magnitude 9.1 megaquake off Sumatra in 2004 spawned catastrophic tsunami, quakes with magnitudes of 7 or 8 started occurring.

“You can’t deny that the Japanese archipelago could be in a similar situation as well,” he said.

Shishikura, however, also said March 11 might have instilled “some sense of panic” in quake researchers, causing them to focus too much on the worst-case combo scenario.

After all, while researchers can only point out the risk of a potential quake, they can’t predict exactly when it will come, he said.

“So the important thing is to learn about the risks and get prepared for disaster, not worry too much about when it will come,” he added.

In the past year, the public has become hypersensitive to new quake predictions.

The daily Yomiuri Shimbun reported Jan. 23 that the risk of southern Kanto, the region encompassing Tokyo, being hit by a magnitude 7 earthquake in the next four years is as high as 70 percent. This kicked off a string of sensational media reports.

The Yomiuri report was based on an estimate by the University of Tokyo’s Earthquake Research Institute, which immediately played down the significance of its own estimate. The institute said its margin of error was too wide and its calculations were based on data from the multitude of aftershocks that followed the March 11 quake through September.

“The figures themselves do not have much meaning,” the institute said on its website.

But it also urged Tokyoites to brace for a major quake, emphasizing that many experts have said one is long overdue in the capital.

Indeed, the science ministry’s Headquarters for Earthquake Research Promotion is fond of repeating that the probability of a magnitude 7 earthquake hitting southern Kanto is “70 percent within 30 years,” without stating the specific year that period starts from.

Separately, the government’s Central Disaster Management Council has predicted that if a magnitude 7.3 quake hits Tokyo, it will kill up to 11,000 people, destroy 850,000 structures and cause ¥112 trillion in economic damage.

“It’s not just Tokyo. If you live anywhere in Japan, there is the possibility of being hit by an earthquake with a magnitude of around 7,” the research center said.

“As long as you live in Japan, you should prepare for a 7-magnitude quake. That’s the minimum necessary thing you should do.”

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  産業技術総合研究所海溝型地震履歴研究チーム長宍倉正展博士作成の地図である。869年（貞観11年）の津波シミュレーションライン"Estimated flood line in 869"と東北大震災津波"Flood area in 2011"が重なっている。
　昨年3月28日に福島県庁でこの津波の危険を説明するスケジュールになっていたが、実際の津波の方が早く来てしまった。"すぐに走って逃げない限り助からないとわかっていた"とインタビューに応えている。地質調査から巨大津波が来ることを予測していたのである。
　今回の津波は想定外ではなかった。2007年にも宍倉氏は研究論文を発表している。なお、貞観11年に陸奥の国で起きた大津波のことは『日本三代実録』に載っている。古記録は日本の財産であると言える。
　ひとつ疑問がわいた。海岸線が内陸側に１㎞ほどずれている。これは日本列島の隆起によるものなのか、それとも海水面が当時よりも下がったからなのか。

　この記事もニムオロ塾の時事英語授業で使うことにしたい。ジャパンタイムズ記事はホットで楽しいテクストだ。

*TBSビデオ映像
　宍倉正展博士が地震後堆積した砂の測量調査を坦々としている、その結果が上記の地図であろう。
http://news.tbs.co.jp/newsi_sp/shinsai2011/tbs_newseye4699986.html

**宍倉正展博士のホームページ
　2007年の学術論文も載っている。
http://staff.aist.go.jp/m.shishikura/index.htm


にほんブログ村

コメント 9

図から、河川の護岸工事が仇になった事が読み取れます。
（中央と右上に注目ください。）

これは、文献や過去の事実からは、
一般人が直接に予測する事はできません。

しかし、この過去のデータがあれば、
コンピュータ解析でハザードマップが作れた事でしょう。

宍倉氏は無念の限りだと思います。
by Hirosuke (2012-03-13 21:16) 

>海岸線が内陸側に１㎞ほどずれている。

これは、
「日本列島の隆起による」
と考えます。

東北地方の奥羽山脈は、
日本列島が乗っているプレートを、
太平洋側の別プレートが圧縮して形成されたものです。

また、
海水面については、
温暖化の影響かどうかは脇に置くとして、
確実に上昇しています。
by Hirosuke (2012-03-13 21:27) 

今回の大地震で、
仙台周辺は80センチ前後も沈降しています。

両年のFLOOD LINEが奇妙に一致しているのは、
この間に地下に蓄積されたプレート隆起が、
一気に全解放されたからだと推測されます。

ただし、
こんな後付け分析なんか、
誰でも出来ます。

僕も無念です。
by Hirosuke (2012-03-13 21:34) 

Hirosukeさんへ

太平洋プレートに押されて奥羽山脈がセリ上がり、海岸線が後退、そして今度の地震で圧力が解放されて沿岸が８０センチ沈降したということですか。

南海地震を経験した四万十川のある高知県中村市の人に聞いた話しですが、帯状に家並みが潰れていたそうです。軟弱な地盤に家を建てたら地震でつぶれる。

南海地震津波で堆積した砂が４センチ、別の地質学者の調査によれば過去には４０センチもの砂の堆積層があるそうです。その量から逆算した津波の高さは４０メートル。

巨大地震があったらすべてを捨ててすぐに逃げるしかないですね。
温泉・火山・地震に恵まれた日本で原発なんてとんでもないことです。そのうちに誰も住めなくなります。
by ebisu (2012-03-13 22:50) 

>高さ40メートルの津波

その地質学者は自らの計算結果を見て、
「有り得ない」
と呟いて計算し直した事でしょう。

当地は内陸なのが、
せめてもの救いです。

by Hirosuke (2012-03-13 23:01) 

奥羽山脈が形成されたのは数万年前ですから、
前回も今回も、
直接には関係していないと思います。

ただし、
同じメカニズムが働いている事は確かであり、
押されて隆起していた分が元に戻って沈降した、
と考えるのが自然でしょう。

「自然」か・・・。

by Hirosuke (2012-03-13 23:11) 

「水蒸気爆発」⇒「オレンジの閃光」について僕がコメントした記事を探しているのですが、見つかりません。

お手数ですが、URLを示して頂けますでしょうか。

by Hirosuke (2012-03-15 07:05) 

Hirosukeさんへ

お問い合わせの「オレンジ色の閃光」のコメントは次のURLにあります。

http://nimuorojyuku.blog.so-net.ne.jp/2011-10-19-1

「#1691 福島県浪江町津島地区で何が起きていたのか：3月11日から数日の記録 Oct. 19, 2011　」
by ebisu (2012-03-15 10:17) 

お手数おかけしました。

トラックバック記事を作成します。

by Hirosuke (2012-03-15 10:50)