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海溝型地震・津波発生における粘土鉱物の役割

Role of clay minerals on tsunamigenic faulting during large earthquakes

亀田 純*; 清水 麻由子  

Kameda, Jun*; Shimizu, Mayuko

The very large slip on the shallow portion of the subduction interface during the 2011 Tohoku-oki earthquake (Mw 9.0) caused a huge tsunami along the northeast coast of Honshu, Japan. In order to elucidate the mechanics of such tsunamigenic slip, the Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project, JFAST), was carried out one year after the earthquake and succeeded in recovering rocks constituting the active plate boundary fault. Mineralogical analyses using X-ray diffraction revealed that the shallow portion of the megathrust is significantly enriched in smectite (60-80wt.%) compared to the surrounding sediments. This mineralogical feature is a fundamental reason for realizing the weak fault zone under various slip conditions as demonstrated by laboratory friction experiments. The smectite-rich deposits are broadly distributed in the northwestern Pacific Ocean, and may therefore potentially enhance conditions for large shallow slip during earthquakes, which would result in large tsunamis for this region.

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