Fractal dimension of debris-avalanche deposits in the Hawaiian submarine landslide deposits

17 landslide deposits on the flanks of the southern Hawaiian Ridge have been classified into two major types: SLUMPS, which moved slowly as a coherent mass, and DEBRIS AVALANCHES, which moved quickly.The debris-avalanche deposits are predominant on submarine flanks of volcanic ocean islands elsewhere in the world. Such huge landslides are considered to produce giant tsunamis and megaturbidites covering large areas of abyssal plains. Based on the small scale topographic elements, we reinvestigated the distribution areas and emplacement styles of the debris-avalanche deposits, which differ from those previously proposed from GLORIA images without benefit of detailed bathymetric data or direct seafloor observations. There are several types of small scale topographic elements in the debris-avalanche deposits previously proposed: source amphitheater, toppled blocks, marginal levee, slide-emplaced blocks, chute, mud wave, hummocky terrain. They are very similar to those appeared in subaerial volcanic debris-avalanche fields. However, no correlation between the collapse height and runout distance are observed in the submarine debris-avalanche deposits. The hummocky terrains can be classified into two types: FLAT-TYPE, which is distributed in the nearly flat abyssal plain, less than 0.5 degree, and SLOPE-TYPE, which located on the lower part of the submarine flanks, greater than 1 degree. The size of hummocks in a slope-type hummocky terrain have an unimodal distribution pattern with a broad peak in the number of hummocks versus height category diagram. On the contrary, the size of hummocks in flat-type hummocky terrains have a power law distribution pattern in the same diagram. The fractal dimensions calculated from these diagrams are 1.19 (Nuuanu landslide), 2.32 (Ka Lae landslide) and 2.96 (Alika 2 debris-avalanche), respectively. They are expected to reflect the processes and degree of fragmentation. Therefore, among the debris_]avalanche deposits proposed previously around Hawaiian ridge, only three debris-avalanche deposits, Nuuanu, Alika 2, Ka Lae, could be a huge landslide deposits accompanied with huge tsunamis. Because fractal dimension indicates degree of the fragmentation, Alika 2 debris flow could be the most powerful turbulent flow among others. The bending trend in the power law distribution pattern of the Nuuanu landslide imply that the hummocks were produced by two different fragmentation: turbulent flows at the toe of the debris-avalanche and translational disruption at proximal part. The hummocks without a power law distribution in the terrain have been produced by a overlapping of small scale debris flow deposits rather than a huge landslide failure. Unimodal size distribution with a broad peak may be interpreted as a gravel-rich submarine fans rather than a huge landslide deposit.