TENSION CRACKS ON THE OCEANWARD SLOPES OF THE NORTHERN JAPAN AND MARIANA TRENCHES

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 diatomaceo us 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 elongate d 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 fracture s. Surface edges of the cracks are generally very sharp, indicating th eir young origin. A pressure ridge caused by mud overflow during closu re of one crack is observed. Considering the available evidence, it se ems likely that the cracks were formed at the horizontally stretched s urface of the down-going subducting oceanic plate under tensional stre ss. This tensional stress may have been caused by a combination of gra vitational slope instability plus additional inertia during earthquake shaking which occurs close to these areas. (C) 1997 Elsevier Science B.V.