Abstract
During the process of initiation and motion of a debris flow triggered by a landslide, failure of clay blocks (megablocks) of in the landslide developed. It is important to clarify the mechanism of this failure in order to develop a model for this type of debris flow.
We studied the fracture of megablocks by surveying deposits of the debris flow that occurred at Hachimantai, Akita Prefecture in May 1997. This study revealed the following.
1) The Landslide C block contained well-weathered tuffs as a source of megablock.
2) The debris flow deposits included many megablocks of several meters in diameter, slightly smaller than the depth of the deposits. The megablocks found in the downstream area had a larger degree of failure.
3) The typical form of failure was due to shear. Thus, as a landslide or debris flow moved downward, shear failure of the megablocks developed. The matrix became fine-grained as a result of the failure of megablocks.
4) The depth of failure (δ) of megablocks was roughly proportional to the distance from the landslide area to the position of a trench (x).
