Quantitative Examination of Hummock Alignment in Debris Avalanche Deposits: Zenkoji Debris Avalanche, Usu Volcano, Japan

Abstract

Debris avalanche hummocks are created by catastrophic volcanic sector collapses. The paucity of quantitative data on their orientation relative to the debris avalanche direction has caused conflicting views among researchers. We evaluated hummock orientation in the Zenkoji debris avalanche deposits at Usu volcano, Japan. Our results show that hummocks within the upper part of the main depositional area, the axial band leading directly from the source to the central extremity of the debris avalanche deposits, tend to be aligned perpendicular to the presumed flow direction. However, hummock alignment varied systematically along the longitudinal path. Although hummocks close to the source were oriented perpendicular to the flow direction, we noted a gradual transition to a parallel orientation with increasing distance from the source. At the same time, hummocks had arisen on compression-dominated features of the debris avalanche surfaces in the marginal areas. These observations are consistent with the following scenario of the debris avalanche movement. To begin with, the origin is explainable for some large-scale hummocks near the source as below. At the moment of collapse, a small number of normal faults develop perpendicular to the landsliding direction, generating a series of ridges and grabens. Then, the alignment of the other hummocks was given rise to by an extensional regime as the runout distance increased and the lateral constraints on the erosional path near the volcano disappeared. As the longitudinal velocity increased further with respect to lateral velocity, hummocks became more strongly aligned at such distances from the proximal area. On the other hand, more-scattered orientations of hummocks have been observed in the marginal areas. This is probably because they were originated from the sediments pushed out of the main flow instead of being transported by the most direct way from the source.