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隕石津波の定量評価に向けて
近貞(山本)直孝
(防災科学技術研究所;防災科研)
背景
海底下で地震が発生した
場合には、海底の地殻変動
に伴い少なからず津波が発
生する。一方で、NOAAの
データベース*に記録されて
いる津波のうち約15%は地
震以外に起因する津波であ
り、海底地すべりや海底火
山の活動によるものとして
研究が進められている。 津波発生要因の割合
https://www.ngdc.noaa.gov/hazard/tsu_db.shtml から作成
目的
• 隕石落下時の津波即時予測手法の開発
• 惑星の表面進化過程に於ける海洋の役割の定量
評価
藤本・今村, 1997 後藤・他, 2013
これまでの研究は、K/T-impact
に限った検討であったり、
簡易的な津波の初期モデル
を構築しているに過ぎず、
衝突現象も含めて数値モデ
ル化されていない(津波の
事典)。
実地形を用いた試計算
• 太平洋のどこに落ちるかによって被害想定箇所
が変わることを確認した。
• 南緯35度、東経190度〜280度
• 半値幅200km,深さ10mのガウシアン波源
S35E190 S35E280
手法
• 隕石落下によってどのような津波が生じるのか、
隕石衝突と津波伝播を数値シミュレーションに
よって高精度に再現するための数値モデルの構
築を行う。
数値衝突計算コードである
iSALEによって出力される海
水の分布を、津波伝播計算
コードであるJAGURSの初期条
件として入力することで、
数値モデルを構築する。
数値衝突計算コード
Wünnemann et al., 2006
津波伝播計算コード
Baba et al., 2017
iSALEを用いた試計算(t=0s)
• 直径640mの小天体を6.5km/sで半無限深の海水面に
衝突させた場合の挙動を確認した。
projectile
iSALEを用いた試計算(t=9.85s)
• 直径640mの小天体を6.5km/sで半無限深の海水面に
衝突させた場合の挙動を確認した。
リムの形成キャビティの形成
破壊されたprojectile
iSALEを用いた試計算(t=49.23s)
• 直径640mの小天体を6.5km/sで半無限深の海水面に
衝突させた場合の挙動を確認した。
リムの崩壊
リバウンドが始まる頃に
はprojectileは海底に沈む
ものとリバウンドと伴に
上空へ吹き上げられるも
のに分かれる。
リバウンドの最大波を津波伝播
計算の入力とする?
iSALEを用いた試計算(t=108.31s)
• 直径640mの小天体を6.5km/sで半無限深の海水面に
衝突させた場合の挙動を確認した。
キャビティの崩壊にリバウンドの戻りが加わって主た
る津波の成分が形成されて伝播していく様子が確認で
きた。
iSALEを用いた試計算(t=128.00s)
• 直径640mの小天体を6.5km/sで半無限深の海水面に
衝突させた場合の挙動を確認した。
1度目よりも非常に鋭いリバウンドが発生し、計算上
10km以上の上空まで達した。ただし、幅が数kmなので
津波(重力波)としては伝播しないと推測される。
伝播する津波
iSALEを用いた試計算(t= 9.85s)
• 直径640mの小天体を6.5km/sで海水面に衝突させた場合の
挙動を確認した。
半無限深海水 水深1,000m
iSALEを用いた試計算(t= 49.23s)
• 直径640mの小天体を6.5km/sで海水面に衝突させた場合の
挙動を確認した。
半無限深海水 水深1,000m
iSALEを用いた試計算(t=128.00s)
• 直径640mの小天体を6.5km/sで海水面に衝突させた場合の
挙動を確認した。
半無限深海水 水深1,000m
iSALEを用いた試計算(t=128.00s)
• 直径640mの小天体を6.5km/sで海水面に衝突させた場合の
挙動を確認した。
半無限深海水 水深1,000m
H=2,000 m H=3,000 m
H=4,000 m
H=5,000 m
衝突地点の地形データ分解能
• GEBCO30で,半値幅を変えたガウシアン初期波源計算
の例
• 30秒角(≒1 km)だと半値幅2 kmのガウシアンは解像度が足
りていない
• 波源域に対するネスティング地形をオンデマンドに作成する
D=2 km D=10 km
津波計算モデルの構築
• 津波の被害予測をする上では、陸域への遡上を
計算する必要がある。
• ASTER G-DEMが全球の陸域を1秒角メッシュで網羅
ASTER G-DEM https://gdex.cr.usgs.gov/gdex/
今後の課題
iSALEのどのタイミングの状態をJAGURSの入力とするのかの検討
津波浸水計算用グローバル地形モデル(Global Tsunami Terrain
Model)の作成
 GEBCOとASTER G-DEMのマージ
海底水圧計での観測可能性(衝突時、津波到来時)の評価
海面到達時の隕石の大きさの評価(大気で崩壊)
3Dシミュレーションコードを用いた斜め突入の評価
謝辞
 iSALE の 開 発 者 で あ る Gareth Collins, Kai Wünnemann,
Boris Ivanov, H. Jay Melosh, Dirk Elbeshausen の各氏に感謝致しま
す。
 iSALEの利用に際しては、黒澤耕介(千葉工業大学惑星探査研究センター)、
末次竜(産業医科大学)、脇田茂(東京工業大学地球生命研究所)の各氏よ
りiSALE講習会でご助言いただきました。
 本研究の一部はJSPS科研費 JP18K04674の助成を受けたものです。
 iSALEの計算は国立天文台CfCAの計算サーバで実施しました。

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隕石津波の定量評価に向けて