Tension cracks on the oceanward slopes of the northern Japan and Mariana Trenches

• Published in: Marine geology Vol. 141; no. 1; pp. 111 - 123
• Main Authors: Ogawa, Yujiro, Kobayashi, Kazuo, Hotta, Hiroshi, Fujioka, Kantaro
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.09.1997
• Subjects: earthquake shaking; gravitational instability; Japan Trench; Mariana Trench; Marine; oceanward slope; open crack; ISEW, Pacific, Mariana Trenches; INW, Japan; Japan Trench; earthquake shaking; oceanward slope; open crack; Mariana Trench; gravitational instability
• ISSN: 0025-3227; 1872-6151
• DOI: 10.1016/S0025-3227(97)00059-5

Many open cracks up to a few meters deep and a few hundred meters long were found by the research submersible Shinkai 6500 on the diatomaceous clayey sediment surfaces of oceanward slopes of the northern Japan Trench. Smaller-scale but similar cracks were also found in a similar setting of the Mariana Trench. The cracks are found on the horizontal or very gentle slopes just above steep cliffs, and are mostly elongated in directions nearly parallel to the strike of the cliffs, although some are aligned en echelon or are branching and merging. Cracks have no vertical nor lateral displacement, suggesting open tension fractures. Surface edges of the cracks are generally very sharp, indicating their young origin. A pressure ridge caused by mud overflow during closure of one crack is observed. Considering the available evidence, it seems likely that the cracks were formed at the horizontally stretched surface of the down-going subducting oceanic plate under tensional stress. This tensional stress may have been caused by a combination of gravitational slope instability plus additional inertia during earthquake shaking which occurs close to these areas.